Neohipparion affine (Leidy, 1869)

Woodburne, Michael O., 2007, Phyletic Diversification Of The Cormohipparion Occidentale Complex (Mammalia; Perissodactyla, Equidae), Late Miocene, North America, And The Origin Of The Old World Hippotherium Datum, Bulletin of the American Museum of Natural History 2007 (306), pp. 1-138 : 37-86

affine , N. coloradense , N. trampasense , N. leptode , N. eurystyle , and N. gidleyi .

REVISED DIAGNOSIS: Based on the type and referred material, within the subgenus Cormohipparion , C. occidentale is distinguished from C. goorisi and C. quinni (fig. 9) in larger cranial (and presumably

TABLE 7

Measurements (mm) of Upper Cheek Tooth Dentition of Cormohipparion occidentale, Merritt Dam Member, Ash Hollow

Formation, Medial Clarendonian, Nebraska

Height is at mesostyle; boldface specimens used in plication count statistics; a 5 approximate; rel. 5 relatively.

body) size; in higher crowned cheek teeth; in having a more complex enamel pattern in the upper cheek teeth; in dP1 being reduced to absent; in lower cheek teeth with stronger protostylids; in a cranium with the DPOF having a generally teardrop shape (dorsoventrally much higher posteriorly than anterior- ly); and in the lacrimal not reaching the rear of the DPOF.

With respect to other members of the C. occidentale group, C. occidentale , s.s., is distinguished in having 4–5 plis on the anterior border of the molar prefossette. The molar posterior border of the prefossette has 8–9 plis; the premolar posterior border of the postfossette has 2–3 plis; the molar posterior border of the postfossette has 2– 3 plis; the molars usually have two plis caballin; and the orbit is posterior to M3. With a range of about 50–66 mm, the unworn MSTHT for P3–M3 is higher than that for all other species, possibly except C. skinneri , n.sp. (table 3B), a segregation strengthened by a comparison of unworn MSTHT for P4/M 1 in these samples, as well.

In addition, and as compared in figures 13, 15, 18, 20, 22, and 24, C. occidentale has the most deeply incised nasal notch (no. 30) of all the other species, and the lacrimal does not extend anteriorly beyond the junction with the maxillo-jugal suture, in contrast to C. merriami , n.sp., and C. johnsoni , n.sp., but not C. fricki , n.sp., C. matthewi , n.sp., or C. skinneri , n.sp. As in C. matthewi , n.sp., the lacrimal in C. occidentale extends across about 60% of the POB (table 3A), greater than all other species except C. skinneri , n.sp., and C. johnsoni , n.sp. Cormohipparion occidentale also has a longer muzzle (parameter 1) than all members of the Cormohipparion group but C. merriami , n.sp. The 1 S.D. (standard deviation) range of the length of the DPOF (no. 33) in C. occidentale incorporates the (low) range of this feature seen in C. merriami , n.sp., and C. skinneri , n.sp., so these populations probably are not distinct in this regard. However, the DPOF is sharply shorter in C. matthewi , n.sp., and C. johnsoni , n.sp., and its 1 S.D. ranges are much smaller in C. fricki , n.sp. None of the other species shows as tall a DPOF (no. 35) as in the Machaerodus and Hans Johnson quarry samples of C. occidentale , and only C. johnsoni , n.sp., has as short a separation of the DPOF and facial crest (no. 36) as in those two quarry samples of C. occidentale .

REFERRED MATERIAL: From Little White River, South Dakota. Medial Clarendonian.

From the Ed Ross Ranch Quarry,?late medial Clarendonian,?Thin Elk Formation, Bennett County, South Dakota. F:AM 71872, Oi skull with right and left C1, P2– M3 (fig. 11).

From Loup Fork beds,?Clarendonian. AMNH 10869, fragmentary skull with right and left dP2–4, right and left M1, RM2 erupting.

TABLE 7 (Continued)

Specimen Wear class; remarks

Cormohipparion occidentale ; Xmas-Kat Quarries, Merritt Dam Member, Ash Hollow Formation, Nebraska

F:AM 71868 Oi DP 2–4 in place; P2–4 from crypt; M2 very early wear. Anterior prefossette plis very small mostly. Nearly true crown height of M1 must be ca 66 mm. P2, 43 mm unworn height; P3, 50 mm; P4, 52 mm; M1, 66 mm; M2, ca 63 mm; M3,? 43 mm.

F:AM 71801 Oi Early juvenile wear; P2–4, M3 erupting; note complex pattern; dP1is absent. Note relatively round protocone on M1. Labial pre- and postfossettes not formed in P2.

F:AM 71807 Early juvenile wear; M3 not fully formed; very complex pattern. P2 pre- and postfossettes separate labially; ovate protocone P2, M1; others more elongate; dP1 is absent.

F:AM 71810 Early adult wear; M3 formed; very complex pattern in all; P2 pre- and postfossettes linked.

F:AM 71800 ♀ Late medial wear; slightly lingually convex protocones in premolars, nearly flat in molars; double pli caballin P2-4; complex pattern in all; pre- and postfossette labially separate in P2.

F:AM 71808 Early medial wear; slightly lingually convex protocones, double pli caballin in P3–M3; complex pattern.

F:AM 71809 Late medial wear; slightly lingually flat to slightly convex protocones, quadruple pli caballin in P2–4; P2 pre- and postfossette separate; fatter protocones in later wear.

F:AM 71813 ♀ Early medial wear; M3 early wear; note complexity; double pli caballinin P2–M3; complex anterior pli prefossettes; 2 plis anterior to small pli protoconule in P4–M2. P2 pre- and postfossettes separate. dP1 is 3.0 mm long.

Cormohipparion occidentale ; Hans Johnson Quarry, Merritt Dam Member, Ash Hollow Formation, Nebraska

F:AM 71862 ♀ Early juvenile wear; M2 barely worn; M3 not erupted. Note moderately complex pattern; elongate protocones, cf. type of C. occidentale . P2 with labially separated pre- and postfossette. dP1 8.0 mm long.

F:AM 71861? ♀ Early adult wear; M3 pattern incomplete; note relatively complex pattern. P2 pre and postfossettes linked labially. Anterior prefossette of P2 cf. type of C. occidentale . Lingual penetration of hyposelene disrupts pli caballin. dP1 ca 7.5 mm long.

F:AM 71860 Oi Early adult wear; M3 pattern incomplete; note relatively complex pattern. P2 pre- and postfossettes separate labially. Note elongate, slanted protocones, lingually concave cf. type of C. occidentale .

F:AM 71857? ♀ Adult wear; M3 pattern complete; still relatively elongate protocones; relatively complex pattern. dP1 is 9.3 mm long.

F:AM 71858 ♀ Late adult wear; protocones still ovate, except M3; triple P3–4, double M2–3 plis caballin; pre- and postfossette P2 labially connected. dP1 7.5 mm long.

F:AM 71856 Oi Late adult wear; still relatively elongate protocones, not concave lingually; connected in P2, nearly connected in M1.

F:AM 71855 Oi Late adult wear; broad protocones; protocone connected in P2, nearly in M1; pre- and postfossette P2 obliterated.

Cormohipparion occidentale ; Machaerodus Quarry, Merritt Dam Member, Ash Hollow Formation, Nebraska

F:AM 71831 Oi Early juvenile wear; M2 barely worn; M3 not erupted. Note relatively simple pattern; more circular protocone M1; short protocone M2, contra type of C. occidentale .

F:AM 71844 Oi Early adult wear; M3 pattern incomplete; note relatively complex pattern. P2 pre- and postfossettes linked labially.

F:AM 71835 Early adult wear; M3 pattern incomplete; note relatively complex pattern. P2 pre- and postfossettes linked labially. Note elongate, slanted protocones, lingually concave cf. type of C. occidentale .

F:AM 71832 Oi Late adult wear; slightly lingually convex protocones premolars ex. P2; protocone connected in P2; riple P3–4, double M1–3 pli caballin; pre- and postfossette P2 labially separate.

F:AM 71834 ♀ Late adult wear; broad protocones; protocone connected in P2, 3, M1; single pli caballins; pre- and postfossette in P2 obliterated.

From Niobrara River, Merritt Dam Member, Ash Hollow Formation, Cherry County, Nebraska. Late medial Clarendonian.

XMas-Kat quarries. F:AM 71800, nearly complete cranium with right and left I1–3, C, P2–M3, adult female, M3 well worn. F:AM 71801, nearly complete cranium with right and left I2–3, C, P2–M3, sub-adult male, M3 just erupting. F:AM 71803, nearly complete cranium with L I2–3, right and left C, P2– M3, adult male, M3 erupted. F:AM 71806, partial cranium with right and left I1–3, P2– M3, adult female, M 3 in medial wear. F:AM 71807, right and left palate with P2–M3, M 3 in early wear. F:AM 71808, partial cranium with right and left P3–M3, M 3 in early wear. F:AM 71809, partial cranium with right and left P2–M2, LM3, M3 well worn. F:AM 71810, partial cranium with right and left P2– M3, M3 well worn. F:AM 71813, nearly complete cranium with right and left I1–3, RC, right and left P2–M3, adult female, M3 well worn. F:AM 71814, badly crushed cranium with L I2–3, C, right and left P3– M3, adult male, M3 well worn. F:AM 71868, anterior cranium with R I2–3, right and left C, dP2–4, LM1–2 erupting, LP2–4 removed from crypt, juvenile male.

Machaerodus Quarry. F:AM 71831, snout and frontal regions preserved, with right and left I1, L12, right and left C, RdP1, right and left dP4, P2–M3, M1–2 erupting, lower mandibles with right and left i1, Ri3, right and left c, p2–m2, juvenile male, p2 and p3 erupting. F:AM 71832, nearly complete cranium with right and left I3, C, LdP1, right and left P2–M3, adult female, M3 well worn. F:AM 71834, snout and facial region with right and left I1–2, RI3, P2–M3, ancient female, M3 very well worn. F:AM 71835, palate with LP2, right and left P3–M3, adult, M 3 in early wear. F:AM 71844, partial snout and palate with R C, P2, right and left P3– M3, adult male, M 3 in early wear, and associated left and right mandibles with p2– m3.

Hans Johnson Quarry. F:AM 71855, laterally compressed cranium with right and left I1–2, LI3, C, RP2, right and left P3–M3, adult male, M3 well worn; mandible with right and left i1–3, c, Pp2–m3. F:AM 71856, nearly complete cranium with right and left I1–3, C, P2–M3, adult male, M3 worn. F:AM 71857, nearly complete cranium with right and left I2–3, dP1–M3, adult female, M3 well worn. F:AM 71858, nearly complete cranium with R I1, C, right and left dP1–M3, adult female, M3 well worn. F:AM 71860, partial cranium with L I3, right and left C, P2–M3, adult male, M3 very well worn. F:AM 71861, partial cranium with right and left P2–M3, adult female, M3 beginning wear. F:AM 71862, crushed cranium with right and left I1, right and left I2-3, C, dP1– M3, subadult female, M3 erupting.

DESCRIPTION OF REFERRED MATERIAL: The cranium of C. occidentale is poorly represented in specimens from localities of

medial Clarendonian age in South Dakota. The following description is based on material from the late medial Clarendonian XMas-Kat quarries, herein taken as the most representative of this species (see below). F:AM 71800 is a nearly complete and undistorted cranium of an adult female (fig. 10A). The skull is slightly crushed dorsoventrally but otherwise nearly complete. The parietal region is somewhat depressed; the maxillary overlaps by about 20 mm its normal contact with the nasal bone; and the lower suture of the lacrimal is obscured. The configuration of the face in this area is more accurately represented by F:AM 71801 (fig. 10B). Figure 10A illustrates the generally elongate configuration of the cranium (snout and facial region) typical of late Miocene hypsodont horses. The large DPOF (no. 33, table 5) characteristic of Cormohipparion species is clearly shown, as are its strongly demarcated borders. The anterior tip of the DPOF is closely associated with the infraorbital foramen, which is located above the anterior margin of P3 and at a level opposite the lower margin of the orbit, and exemplifies the anteroventral orientation of this fossa (state 0, character 5, table 1). Figure 10B demonstrates the extent to which the DPOF is pocketed in C. occidentale , reaching posteriorly about 10 mm to a point internal to the tip of the lacrimal bone; the anterior tip of the lacrimal is about 15 mm posterior to the rear of the DPOF. The lacrimal is not pointed anteriorly, but its anterior edge (maxillo-lacrimal suture) is nearly vertically oriented and approximately aligned with the maxillo-jugal suture. The nasal notch is well developed and extends posteriorly to a point above the anterior margin of P2. The anterior edge of the large orbit is located above the rear of M3 and is well separated from the DPOF by a wide POB (no. 32, table 5). As shown in table 5, the orbit tends to be somewhat longer (no. 28) than high (no. 29). The facial crest is well developed and extends anteriorly to a point above M1. Although crushed in F:AM 71800, F:AM 71801 shows that the frontal region is slightly domed anterior to the parietal crest. The occipital condyles are slightly below the level of the tooth row in lateral view, and the paroccipital processes are strongly developed.

Crania from the Machaerodus and Hans Johnson quarries are comparable to those from XMas-Kat. Figure 13 shows the logratio values for the C. occidentale samples from the Machaerodus and Hans Johnson quarries relative to that from XMas-Kat. Parameter 4 is a measure of the basicranium and thus of fundamental cranial size. The correspondence between the samples indicates that they are all about the same size. Although the Machaerodus Quarry occasionally is represented by only one specimen (in which case it is assumed that the dimension represents the log mean for its sample), the relatively close correspondence of the log means of all three samples indicates that range of variation (± 1 standard deviation) would overlap extensively. Parameters in which the S.D. range of the XMas-Kat sample overlaps the other two are no. 22 (inion height), no. 30 (nasal notch), no. 34 ( IOF relative to rear of DPOF), and no. 37 ( IOF height relative to alveolar border). Parameters in which the S.D. range of the XMas-Kat sample is overlapped by the other two (assuming the S.D. range for the sample represented by a single Machaerodus specimen is comparable to that from the Hans Johnson Quarry) are no. 1 (muzzle length), no. 3 (postpalatal length), no. 12 (choanal width), no. 13 (palatal width), no. 19 (transglenoid width), no. 25 (muzzle height; assumes a greater range for the Hans Johnson sample), no. 31 (facial length; log mean only in Machaerodus Quarry; includes no. 2 in part), no. 32 ( POB length), and no. 39 (lacrimal tip to rear of DPOF; see also table 3A). Parameters in which the Machaerodus and Hans Johnson samples are smaller or shorter than, and beyond the S.D. range of, the XMas-Kat sample are no. 2 (palatal length), no. 6 (total cranial length, in part includes no. 2), and no. 36 (facial crest below DPOF; inversely correlated with no. 35). Parameters in which the Machaerodus and Hans Johnson samples are slightly shorter or lower (but at least partly within the S.D. range of) XMas-Kat are no. 5 (foramen magnum to rear of palate; combines nos. 3 and 4), no. 7 (premolar length; includes no. 2 in part), no. 8 (molar length; includes no. 2 in part), no. 9 (premolar-molar length; includes no. 2 in part), and no. 28 (orbital length). Parameters in which the Machaerodus and Hans Johnson samples are much greater or higher than the S.D. range of XMas-Kat are no. 35 ( DPOF height; inversely correlated with no. 36; DPOF relative to facial crest). Those in which the above two samples are slightly higher than (but S.D. ranges overlap with) that from XMas-Kat are no. 18 (frontal width) and no. 29 (orbital height). In most cases where parameters differ, samples from Machaerodus and Hans Johnson quarries segregate together. In no. 33 (length, DPOF) and no. 38 (height of rear of DPOF relative to alveolar border), the Hans Johnson sample is partly (no. 33) to completely (no. 38) shorter than that from Machaerodus and XMas-Kat. In nos. 14–15, the S.D. range for the Machaerodus sample is overlapped by that from XMas-Kat, but the Hans Johnson sample overlaps that from XMas-Kat .

Except for parameter 2 (which likely is associated in part with nos. 7–9 and possibly with nos. 31, 33, and 34), no. 6 (in part includes no. 2), no. 36 (inversely correlated with no. 35), cranial parameters of the three quarry samples are generally similar. Thus, even though the three quarry samples are within about one mile of each other ( Skinner and Johnson, 1984: fig. 16) and are nominally correlative, it appears that the Machaerodus and Hans Johnson samples have a slightly shorter face/palate and slightly taller DPOF than does the sample from XMas-Kat. As indicated in table 5, both sexes are represent- ed in each of the three quarry samples, so these differences cannot be ascribed to sexual dimorphism. The possible significance of this is discussed below. In having the greatest number of specimens (table 5), the sample from XMas-Kat quarries is taken herein as the most representative of C. occidentale from Nebraska.

The upper incisors are well developed and ovate in cross section, with I3 tapering laterally. All have cement-filled infundibula. Lengths for I1–3 are 15.5 mm, 16 mm, and 15 mm, respectively; widths are 9.8 mm, 9 mm, and 8.4 mm. In F:AM 71800, the canine is relatively small, indicating the female sex of the specimen (in contrast to its much larger size in F:AM 71801; table 6). The canine is located 32 mm posterior to the rear of I3, or 36% of the length of the diastema (table 6) between I3 and P3. DP1 is absent in this as well as in all adult XMas-Kat quarry specimens. As indicated in table 6, the canine is located about 34% of the length (mean) of the diastema in XMas-Kat and Hans Johnson material and about 31% of the length in Machaerodus Quarry specimens.

As considered from all quarry sites, the upper cheek tooth dentition is relatively complex. Protocones tend to be elongate, but only P3 and P4 have protocones with slightly lingually concave borders at this stage of wear. As indicated in table 7, there are two plis caballin in both the premolars and molars; fossette borders are relatively complex, except for the posterior border of the postfossette (usually with 2 plis only). Although some specimens of C. quinni in early wear stages show multiple plications on the anterior border of the prefossette ( Woodburne, 1996b), this trait is especially characteristic of the C. occidentale group, with mean counts of three or more in most samples of P2–M1 (table 7). Overall, the sample from XMas-Kat Quarry has the most complex fossette borders at all tooth positions relative to the samples from the Machaerodus and Hans Johnson quarries. The fossette borders of P2, P3, and M2 appear to be more complex in the Machaerodus samples than in the Hans Johnson samples, with the reverse being the case for P4 and M1. M3 is not considered relevant in this regard as being poorly represented in the adult state.

The dP1 is absent from all adult skulls at the XMas-Kat quarries regardless of sex. The upper dentition of C. occidentale from the XMas-Kat quarries is relatively complex and high crowned (table 3B). As reconstructed from table 7, the unworn P2 is about 43 mm tall; P3 is 50 mm; P4 is 52 mm; M1 likely was about 66 mm; M2 was 65 mm; and M3 probably was about as tall as P2 (ca. 45 mm). At about medial wear (25–40 mm; table 7, excluding F:AM 71809, F:AM 71813), and as indicated in figures 9C and 10D, the mean plication count for P2 is about 6:8:6:3. In P3, this is 5:10:7:2; in P4, 7:9:7:2; in M1, 4:8:7:2; in M2, 4:8:6:2; and in M3, 4:7:4:1. For the entire sample, the protocone tends to be relatively elongate to subrounded (narrower in M2–3; table 3C), and the hypoconal groove is open and not spurred. The anterior border of the prefossette is relatively complex (more than 3 plis), commonly with many plications that are of small amplitude rather than there being a smaller number of relatively deep plis; the pli caballin is double or more in premolars and typically double in molars. Of eleven specimens, the P2 pre- and postfossette score is confluent in three specimens, separate in four specimens, and not applicable in four specimens.

FROM MACHAERODUS QUARRY: The dP1 is absent in all (two) adult male skulls and one adult female. It is present in one adult female skull. The upper dentition of C. occidentale from the Machaerodus Quarry is somewhat smaller than the sample from XMas-Kat Quarry and slightly less complex. The unworn cheek tooth crown height is difficult to assess in the available sample. Based on F:AM 71844, P 3 in an adult stage of wear is about 40 mm tall (table 7). M3 (very early wear) is 41 mm tall in F:AM 71835 (table 7), so the unworn crown height would have been at least 43 mm. This is comparable to the MSTHT in the XMas-Kat and Hans Johnson samples, so it is likely that the other cheek teeth were about as tall as in those samples. In about medial wear (ca. 30 mm; F:AM 71844, F:AM 71835, table 7), complexity in P2 reaches a mean of 5:6:3:1; in P3, 5:7:7:1; in P4, 3:8:6:2; in M1, 2:7:7:2; in M2, 3:8:6:2; and in M3, 1:7:5:1. With regard to the entire sample, the protocone tends to be elongate in the premolars but somewhat more ovate in the molars; the protocone retains a slight lingual concavity into later wear; the hypoconal groove is about equally spurred as un-spurred and open; the anterior border of the prefossette tends to be relatively complex and well delineated in medial wear, especially in the premolars; and the pli caballin is typically double in both premolars and molars but tends to be somewhat more complex in the premolars. Overall, the upper cheek teeth are less complex than in the XMas-Kat Quarry sample. The sample apparently is about as hypsodont as that in the Hans Johnson Quarry. Of five specimens, the pre- and postfossette of P2 score confluent in two, separate in one, and not applicable in two.

FROM HANS JOHNSON QUARRY: The dP1 is absent in all three adult male skulls and is present in all four adult female skulls. The upper dentition of C. occidentale from the Hans Johnson Quarry is relatively complex and high crowned. The unworn P2 is about 45 mm tall, judging from the very early wear stage of F:AM 71862 (table 7), and P3 is likely about 50 mm tall, judging from the very early wear in F:AM 71862 and compared with materials from the XMas-Kat quarries. On the same basis, likely unworn crown heights for P4 are 52 mm; M1 likely was about 64 mm; M2 was about 65 mm; and M3 probably was about as tall as P2 (ca. 45 mm). At about medial wear (25–40 mm; F:AM 71862, F:AM 71861, F:AM 71860, table 7), the plication count for P2 is about 4:5:5:1; for P3, 3:6:5:1; for P4, 5:6:5:1; for M1, 3:8:5:2; and for M2, 2:5:5:1. M3 is not represented at this wear interval. In the total sample, the protocone tends to be relatively elongate (table 3C) and lingually concave; the hypoconal groove is open and not spurred. The anterior border of the prefossette is relatively complex (3 or more plis in P2–M1), usually being represented by relatively deep plications rather than by a larger number of plications having a very shallow amplitude, and the pli caballin is double or more in premolars, typically double in molars. Overall, the dentition of the Hans Johnson sample is most similar to that of the type of C. occidentale than any other sample under study here. This similarity includes, in F:AM 71861 and F:AM 71860, the disconnection of the pli caballin from the hypoconal selene, with that selene penetrating labially to incorporate or lie in proximity with the pli protoconule, also as seen in P3 of the holotype. Of seven specimens, the pre- and postfossette in P2 score confluent in three, separate in three, and not applicable in one.

FROM ED ROSS RANCH QUARRY, SOUTH DAKOTA: F:AM 71872 from?late medial Clarendonian sites (referred by Skinner and MacFadden to Hollow Horn Bear Quarry [sic]), apparently in the Thin Elk Formation (Oi skull with right and left C1, P2–M3; fig. 11; tables 3A, 4C, 5), also indicates the presence of taxa with the morphology of C. occidentale in this district of South Dakota. The unworn MSTHT likely was in the range of 60 mm (table 4C). The cranial morphology shows that the ratio of facial length of the lacrimal (nos. 32, 39, table 5) to the length of the POB (no. 32, table 5) is relatively large (0.75; table 3A), and the ratio of the length of the lacrimal tip to the rear DPOF (no. 39, table 5) to the POB length is generally comparable (0.25; table 3A) to the later Clarendonian taxa C. occidentale or C. skinneri , n.sp. AMNH 10869, also possibly from the Thin Elk Formation, has an unworn MSTHT (M1) of about 65 mm (table 4C). As indicated above, this material is a likely correlate of that from the XMas-Kat quarries. P2 pre- and postfossettes are confluent on the right side of F:AM 71872 and separate on its left side; in AMNH 10869, the condition is separate.

The mandible of C. occidentale at the XMas-Kat Quarry is represented by F:AM 71800. As shown in MacFadden (1984: fig. 135) and table 8 (nos. 10–12), this female mandible is relatively shallow below the cheek tooth row. The symphysis extends for about half the length of the diastema from p2 to c1. The mandibular foramen is sited about halfway in the diastemal region. The lower incisors are arranged in an arcuate fashion and display cement-filled infundibula. The lower canine is nearly directly posterior to i3. The lower cheek tooth dentition is relatively slender in overall proportions. Protostylids are present on all cheek teeth except p2 and take the shape of a labially projected enamel loop. The metaconids and metastylids are well developed, of equal size, and essentially subrounded, except in p2, where the metaconid is slightly smaller than the metastylid and subtriangular. C. occidentale differs from C. quinni in the clear separation of p2 metaconid and metastylid and in their larger and subequal size, whereas in C. quinni the metaconid tends to be larger than the metastylid and both cuspids are relatively smaller. The labial margins of the protoconid and hypoconid are relatively flat in p2 and p3 but more convex in the remaining cheek teeth. A single pli caballinid is present in each of the premolars but not in the molars. The ectoflexids are labial in position in all cheek teeth and do not penetrate the metaconid– metastylid isthmus. The linguaflexid is relatively shallow in premolars and deeper in molars, but still sub V-shaped rather than Ushaped.

F:AM 71844, from the Machaerodus Quarry, is similar overall to F:AM 71800. The diastema in this male mandible is slightly shorter than in F:AM 71800, but other dimensions are comparable (table 8). The rear of the symphysis is about level with the anterior border of the mandibular foramen instead of being located posterior to the foramen in F:AM 71800. F:AM 71844 is in a slightly earlier wear stage than F:AM 71800, as judged by the less worn heel of m3. This may account for the otherwise unusual penetration of the isthmus by the ectoflexid in p2, p3, and m 1–3 in the Machaerodus specimen and the overall less prominently developed metaconids and metastylids. Woodburne (2003: 402) indicates that the penetration of lower premolar isthmuses by the ectoflexid is a plesiomorphic trait that is found in Merychippus insignis but not, for example, in C. goorisi . As indicated in the present report, this trait is typically absent in juvenile or adult (but not ancient) specimens of other species of Cormohipparion , so its presence in F:AM 71844 apparently is an atavistic occurrence.

F:AM 71855, from the Hans Johnson Quarry, is a male in late wear. The mandible is somewhat crushed dorsoventrally, but its overall dimensions (table 8) appear comparable to those of the other two specimens. As in F:AM 71844, the symphysis ends anterior to the mandibular foramen, which is located near the mid-length of the diastema. The cheek teeth in F:AM 71855 show that the ectoflexids penetrate the isthmus in late wear on all cheek teeth. Ectoflexids are present in all teeth except p2 but are nearly obscured by wear in p3–m1.

Cormohipparion matthewi , new species figures 14–15; tables 2–3, 6, 8–10

TYPE SPECIMEN: F:AM 71802, nearly complete skull with right and left I1–3, C, P2–M3, female, late adult wear.

TYPE LOCALITY: XMas-Kat quarries, Merritt Dam Member, Ash Hollow Formation, Cherry County, Nebraska.

AGE: Medial Clarendonian.

DISTRIBUTION: Xmas-Kat quarries, northcentral Nebraska.

DESCRIPTION OF TYPE SPECIMEN: The cranium of F:AM 71802 (fig. 14) generally resembles that of C. occidentale (fig. 10). It is slightly crushed dorsoventrally and medially above the facial crest. The frontals are slightly domed above the dorsal skull profile above the nasal bones. The canine is situated about two-thirds of the length of the diastema anterior to P2. The DPOF is relatively small (short in length as well as height; nos. 33 and 35, table 9) and appears to have been pocketed to a depth of about 13 mm posterior to the rear of the DPOF and, thus, anterior to the tip of the lacrimal. The IOF is located above the anterior half of P3 and below the anterior end of the DPOF, as in Cormohipparion generally. The anterior end of the orbit is located above M3 rather than posterior to it. The lacrimal is blunt anteri- orly, with its anterior end being about aligned with the maxillo-jugal suture rather than extending anteriorly from that point. It extends across about 60% of the POB (length 2, table 3A). The anterior tip of the lacrimal is about 20 mm posterior to the rear of the DPOF so that the facial extent of the lacrimal is about 27.5 mm, or 40%, of the DPOF (length 1, table 3A). DP1 appears to be absent. As indicated in table 10, P2 is at least 35 mm tall, and P3 is at least 33 mm tall. Based on the hypodigm, the cheek teeth appear to have been 40–45 mm tall in the unworn condition (table 3B), in contrast to 50–66 mm in C. occidentale . The fossette plications appear to have been relatively complex, as discussed below, but not as complex as in C. occidentale .

DIAGNOSIS: Based on the type and referred material, and as indicated in figure 15 and in comparison with figure 13, C. matthewi differs from C. occidentale in the greater length of the postpalatal dimension of the cranium (no. 3), and thus somewhat greater length of no. 5 (which includes no. 3). Effectively, all other measured parameters in C. matthewi are smaller, narrower, or shorter than in C. occidentale . This is exemplified by the shorter muzzle (no. 1) in C. matthewi . The palatal length (no. 2) and total cranial length (no. 6) in C. matthewi is also smaller than in C. occidentale from XMas-Kat Quarry but not in the samples from the Machaerodus and Hans Johnson quarries. Similarly, the cheek-tooth lengths (nos. 7–9) in C. matthewi are at the lower end of the range for these features in the XMas-Kat Quarry sample of C. occidentale but more in line with those from the Machaerodus and Hans Johnson quarries. The snout in C. matthewi is much narrower (no. 14) than in C. occidentale and all other species of the C. occidentale group except C. johnsoni , n.sp., which is even narrower. C. matthewi has the narrowest transglenoid dimension (no. 19) of any species discussed here. The snout (no. 25) is relatively much shorter vertically in C. matthewi , C. johnsoni , n.sp., and C. fricki , n.sp., than in C. occidentale or the other species. The nasal notch is relatively less developed in C. matthewi , C. johnsoni , n.sp., C. merriami , n.sp., C. fricki , n.sp., and C. skinneri , n.sp., compared with C. occidentale . The DPOF (no. 33) is sharply shorter in C. matthewi and C. johnsoni , n.sp., and the S.D. ranges are shorter in C. fricki , n.sp., and C. merriami , n.sp. The separation of the IOF relative to the rear of the DPOF (no. 34) is much less in C. matthewi compared with C. occidentale or the other species, except C. johnsoni , n.sp., and the DPOF is much shorter vertically (no. 35) in C. matthewi than in C. occidentale . The rear of the DPOF is nearer the alveolar border (no. 38) in C. matthewi than in C. occidentale and is exceeded in this only by C. johnsoni . C. matthewi has a much shallower DPOF (no. 40) than any other species.

The unworn dentition is not represented; overall plications are less numerous in medial adult wear; and protocones are perhaps somewhat more elongate, but the sample is small. The unworn MSTHT for the upper cheek teeth is estimated at 40–45 mm (tables 3B, 10).

REFERRED MATERIAL: From XMas- Kat quarries, eastern Cherry County, Nebraska ( Skinner and Johnson, 1984: 315, fig. 16). F:AM 71804, nearly complete skull, RI3, right and left P2–M3, female, adult wear. F:AM 71805, partial cranium, RI2–3, right and left C, P2–M3, male, early adult wear.

P2 Protocone Pli Hypoconal Specimen Height Length Width Ratio Index Length Width Ratio caballin groove XMas-Kat Quarry F:AM 71805 Oi 35.0a 28.4 21.6 0.76 3:9:5:1 6.7 3.0 0.45 2 open; simple F:AM 71802 ♀ 29.0 28.9 22.5 0.78 3:7:7:2 7.2 3.7 0.51 1 spurred F:AM 71806♀ 22.0 29.7 21.8 0.73 1:4:6:1 7.5 4.1 0.55 2 open; narrow F:AM 71804♀ 12.0a 27.2 20.9 0.77 0:1:3:0 7.4 3.2 0.43 0 open; narrow Range 27.2–29.7 20.9–22.5 0.76–0.78 6.7–7.5 3.0–4.1 0.43–0.55 Mean 28.5 21.7 0.80 3:8:6:2 7.2 3.5 0.49 2 SD 1.05 0.66 0.02 0.36 0.45 0.05 CV 3.67 3.03 2.38 4.94 14.2 11.2 N 4 4 4 4 2 4 4 4 2 4 Hans Johnson Quarry F:AM 71863 39.0 barely worn not formed XMas-Kat Quarry F:AM 71805 Oi 33.0a 25.7 23.6 0.92 5:7:6:1 8.8 3.5 0.40 3 spurred F:AM 71802 ♀ 32.0a 26.2 24.7 0.94 6:9:4:2 7.5 3.7 0.49 2 narrow; simple F:AM 71806♀ 26.0 23.7 23.5 0.99 1:8:6:2 8.6 3.8 0.44 2 open F:AM 71804♀ 16.0a 22.9 22.6 0.99 1:5:3:1 7.3 3.0 0.41 3 open; narrow Range 22.9–26.2 22.6–24.7 0.92–0.99 7.3–8.8 3.0–3.8 0.40–0.49 Mean 24.6 23.6 0.96 6:8:6:2 8.1 3.7 0.33 3 SD 1.58 0.86 0.04 0.76 0.15 0.23 CV 6.41 3.65 3.69 9.43 4.17 67.7 N 4 4 4 4 2 4 4 4 2 4 XMas-Kat Quarry F:AM 71805 Oi 23.8 22.7 0.95 1:6:3:1 8.0 3.3 0.41 2 spurred F:AM 71802 ♀ 24.0 23.0 0.96 3:7:4:1 8.9 3.3 0.37 3 open; simple F:AM 71806♀ 26.0 est. 22.4 23.7 1.06 4:8:8:1 7.5 4.2 0.56 2 open F:AM 71804♀ 21.5 22.3 1.04 1:5:3:1 7.3 3.2 0.44 2 open Range 21.5–24.0 22.3–23.7 0.95–1.06 7.3–8.9 3.2–4.2 0.37–0.56 Mean 22.9 22.9 1.00 2:7:4:1 7.9 3.5 0.45 3 SD 1.19 0.59 0.05 0.71 0.47 0.08 CV 5.18 2.58 5.35 9.00 13.4 18.3 N 1 4 4 4 2 4 4 4 2 4

P3 P4

From Machaerodus Quarry, eastern Cherry County, Nebraska ( Skinner and Johnson, 1984: 315, fig. 16). F:AM 71833, juvenile cranium, with LI3, LdP1, right and left dP2– 4, M1, M2 erupting.

From Hans Johnson Quarry, eastern Cherry County, Nebraska ( Skinner and Johnson, 1984: 315, fig. 16). F:AM 71859, nearly complete Oi skull; snout displaced; dorsoventrally somewhat flattened, laterally expanded, right and left I1–2, LC, right and left P2–M3. F:AM 71863, nearly complete ♀ cranium of immature individual, LI1–3, R, C erupting, right and left dP1–dP4, M1 erupting.

DESCRIPTION OF REFERRED MATERI- AL: F:AM 71895 and F:AM 71863 are too crushed to yield significant new morphological information, but F:AM 71804 and F:AM 71805 confirm that the blunt end of the lacrimal is effectively aligned with the nearly vertical maxillo-jugal suture and ends about 18 mm and 13 mm, respectively, posterior to the rear rim of the DPOF (no. 39, table 9). F:AM 71863 is somewhat crushed in this region, but its right side confirms that the lacrimal terminates about 23 mm posterior to the rim of the DPOF. F:AM 71833 is a juvenile cranium and yields no significant information.

DP1 appears to be absent, except in the juvenile cranium F:AM 71833, where the tooth is very well worn and about 9 mm long, and in the equally juvenile cranium F:AM 71863 (female), where the tooth is also worn and only about 5 mm long. DP1 is absent in all adult crania, regardless of sex. Compared with the average length of P 2 in the adult specimens from the XMas-Kat quarries (28.6 mm; table 10), the dP1–P2 length ratio would be 0.32–0.18, generally consistent with the late medial Clarendonian age of this sample. The cheek teeth appear to have been 40–45 mm tall in the unworn condition, in contrast to 50–66 mm in C. occidentale (tables 3B, 10). As indicated in table 10, the anterior border of the prefossette in C. matthewi has a mean of at least 3 plis in P2 (mean of 6), P3, M2, and M3 (mean of 5), but a mean of 2 in P4 and M1. For P2–M2, the posterior border of the prefossette usually has a mean of at least 7 (up to 8) plis, with a mean of 2 in M3. The anterior border of the postfossette usually has a mean of 6 plis in P2, P3, and M1, but a mean of 5, 4, and 2 in M2, P4, and M3. The posterior border of the postfossette has a mean of 2 plis in P2 and P3 but a mean of 1 pli in the other cheek teeth. Regarding the confluence of pre- and postfossettes of P2, the three XMas-Kat specimens are scored as one yes, one no, and one not applicable; for the Machaerodus Quarry, the score is one not applicable; for the Hans Johnson Quarry, the score is one not applicable and one no. Based on the very small sample, it appears that the pre- and postfossettes in P2 may be confluent somewhat less than 50% of the time.

At present, no mandibles or lower dentitions can be directly associated with crania referred to C. matthewi .

Cormohipparion johnsoni , new species figures 16–18; tables 2–3, 6, 11–12

TYPE SPECIMEN: F:AM. 71891, Oi skull, missing snout and occiput, somewhat crushed dorsoventrally and laterally skewed, dentition in very late wear.

TYPE LOCALITY: Burge Quarry, Burge Member, Valentine Formation, Nebraska.

AGE: Early Clarendonian, Burge Local Fauna, Nebraska.

DISTRIBUTION: Type locality.

DESCRIPTION OF TYPE SPECIMEN: The basic morphology of the type cranium is as for C. occidentale . The left side best preserves the morphology in lateral view (fig. 16). As indicated in table 11, and in comparison with the juvenile cranium F:AM 71894 (fig. 17), the orbit is somewhat attenuated anterodorsally, and its anteroventral portion faces more posterolaterally than otherwise normal. The anteroventral margin of the orbit is less sharply demarcated than in F:AM 71894, for instance. The frontal region of the cranium is depressed in the type, and the postorbital fossa is distorted due to the adjacent lateral wall of the cranium having been displaced somewhat ventrally and laterally. As indicat- ed in figure 16, the lacrimal exits the orbit just anterior to the end of the rugose dorsal orbital border and extends anterodorsally to join the fronto-nasal suture, then descends shallowly anteroventrally toward the rear of the DPOF. The lacrimal extends anteriorly about to the maxillo-jugal suture and nearly reaches, but does not penetrate, the rear of the DPOF. The lacrimo-jugal suture descends posteroventrally to enter the orbit well below the lacrimal foramen. As indicated in table 11, the POB is about 40 mm long. The DPOF is well defined posteriorly. Its dorsal margin extends anteroventrally to the relatively well-defined anterior end, situated above and in close proximity to the IOF. The bony surface of the DPOF is broken so that the canal for the maxillary nerve is exposed posterior to the infraorbital foramen, but the continuous ventral margin of the DPOF can be recognized as it joins the posterior margin of the fossa. The IOF is located about 44 mm (table 11) above the rear half (metacone) of P3 and is aligned about with the orbital bisector. Allowing for distortion, it appears that the DPOF is positioned relatively high on the face, anterior to the orbit in C. johnsoni .

In lateral view (fig. 16A), the robust canine of this male cranium can be seen, but the snout anterior thereto is not preserved. As indicated in table 6, it appears that the diastema would have been as extensive as in F: AM 125899 but probably more robust, reflective of its male sex. The basic morphology is similar to that of C. occidentale . Roots for dP1 are preserved, but the crown is absent. Due to the advanced age of the specimen, the dental morphology is poorly preserved. That for P2 is virtually obliterated, with the protocone confluent with the protoloph at this advanced stage of wear (MSTHT 10 mm, table 12). In P3, the protocone is confluent to still virtually isolated (left side; fig. 16B); it is barely open to the protoloph in M1 but isolated in all other teeth. Tables 3B and 12 show that the protocone is ovate in all teeth except for M3, where it is more elongate (protocone width to length ratio of 0.47), with M3 preserving a remnant of what likely was a fairly complex fossette border morphology throughout. Due to the late stage of wear shown by this specimen, the teeth are

TABLE 12

Measurements (mm) of Upper Cheek Tooth Dentition of Cormohipparion johnsoni, Burge Member, Valentine Formation ,

Early Clarendonian, Nebraska

very low crowned. Cranial dimensions are summarized in table 11.

DIAGNOSIS: Based on the type and referred material, Cormohipparion johnsoni is distinguished from all other species of the subgenus Cormohipparion in having an un- worn mesostyle crown height of 40–50 mm (table 3B). In addition, figure 18 in comparison with figure 13 indicates that C. johnsoni differs from C. occidentale and the other species in having a much shorter muzzle (no. 1) and narrower muzzle at the I3 (no. 15). C. johnsoni differs from the XMas- Kat sample of C. occidentale , but from not the Machaerodus and Hans Johnson samples, in the shortness (no. 2) and lesser palatal breadth (no. 13) and in having shorter premolars (nos. 7, 9). It differs from all samples of C. occidentale in the diminished breadth of the frontals (no. 18) and transglenoid region (no. 19) and from all other species in these parameters, as well (except for no. 19 in C. matthewi ). C. johnsoni also has smaller orbit (nos. 28–29) than in any other species of the group, as perhaps befits its plesiomorphic status (see below). The POB is shorter in C. johnsoni and C. merriami , n.sp., than in the other species. The DPOF (no. 33) is sharply shorter in C. johnsoni (and C. matthewi ) and ranges to a comparably small size in C. merriami , n.sp., and C. fricki , n.sp. The separation of the IOF relative to the rear of the DPOF (no. 34) is much less in C. johnsoni as compared with C. occidentale or the other species, except C. matthewi , and the lacrimal nearly reaches the rear of the DPOF (no. 39) and extends across about 80% of the POB (table 3A), in contrast to any other species.

Especially as regards parameters 2 and 39, the very large range for the standard deviation is influenced by the small number of specimens, and the overall smaller dimensions can be related to the overall smaller size of the skull of C. johnsoni compared with C. occidentale . Still, the very diminished size of parameters 14 (narrow diastema) and 39 (tip to rear of DPOF; table 11) is noteworthy, as is the fact that other parameters are near or contained within the range of those in C. occidentale (fig. 18): no. 7, premolar row length; no. 8, molar row length; no. 12, choanal width; no. 31, facial length; no. 35, height of DPOF relative to the facial crest; no. 36, relative position of the facial crest and DPOF; no. 37, relative height of the IOF from the alveolar row; no. 38, height of rear of DPOF relative to alveolar row; and no. 40, medial depth of DPOF.

Cormohipparion johnsoni (fig. 18) differs from C. matthewi (fig. 15) in having a much shorter snout and palate (nos. 1–3) in contrast to comparably sized premolar and molar rows (nos. 7–9). C. johnsoni also has a narrower palate (no. 13); a greatly narrower diastemal region (no. 14) and muzzle (no. 15); a much narrower frontal width (no. 18); and a narrower transglenoid width (no. 19). In contrast to an equally high snout (no. 25) and equally incised nasal notch (no. 30) relative to C. matthewi , C. johnsoni has a smaller orbit (nos. 28–29), a shorter facial region (no. 31), a relatively shorter POB (no. 32), and a somewhat shorter DPOF (no. 33). However, the IOF location relative to the DPOF is similar (no. 34) in C. johnsoni and C. matthewi , but C. johnsoni has a taller DPOF (no. 35), a diminished separation of the facial crest from the DPOF (no. 36), a decreased separation between the IOF and alveolar border (no. 37), a greater separation between the DPOF and alveolar border (no. 38), a lacrimal that is very close to the rear of the DPOF (no. 39; table 11), and a medially deeper DPOF (no. 40).

In comparison with C. merriami , n.sp. (fig. 20), C. johnsoni (fig. 18) has a much shorter snout and palate (nos. 1–2); a much shorter premolar and molar row (nos. 7–9); a much narrower palate (no. 13), diastemal region (no. 14), muzzle (no. 15), frontal region (no. 18) and transglenoid region (no. 19); a much smaller snout (no. 25); a smaller orbit (no. 29); a shallower nasal notch (no. 30); and a much shorter facial region (no. 31). Whereas the POB is comparable (no. 32) in the two species, C. johnsoni has a much shorter DPOF (no. 33); a decreased separation between the IOF and rear of the DPOF (no. 34); a much reduced DPOF height (no. 35); a somewhat diminished separation of the facial crest and DPOF (no. 36); and of the IOF relative to the alveolar border (no. 37), a more normative (cf. C. occidentale ) separation of the alveolar border and rear of DPOF (no. 38); a lacrimal anterior tip very close to the rear of the DPOF (no. 39, table 11); and a comparable DPOF medial depth.

In addition to overall size and crown height, as well as dental complexity, C. johnsoni differs from C. quinni in the more anterior position of the DPOF, reflected in the fact that the lacrimal barely reaches, but does not penetrate, the DPOF. C. johnsoni is the most plesiomorphic member of the C. occidentale group.

REFERRED MATERIAL: F:AM 71894, nearly complete immature? ♀ cranium, with dP1–4 present and well worn. F: AM 125899 , laterally compressed cranium with occipital region obscured, right and left C, dP1–M2 ; M3 not erupted. F: AM 119095 , right maxilla with P3–M3 .

DESCRIPTION OF REFERRED MATERIAL: F:AM 71894 (fig. 17) is useful in demonstrating the presence of dP1 as a relatively small, but still functional, tooth (table 12). The deciduous premolars are in an early stage of wear, with the pattern not fully developed on dP2 or dP4. The dP3 and dP4 show double plis caballin and complex opposing borders of the pre- and postfossette. F: AM 125899 indicates that the ratio of the length dP1 to the length of P2 is 0.3 (table 12).

In lateral view (fig. 17), the well-developed and strongly demarcated DPOF is clearly visible, as is the wide POB and the pointed lacrimal that terminates a short distance posterior to the rear of the DPOF (5.5 mm, no. 39, table 11). Although not as well shown, a similar arrangement for the lacrimal is interpreted to have obtained in the type and in F: AM 125899 , with the anterior tip of the lacrimal 11 mm posterior to (type) or 6.5 mm to the rear of the DPOF (F: AM 125899 ).

F: AM 125899 and F: AM 119095 indicate that the unworn cheek tooth MSTHT dimensions apparently were about 45– 50 mm (tables 3B, 12). The cheek teeth of F: AM 125899 were removed from the cranium under the direction of the late Morris F. Skinner, and measurements were taken before they were sectioned at three intervals at progressively deeper levels in each tooth. The tooth segments were preserved by being glued to a plate of glass. From these segments it can be seen that M3 is completely unworn and is not furnished with a cement cover, as are the other teeth. The MSTHT for M3 is not available (encased in matrix). P4, the next tooth in the wear progression, shows only incipient wear on its highest crests and is 48.6 mm tall. In further order of increasing wear, P2 MSTHT is 36.7 mm; P3 is 42.2 mm; M2 is 45.6 mm; and M1, with a nearly complete wear pattern, is 42.4 mm tall. It is likely that M1 originally was about as tall as P4. Based on its currently unworn state, the MSTHT for P4 appears to have been no greater than 50 mm. Based on F: AM 125899 , the unworn MSTHT for upper cheek teeth in C. johnsoni appears to have been 40–50 mm. These parameters are basically verified by AMNH 119095 (table 12), which also shows the moderate complexity of the pre- and postfossettes. In P3–M3, the anterior border of the prefossette and posterior border of the postfossette typically have a single pli (mean value, table 12). In P3, opposing borders of the pre- and postfossette have mean values of 5 and 4 plis, respectively. In P4, comparable values are 6 and 5; in M1 and M2, 6 and 6; and in M3, the values are 7 and 3. The protocones are ovate to slightly elongate and ovate, with somewhat flattened lingual borders in P3 and P4 but lingually rounded in the molars. As indicated in table 3C, the mean width-to-length ratios for the cheek teeth of C. johnsoni are generally similar to those for C. merriami , n.sp., and generally wider (at or above 50% in P2–M2) than in all other species of the C. occidentale group. AMNH 119095 displays a pli protoconule that is isolated from the posterolingual border of the prefossette in early wear of P3–4 and a large and distinct loop in this position in M1–3. The pre-and postfossettes were separated from one another in P2 of F: AM 125899 . Plis caballin are more complex (mean value of 2, table 12) in the P3 and P4 compared with the other cheek teeth. The hypoconal groove is usually simple (bears a spur in the juvenile P2 and P 3 in F:AM 71984). In early wear (F: AM 125899 ), P2 shows separated pre- and postfossettes, but in late wear (type specimen) these fossettes are confluent.

At present, no mandibles or lower dentitions can be directly associated with crania referred to C. johnsoni .

DISCUSSION: As discussed below, C. johnsoni is the basal member of the C. occidentale group.

Cormohipparion merriami , new species figures 19–20; tables 2–3, 6, 11, 13

Neohipparion cf. coloradense Webb 1969: 98 .

TYPE SPECIMEN: AMNH 141219, nearly perfect ♀ skull, right and left I3, C, dP1–M 3 in about mid wear.

TYPE LOCALITY: June Quarry, Burge Member, Valentine Formation, Nebraska.

AGE: Early Clarendonian, Burge Local Fauna, Nebraska.

DISTRIBUTION: Midway Quarry, Burge Member, Valentine Formation, Nebraska.

DESCRIPTION OF TYPE SPECIMEN: AMNH 141219 is generally similar to C. occidentale in basic skull morphology (fig. 19A). The IOF is situated above the anterior half of P3 and below the anterior end of the DPOF, as is typical of the C. occidentale group of Cormohipparion . The anterior edge of the orbit is located above M3 rather than more posteriorly, as in C. occidentale . The lacrimal is pointed anteriorly and extends about 11 mm anteriorly beyond the maxillo-jugal suture but ends about 15 mm (mean value) posterior to (and does not reach) the rear of the DPOF (no. 39, table 11). It appears that the DPOF is pocketed posteriorly to about the anterior tip of the lacrimal. In contrast to C. occidentale as represented by the XMas-Kat sample, the MSTHT is estimated as having been ca. 55 mm (i.e., much lower crowned; tables 3B, 13); dP1 is present (the ratio of the length of dP1 to the length of P2 is 0.37) in the type specimen but apparently has been lost in the juvenile cranium F:AM 71893. As indicated in table 3C, the protocones are relatively wider than in all members of the C. occidentale group except for C. johnsoni . Whereas P2 is subovate, the protocone in the other cheek teeth is more elongate (fig. 19B). Fossette plications are discussed in conjunction with the referred material (see below).

DIAGNOSIS: Based on the type and referred material, Cormohipparion merriami is distinguished from other species of the subgenus Cormohipparion in having lacrimal that extends anteriorly beyond the maxillo-jugal suture but still does not reach the rear of the DPOF. The MSTHT is relatively low (ca. 55 mm) in C. merriami ; the protocones are relatively wide and not so elongate. As indicated in figure 20 and in comparison with figure 13, C. merriami is greater than (or likely would range much higher than) all other species in length of palate (no. 2) and has a relatively greater premolar (no. 7) and thus cheek tooth (no. 9) length than in C. occidentale (unknown in C. johnsoni ), with the exception of the palate and cheek teeth of C. skinneri , n.sp. Cormohipparion merriami tends to have a smaller orbit than in C. occidentale (nos. 28–29) and all other species, except for C. johnsoni , in which the orbit is still smaller. The nasal notch in C. merriami is less strongly incised than in C. occidentale (no. 30) but more so than in all other species. The facial length (no. 31) apparently is (or likely would range) larger than in C. occidentale and all other species except C. skinneri , n.sp. The POB (no. 32) tends to be shorter in C. merriami than in C. occidentale and all species except the comparably sized C. johnsoni .

REFERRED MATERIAL: From June Quarry, Burge Member, Valentine Formation, Nebraska. AMNH 141273, left maxillary fragment with dP1–dP3; AMNH 141274, right upper cheek tooth row in very early wear, P2–M3 and LM2; AMNH 141272, right upper cheek tooth row in medial wear, P3–M3.

From Midway Quarry, Burge Member, Valentine Formation, Nebraska. AMNH 141220, old adult, cracked and crushed skull with relatively intact facial region; F:AM 71893, juvenile partial cranium LI1–2, right and left I3, alveoli for the canines, right and left dP2–4 erupted and worn to the degree that the enamel pattern is almost completely developed; right and left M1 are barely exposed above the gum line. The cranium is largely undistorted but lacks the occipital region and paroccipital processes. F:AM 71983 also shows the triangular lacrimal bone seen in the type and is similar in that the lacrimal does not reach the rear of the DPOF .

DESCRIPTION OF REFERRED MATERIAL: The partial cranium AMNH 141220 retains the muzzle but lacks the intervening nasal elements to the cheek teeth, and the skull itself is conspicuously cracked, with most of the left side of the facial region missing. Overall, AMNH 141220 appears similar to the type skull. The POB is intact and shows the lacrimal as about subtriangular in aspect, with its anterior end about 17 mm posterior to the rear of the DPOF (no. 39, table 11), comparable to that of the type specimen. The DPOF in AMNH 141220 is somewhat longer than in the type and somewhat shorter dorsoventrally but otherwise similar, with the differences (including the apparently greater DPOF-IOF distance, no. 34) due

TABLE 13 Measurements (mm) of Upper Cheek Tooth Dentition of Cormohipparion merriami , and Cormohipparion sp.

largely to the cracked and distorted condition in AMNH 141220.

The upper cheek tooth dentition displayed by the referred material includes the presence of dP 1 in AMNH 141273, in association with a strongly worn dP2–3, so that its persistence in the adult condition as shown by the type specimen is to be expected. In AMNH 141273, dP1 is 9.8 mm long and 6 mm wide, somewhat smaller than the dimensions (12.1 mm and 6.7 mm) in the type. As indicated in table 13, the ratio of the length of dP1 to the length of P2 is 0.37 in the type. AMNH 141274 is the least worn of the

TABLE 13

(Continued)

available specimens and suggests that the unworn MSTHT for the cheek tooth dentition was likely 45–55 mm tall, with P4 and M1 having been 52–55 mm tall (table 3). The upper cheek tooth protocones were uniformly elongate and ovate, except in P3 and P4, where the lingual border is essentially flat (or slightly concave) and, in late wear of P2, where the protocone is connected to the protoloph in AMNH 141220. As indicated in tables 3C and 13, the mean width-tolength ratios are 0.67, 0.50, 0.47, 0.51, 0.53, and 0.46 for P2–M3. The type (left side, but not the right side) and AMNH 141274 show that the pre- and postfossettes are confluent labially in P 2 in relatively early wear (16% and 25% wear, respectively), but in late wear (AMNH 141220) these fossettes are separate.

Tables 2 and 13 show that the fossette borders overall are simpler than in C. occidentale or C. matthewi . The mean values in table 13 indicate that C. merriami has a more complex plication pattern than C. johnsoni (table 12). In table 13, it can be seen that the anterior border of the prefossette in P2 has a mean value of 5 plis, but in all other cheek teeth this is reduced to 2 plis or 1 pli (M1). In premolars, the posterior border of the prefossette contains a mean value of 5– 10 plis; in the molars, this feature is represented by 6–9 plis. Remaining mean values for fossette plications are anterior border of the postfossette 4–6 in premolars and 3–7 in molars. The mean value of the plis in the posterior border of the postfossette is a single pli in all cheek teeth but M2 (where the mean value is 2).

DISCUSSION: Webb (1969) discussed the morphology of a sample from Burge Quarry, Valentine Formation, Nebraska, allocated to Neohipparion cf. coloradense . MacFadden (1984) reviewed the species referable to Neohipparion and defined N. coloradense as having an MSTHT of 35–43 mm, a simple cheek tooth occlusal pattern, and a relatively ovate protocone. These aspects of Webb’s Burge sample (MSTHT 47–52 mm; cheek teeth with relatively complex occlusal pattern, and a more elongate protocone) are most similar to C. merriami as herein defined and characterized.

C. merriami is comparable to C. johnsoni in retaining dP1 into the adult condition but more advanced in being taller crowned and in the greater complexity of the fossette borders of the upper cheek teeth. It also is of larger size than C. johnsoni , with which it was contemporaneous. See below for further discussion of C. merriami relative to the other species of the C. occidentale group.

At present, no mandibles or lower dentitions can be directly associated with crania referred to C. merriami .

Cormohipparion fricki , new species figures 8, 21–22; tables 2–3, 6, 14–15

TYPE SPECIMEN: F:AM 73912, partial skull with right and left C–M3, adult male, M3 well worn.

TYPE LOCALITY: MacAdams Quarry, Clarendon beds, Texas.

AGE: Medial Clarendonian, Nebraska, Texas, South Dakota.

DISTRIBUTION: Type locality and Hollow Horn Bear Quarry, Ash Hollow Formation undifferentiated (Skinner and Johnson,

1984; figs. 2A, 3; table 1), Todd County, South Dakota.

DESCRIPTION OF TYPE SPECIMEN: As indicated in figure 21, the specimen is relatively complete. The nasal bones above the snout are missing, as are the upper borders of the orbits. The cranium is slightly depressed by postmortem action, so that certain cranial width dimensions are exaggerated. In comparison, F:AM 71800 from the XMas-Kat sample is somewhat less distorted. The upper cheek tooth dentition is well preserved. The overall quality of F:AM 73912 qualifies its being chosen for the type specimen, even though some others (e.g., F:AM 73920) better preserve the incisors.

In overall morphology, the cranium of C. fricki is comparable to that of many contemporaneous equids. The nasal notch appears to have been located about midway between the canine and the anterior tip of P2, but the relatively elongate facial region as compared with, for example, C. quinni ( Woodburne, 1996b) , is shown by the anterior margin of the orbit being located above the rear portion of M 3 in adult wear. In lateral view (fig. 21A), the profile overall is relatively regular, with the dorsal surface (lacking the nasal bones) being gently curved. Although depressed postmortem, the frontal bones do not appear to have been dome-like. The orbits are relatively large (nos. 28–29; table 14) and the POB (no. 32) is relatively wide. The DPOF is thus well anterior to the orbit and is higher posteriorly than anteriorly but not as distinctly teardrop shaped as, for instance, in the XMas-Kat taxon. As in Cormohipparion generally, the IOF is located just below the anteroventral margin of the DPOF, slightly posterior to the actual anterior end of the fossa, as is the usual case. The anterior end of the DPOF is not as pronounced as typically seen in XMas-Kat materials of Cormohipparion . The anteriorly wide POB is reflected in the lacrimal usually being exposed on the posterior one-half of the preorbital bar (length 2, table 3A; no. 39, table 14) and not entering the rear of the DPOF, as in C. quinni . In the type, dorsoventral compression of the facial region results in the ventral boundary of the lacrimal being distorted. Based on other specimens, the morphology of the lacrimal would resemble that in F:AM 71800 (fig. 8A), further described below, rather than being narrowed anteriorly. As in C. quinni and the XMas-Kat materials, the facial crest ends about in alignment with the premolar/molar boundary at an elevation approximately midway between the alveolar border and the ventral margin of the DPOF. In spite of the wider POB, the DPOF is still situated about as in C. quinni ; the rear of the DPOF is still located about above the mesostyle of M1. In the type and other specimens of C. fricki , the lacrimal is approximately as long as in C. quinni , so that the extension of the POB appears to have taken place between the lacrimal tip and the rear of the POB. This is reflected in dimension no. 32 being essentially twice that in C. quinni (table 14, in comparison with table 16, no. 32, in Woodburne, 1996b), even though the facial length of the lacrimal is about 34 mm in C. fricki versus about 25 mm in C. quinni .

In ventral aspect, the incisor arcade is smoothly rounded (best seen in F:AM 73920), the incisors having cement-filled infundibula (also in F:AM 73910). The canines are situated about midway between I3 and P2 (fig. 21A). DP1 is absent in this adult skull. Premolars are larger than the molars and tend to have a somewhat more complex enamel pattern, comparable to the general

TABLE 14

Measurements (mm) of Cranial Parameters for Cormohipparion fricki, Medial Clarendonian, Texas and South Dakota

Characters are illustrated in figures 3–5.

TABLE 15 Measurements (mm) of Upper Cheek Tooth Dentition of Cormohipparion fricki MacAdams Quarry, Clarendon Beds, Early

Clarendonian, Texas

situation in the C. occidentale group. The dentition in F:AM 73912 (fig. 21B) is in a relatively mature state of wear (P2 seems to be more than half worn; table 15), but the bifid versus the single pli caballin distinguishes premolars from molars. At this wear stage, the pre- and postfossettes of P2 are separate rather than confluent along their labial margins; cheek tooth protocones are elongate and isolated but relatively broad and slightly concave lingually in the premolars versus nearly flat in the molars.

The lower dentition and mandible is not represented in the type material of C. fricki .

DIAGNOSIS: Based on the type and referred material, C. fricki (fig. 22) has a much shorter muzzle (no. 1) than C. occidentale and, apparently, C. merriami , but not C. matthewi or C. skinneri , n.sp., and is longer than in C. johnsoni . Cormohipparion fricki has a much longer palatal region than in the Machaerodus and Hans Johnson samples of C. occidentale , C. matthewi and C. johnsoni , and a shorter overall cranial length (no. 6) than C. merriami , C. skinneri , n.sp., C. matthewi and the XMas-Kat (but not Machaerodus or Hans Johnson samples) of C. occidentale . The choanae and palate are somewhat wider (nos. 12–13) in C. fricki than in C. occidentale . C. fricki apparently is much wider across the frontals (no. 18) than any other species and is wider across the glenoid fossae (no. 19) than C. occidentale , C. matthewi , C. johnsoni , and C. merriami but not C. skinneri , n.sp. The snout (no. 25) is relatively much shorter vertically in C. fricki and C. matthewi than in C. occidentale or the other species. The nasal notch is less incised than in C. occidentale (no. 30) in common with all other species, to varying degrees. The DPOF (no. 33) ranges much smaller in C. fricki than in all but C. johnsoni and C. matthewi , and the rear of the DPOF is much farther from the tip of the lacrimal (no. 39) in C. fricki than in all other species (tables 5, 9, 11, 16).

Relative to C. matthewi (fig. 15), the cranium of C. fricki (fig. 22) differs in having a longer palate (no. 2), a somewhat greater cheek tooth length (nos. 7–9), a somewhat wider palate (no. 13), a distinctly greater separation between the IOF and rear of the DPOF (no. 34), and a much greater separa- tion of the anterior tip of the lacrimal and rear of DPOF (no. 39, table 9 versus table 14). C. fricki also differs in having relatively shorter palatal fenestrae (no. 3), a shorter postpalatal region (no. 5), a distinctly wider diastemal region (no. 14), a relatively wider muzzle (no. 15), an apparently wider frontal region (no. 18) and transglenoid width (no. 19), a higher DPOF (no. 35) and IOF-alveolar border distance (no. 37), and a medially deeper DPOF (no. 40). The two species are comparable in snout length (no. 1) and height (no. 25), basicranial length (no. 4), total cranial length (no. 6), choanal width (no. 12), orbital dimensions (nos. 28– 29), nasal notches (no. 30), facial lengths (no. 31), POBs (no. 32), and DPOF lengths (no. 33) and facial crest elevation (no. 36), and DPOF-alveolar border distance (no. 38).

Cormohipparion fricki (fig. 22) differs from C. johnsoni (fig. 18) in having a distinctly longer muzzle (no. 1) and palate (no. 2), longer premolar (no. 7) and cheek tooth (no. 9) lengths, longer facial region (no. 31) and POB (no. 32), and a longer IOF–DPOF distance (no. 34), as well as larger orbits (no. 28). The lacrimal is much more widely separated from the rear of the DPOF (no. 39, table 11, versus table 14), and the IOF is more widely separated from the alveolar border (no. 37). C. fricki also differs from C. johnsoni in having a wider palate (no. 13), diastema (no. 14), muzzle (no. 15), fontal region (no. 18), and transglenoid width (no. 19). Relative to C. johnsoni , C. fricki has a greater vertical dimension for the DPOF (no. 35), as well as vertical separation of the IOF from the alveolar border (no. 37), in contrast to the DPOF actually lying nearer the alveolar border (no. 38). C. fricki and C. johnsoni are comparable in snout height (no. 25), postpalatal dimensions (no. 3), molar lengths (no. 8), relatively short length of the DPOF (no. 33), depth of nasal notches (no. 30, probably in general context to no. 25), and medial depth of the DPOF (no. 40).

In comparison with C. merriami (fig. 20), C. fricki (fig. 22) differs in having a much shorter muzzle (no. 1) and palate (no. 2) and overall cranium (no. 6), a somewhat shorter facial region (no. 31), a much lower snout height (no. 25), somewhat longer (no. 28) but equally tall (no. 29) orbits, a distinctly longer POB (no. 32), and an apparently greater separation of the lacrimal from the rear of the DPOF (no. 39; table 11 versus table 15). C. fricki also differs from C. merriami in having a distinctly wider frontal (no. 18) and transglenoid region (no. 19). These two species are comparable in postpalatal length (no. 3), basicranial length (nos. 4–5), cheek tooth lengths (nos. 7–9; slightly shorter in C. fricki ), palatal width (no. 13), muzzles (no. 15), likely diastemal width (no. 14), low inion height (no. 22), and small nasal notch incision (no. 30). Based on an overlap in 1 S.D. ranges, the two species apparently are comparable in having a similar DPOF length (no. 33), IOF positions (no. 34), DPOF heights (no. 35), facial crest–DPOF positions (no. 36), IOF–alveolar border distances (no. 37), and IOF–DPOF distances (no. 38). Also, the DPOFs are comparably deep medially (no. 40).

REFERRED MATERIAL: MacAdams Quarry: F:AM 73913, snout and facial region, right and left I1, LI2, RI3, right and left C, LdP1–M3, RP2–M3, subadult male, M3 erupting. F:AM 73914, partial cranium, LI2, right and left dP1–4, M1, juvenile female, M1 erupting. F:AM 73915, partial cranium, RI2– 3, right and left C, P2–M3, adult male, M3 medial wear. F:AM. 73920, a partial cranium with right and left I1–2, LI3, right and left C, dP1–M3, subadult male, M3 early wear. F:AM 73921, snout and facial region, right and left I1–3, C, P2–M3, adult female, M3 well worn. F:AM 73925, facial region, RP3– M3, LP2, M3 early wear. F:AM 73938, cranial fragment.

Hollow Horn Bear Quarry, Ash Hollow Formation undifferentiated ( Skinner and Johnson, 1984; figs. 2A, 3; table 1), Todd County, South Dakota: F:AM 71880, partial cranium with right and left P2–M3; F:AM 71881, LdP2–4, M1 erupting; F:AM 71873, juvenile cranium and mandibles with right and left dP1–P4, M1 erupting, right and left dp1–4; F:AM 71874, juvenile palate with right and left dP1–4; F:AM 71875, LP2–M3, late wear; FAM: 71876, R maxilla with dP2– 4, M1, M2 erupting; F:AM 71879, dorsally flattened skull with RI1, right and left C1, P2–M3.

DESCRIPTION OF REFERRED MATERIAL: Based on the material from MacAdams Quarry, major features of the cranium are comparable to those of the type specimen. The upper cheek tooth dentition is relatively complex (fig. 21B). As indicated in table 15, the tallest P2 is about 41 mm tall and P3 is about 46 mm tall in F:AM 73913, which show an incipiently developed occlusal pattern. Based on this specimen, the unworn MSTHT of P2 likely was about 45 mm and that of P3 was likely about 55 mm, from which it may be suggested that P4–M2 also were about 55 mm tall in the unworn condition, with M3 being somewhat shorter. If this is reliable, it suggests that C. fricki was somewhat lower crowned than the XMas- Kat C. occidentale (table 3B). The upper cheek tooth dentition of C. fricki also appears to have been somewhat less complex than in the XMas-Kat C. occidentale , especially as regards the anterior border of the prefossette, which usually bears only 1–2 plis after about medial wear. Similarly, the mean values of plis on the opposing borders of the pre- and postfossettes are 5:4, 9:5, 7:4, 7:6, and 7:5 for P2–M2, respectively, based on the relatively small sample (table 15). The labial margins of the pre- and postfossette in P2 are separate rather than confluent, except for F:AM 73925. The simple morphology of P 2 in C. fricki contrasts with the more complex morphology in the type of C. occidentale . Similarly, the commonly spurred hypoconal groove in P2–4 of C. fricki is more complex than in XMas-Kat and in the type of C. occidentale .

As indicated in F:AM 73913, dP1 persists into early adult wear in which M3 is in the process of being erupted, but P2–3 are in early phases of wear, with MSTHTs of about 41 mm and 46 mm, respectively (table 15). Based on F:AM 73913 and F:AM 73920, the ratio of the length of dP1 to the length of P2 is 0.38–0.41 (table 15). This compares with a ratio of 0.3 in C. johnsoni (table 12), 0.37 in C. merriami (table 13), and 0.3 in juvenile XMas-Kat specimens of C. occidentale (ta- ble 7). DP1 is unknown in adult C. matthewi and C. skinneri , n.sp. The relatively large size of dP 1 in C. fricki and C. merriami appears to be plesiomorphic in those species. The dP1 is present in about 60% of the specimens from MacAdams Quarry, regardless of sex. It also is absent in both female and male crania.

Upper cheek tooth protocones tend to be relatively elongate as in the type specimen, except in F:AM 73915, where they are shorter (table 15). Premolar plis caballin in C. fricki appear to be about as complex as in C. occidentale but less complex in the molars (tables 7–8), comparable to the other species of the C. occidentale group. Except where not applicable (deciduous premolars; two specimens), the pre- and postfossettes of P2 are about equally confluent or separate in the hypodigm (N 5 3 for each).

F:AM 71880 (fig. 8) is a partial skull from the Hollow Horn Bear Quarry that preserves the anterior portion of the orbits, the facial region containing the DPOF and palate wherein right and left P2–M3 are retained. As indicated in table 3B, the measured crown height of P 2 in F:AM 71880 is within the range for C. fricki (table 15). The upper cheek tooth morphology of F:AM 71880 is compatible with that for C. fricki (figs. 8B, 21B), considering the earlier wear stage of the Hollow Horn Bear Quarry specimen. The nearly unworn M3 of this specimen is likely to have been about 37 mm tall (table 4B) and about 45 mm in the unworn condition. This suggests that the unworn crown heights of the tallest cheek teeth (P4, M1) would have been on the order of 55 mm, comparable to the MacAdams sample of C. fricki . The unworn MSTHT of M1 and M 2 in F:AM 71876 is circa 55 mm and 50 mm, respectively (table 3B). Specimens from the Hollow Horn Bear Quarry show a confluent pre- and postfossette in P 2 in three specimens (not applicable in four) and separate in none.

F:AM 71880 preserves the facial part of the cranium (fig. 8A), which displays a welldeveloped DPOF that is situated well anterior to the orbit and to the anterior tip of the lacrimal bone. Note that the lacrimal is separated from the maxillo-nasal suture by a jugo-nasal suture that is about 5 mm long. The lacrimal therefore does not reach the maxillo-nasal suture. The DPOF is slightly pocketed posteriorly and has a well-developed anterior rim adjacent to, but above, the IOF. The IOF is located above the boundary between P2 and P3. Other specimens from the Hollow Horn Bear Quarry (referred material) demonstrate a comparable morphology.

DISCUSSION: Based on these specimens, the unworn crown height for C. fricki is on the order of 55 mm for M1 and somewhat less for P2 and M3 (table 3B). The anterior border of the prefossette and the opposing borders of the pre- and postfossettes are relatively complex, even in medial wear (e.g., F:AM 73925, F:AM 73901, F:AM 74915, table 15). For the overall sample, premolar plis caballin tend to be at least double; protocones are ovate, commonly lingually concave, and remain isolated from the protoloph in late wear; at comparable wear stages, cheek teeth tend to be relatively wide. P2 pre- and postfossettes are confluent in all three of the specimens in which it can be appraised (not applicable in four).

Webb (1969: 102) allocated material from Little Beaver B and Little Beaver A sites, Minnechaduza Fauna, Nebraska, to Neohipparion occidentale . As revised by Skinner and MacFadden (1977), this species is referable to Cormohipparion . In the present revision, the Minnechaduza material is referred to C. fricki . This is based chiefly on the crown height of the upper cheek teeth (unworn MSTHT 51–56, according to Webb, 1969: 102). This referral also is compatible with the degree of enamel pattern complexity and the relative elongation of the protocone of the Minnechaduza sample as described by Webb (1969). The Minnechaduza sample does not reach the crown height and enamel complexity characteristic of C. occidentale as described herein.

At present, no adult mandibles or lower dentitions can be directly associated with crania referred to C. fricki . Based on the crown height of the upper cheek teeth, the confluence of the pre- and postfossettes of P2, the ovate protocone proportions in the upper cheek teeth, general cranial characters, and the retention of dP1, C. fricki possesses many features likely prophetic of the North American ancestry of the Hippotheriuim Datum. This is discussed further below.

Cormohipparion skinneri , new species figures 23–24; tables 2–3, 6, 16–18

TYPE SPECIMEN: F:AM. 73909, nearly complete undistorted cranium and mandible, with right and left I1–3, C1, P2–M3, right and left i1, Li2–3, right and left c1, Rp2–m3, Lp3–m3.

TYPE LOCALITY: Gidley Horse Quarry, Clarendon beds, Texas.

AGE: Medial Clarendonian, Texas.

DISTRIBUTION: Type locality.

DESCRIPTION OF TYPE SPECIMEN: F:AM 73909 is comparable overall to C. occidentale . The type cranium is slightly crushed at the frontal region so that the facial area anterior to the orbit is distorted. The nasal bones and the dorsal border of the facial region are preserved, but the frontal region is somewhat depressed, and the parietals are broken. The lateral surface of the face above the facial crest is depressed so that the lacrimal morphology is somewhat distorted. The dorsal half of the lacrimal is best preserved on the left side. The lower course of this element is partially obscured on the right side, as well. The morphology shown in figure 23A is a composite of both sides. In comparison, F:AM 71800 from the XMas-Kat sample is less distorted. The upper cheek tooth dentition is well preserved, as is the mandible and lower dentition. The angular region of both mandibles is missing (restored), as is the tip of the left coronoid process.

In overall morphology, the cranium of C. skinneri is comparable to that of many contemporaneous equids. The nasal notch extends posterior to the canine, but the relatively elongate facial region as compared with, for example, C. quinni (fig. 26; see also Woodburne, 1996b) is shown by the anterior margin of the orbit being located above the anterior edge of M 3 in adult wear. In lateral view (fig. 23A), the overall profile is relatively regular, with the dorsal surface gently recurved. Although depressed postmortem, the frontal bones do not appear to have been domed dorsally. The orbits are relatively large (table 16), and the POB is relatively wide. The DPOF is thus well anterior to the orbit, is higher posteriorly than anteriorly but not as distinctly teardrop-shaped as, for instance, in the XMas-Kat C. occidentale . As in Cormohipparion generally, the IOF is located just below the anteroventral margin of the DPOF, usually slightly posterior to the actual anterior end of the fossa. In C. skinneri , the IOF is situated above the rear half of P2, comparable to the condition in C. fricki . The anterior end of the DPOF is not as pronounced as typically seen in XMas-Kat materials of Cormohipparion . The anteriorly wide POB (no. 32, table 16) is reflected in the lacrimal usually being exposed on the posterior three-fifths of the bar (length 2, table 3A) and not entering the rear of the DPOF, as recorded in C. quinni . As in C. quinni and the XMas-Kat materials, the facial crest ends above the premolar–molar boundary at an elevation approximately midway between the alveolar border and the ventral margin of the DPOF. In spite of the wider POB, the DPOF is still situated about as in C. quinni ; the rear of the DPOF still is located essentially above the mesostyle of M1. In C. skinneri , the lacrimal is longer than in C. quinni , but the extension of the POB appears to have taken place as well between the lacrimal tip and the rear of the POB. This is reflected in dimension no. 32 being about twice that in C. quinni (table 16 in comparison with Woodburne, 1996b: table 16 no. 32), even though the facial length of the lacrimal (nos. 32–39, table 16) is 39 mm in C. skinneri versus about 25 mm in C. quinni . In contrast to that in C. fricki , the lacrimal in C. skinneri is blunt anteriorly and nearly vertical. The maxillo-lacrimal and maxillo-jugal sutures thus are effectively aligned with one another. In addition, the lacrimal appears to have been more broadly exposed on the face (length 2, table 3A), and has a blunt, rather than pointed, anterior end. Consequently, the lacrimal does not extend appreciably anterior to the maxillo-jugal suture.

In ventral aspect, the incisor arcade is smoothly rounded, the incisors having cement-filled infundibula. The canines are sited about two-thirds the distance between I3 and P2. DP1 is absent. The premolars are generally larger than the molars and tend to have a slightly more complex enamel pattern (fig. 23B). The dentition of F:AM 73099 is in a mature stage of wear (table 17). With a MSTHT of about 30 mm, P2 seems to be one-quarter worn, if this tooth was about 40 mm tall in the unworn state, comparable to C. occidentale . In P2–3, the anterior border of the prefossette is moderately complex (up to 4 plis) but simpler in the more posterior teeth (table 17). The posterior border of the prefossette displays 5–8 plis in the premolars and 4–6 plis in the molars. For the anterior border of the postfossette, comparable numbers are 2–5 plis in the premolars and 4– 6 plis in the molars. All have but a single pli on the posterior border of the postfossette. The bifid versus the single pli caballin distinguishes P2, P3, and M1 from the other cheek teeth. At this wear stage, the pre- and postfossettes of P2 are separated rather than confluent along their labial margins; cheek tooth protocones are elongate and isolated but relatively broad and slightly concave labially in both premolars and molars.

The mandibles are well developed, except for missing their angular regions (fig. 23D). As indicated in table 18, the mandible is relatively deep beneath the molars but becomes shallower anteriorly. The lower dentition is remarkable in having a relatively strong (premolars) to strong (molars) penetration of the isthmus by the ectoflexid (see fig. 6 for terminology). In P3 and P4, the base of the metaconid and metastylid are constricted, respectively, into right and left halves of the upper arms of an ‘‘X’’ pattern, as shown in figure 23C. In M1 and M2, the metaconid and metastylid respectively form the right and left halves on an ‘H’ pattern, with the cross bar of the H formed by the anteroposteriorly elongate labial end of the ectoflexid (fig. 23C). Protostylids are present on p3–m3. The ectoflexid development is exhibited by a specimen judged to be in adult wear (upper dentition about 25% worn) and is present in all molars, as well as in the premolars. If borne out by additional specimens, if and when recovered, this would represent an evolutionary novelty not found in other species of Cormohipparion .

DIAGNOSIS: Cormohipparion skinneri is distinguished from all other species of the subgenus Cormohipparion in having the characteristic strong development of premolar and molar ectoflexids and the development of the ‘‘X’’ and ‘‘H’’ patterns described above (assuming the type is representative of its species population). As shown in figure 24, C. skinneri is represented only by a single specimen in parameters 1–25 and by two specimens in parameters 28–40. Parameters in which C. skinneri falls within the range indicated on figure 24 for C. occidentale from XMas-Kat Quarry and in figure 13 from the XMas-Kat, Machaerodus, and Hans Johnson quarries are not considered further here. In that context, C. skinneri differs from C. occidentale in having a somewhat shorter muzzle (no. 1), a somewhat longer palate (no. 2), and a much shorter postpalatal dimension (no. 3), even though the basicranial dimension (no. 4) is similar. Cormohipparion skinneri also differs from C. occidentale in having longer premolar, molar, and cheek tooth row lengths (nos. 7–9); a wider choanae (no. 12), palate (no. 13), and muzzle (no. 15); and a very much wider transglenoid dimension (no. 19), in spite of the frontal width (no. 18) being comparable. Although the standard deviation for C. skinneri overlaps that for C. occidentale , C. skinneri has a somewhat larger orbit (no. 29), a somewhat longer facial region (no. 31), and a somewhat taller DPOF (no. 35). Cormohipparion skinneri also differs from C. occidentale in having a much less incised nasal notch (no. 30), a shorter DPOF (no. 33), and a much greater separation of the rear of the DPOF from the alveolar region (no. 38).

In comparison with C. matthewi (fig. 15), C. skinneri (fig. 24) has a shorter muzzle (no. 1), a much longer palate (no. 2), a much shorter postpalatal dimension (no. 3), a smaller basicranial dimension (nos. 4–5) (but a relatively longer cranium overall [no. 6]), longer cheek tooth row dimensions (nos. 7– 9), a wider choanae (no. 12) and palate (no. 13), a much wider snout (no. 14) and muzzle (no. 15) (but a narrower frontal [no. 18] versus a much wider transglenoid region [no. 19]), a much taller snout (no. 25), comparably sized orbits (nos. 28–29), comparably weakly incised nasal notches (no. 30), a somewhat longer facial region (no. 31), a longer POB (no. 32), a comparably short DPOF (no. 33), a much greater separation between the IOF and rear of the DPOF (no. 34) and a much taller DPOF (no. 35), a greater separation of the facial crest and the DPOF (no. 36) (but a comparable separation of the alveolar border and IOF [no. 37]), a very much taller DPOF relative to the alveolar border (no. 38), a somewhat greater separation of the anterior tip of the lacrimal and rear of the DPOF (no. 39; table 16), and a somewhat medially deeper DPOF (no. 40).

In comparison with C. merriami (fig. 20), C. skinneri (fig. 24) has a somewhat shorter muzzle (no. 1), a comparable palatal length (no. 2); a shorter postpalatal (no. 3) and basicranial region (no. 4); and a comparable dimension no. 5, overall skull length (no. 6), cheek tooth rows (nos. 7–9), and palatal width (no. 13), but a relatively wider snout (no. 14) and muzzle (no. 15). In contrast, the frontals are comparably wide (no. 18), but the transglenoid dimensions are distinctly greater (no. 19) and the inion height smaller (no. 22) in C. skinneri , with the snout height (no. 25) being comparable in the two species. The orbit is somewhat smaller in C. merriami (nos. 28–29); the nasal notch is somewhat more incised (no. 30); and the facial region is longer (no. 31). The two species have a comparably short POB (no. 32) and DPOF (no. 33), whereas the IOF location (no. 34), DPOF height (no. 35), and facial crest– DPOF separation (no. 36, strongly) are greater in C. skinneri . The IOF–alveolar border separation (no. 37) and DPOF–alveolar border separation (no. 38) are comparable in the two species, whereas the DPOF is farther from the lacrimal (no. 39, table 16), but the DPOF is shallower in C. skinneri .

In comparison with C. johnsoni (fig. 18), C. skinneri (fig. 24) has a longer muzzle (no. 1) and palate (no. 2); a shorter postpalatal dimension (no. 3); longer cheek tooth rows (nos. 7–9); a wider palate (no. 13), snout (no. 14), muzzle (no. 15), frontal (no. 18), and transglenoid (no. 19); a taller snout (no. 25); and larger orbits (nos. 28–29), but a comparably little incised nasal notch (no. 30). The facial region (no. 31) and POB (no. 32) are longer in C. skinneri , but the DPOF (no. 33) is comparably short in the two species. The IOF location (no. 34), DPOF height (no. 35), facial crest–DPOF separation (no. 36), IOF–alveolar border separation (no. 37), DPOF–alveolar border separation (no. 38), and separation of DPOF and anterior lacrimal tip (no. 39; much greater; see table 16) are greater, but the medial DPOF depth (no. 40) is smaller, in C. skinneri .

In comparison with C. fricki (fig. 22), C. skinneri (fig. 24) has a similarly short snout (no. 1), similar palatal length (no. 2), shorter postpalatal dimension (no. 3), and somewhat shorter basicranium (nos. 4–5), but a somewhat longer skull overall (no. 6). The cheek tooth rows are longer in C. skinneri (nos. 7– 9), but the choanae (no. 12) and palates (no. 13) are about equally wide in the two species. Cormohipparion skinneri has a somewhat wider snout (no. 14) and muzzle (no. 15) but a much narrower frontal (no. 18) and a slightly wider transglenoid dimension (no. 19), as well as a slightly lower inion height (no. 22). Cormohipparion skinneri has a great deal taller snout (no. 25) and a somewhat longer orbit (no. 29), but a comparably weakly incised nasal notch (no. 30). The facial region (no. 31), POB (no. 32), and DPOF (no. 33, relatively short) are about equally long in the two species, but the IOF is relatively more anterior (no. 34), the DPOF is taller (no. 35), the facial crest is farther from the DPOF (no. 36) (but the IOF is nearer the alveolar border [no. 37]), the DPOF is farther from the alveolar border (no. 38), the anterior tip of the lacrimal is much nearer the rear of the DPOF (no. 39, table 16), and the DPOF is medially shallower (no. 40) in C. skinneri .

REFERRED MATERIAL: F:AM 73910, distorted cranium with right and left I1, LI2–3, right and left P2–M3. Gidley Horse Quarry, Clarendon beds, Texas.

DESCRIPTION OF REFERRED MATERIAL: F:AM 73910 is badly distorted in having been crushed dorsoventrally and skewed labially. It seems to have the same basic morphology as in the type specimen, including a lacrimal that is narrower anteriorly than at the orbit but that appears to be blunt anteriorly. At this mature stage of wear (P2 appears to be about one-half its unworn MSTHT), the cheek tooth protocones still are elongate and isolated. In P 3 and P4, the lingual margin of the protocone is slightly concave ; otherwise, this protocone border is slightly convex to flat in the other teeth. Plis caballin are more complex (trifid) in P2 and P3 than in P4–M3 (bifid, table 17). The dP1 is absent in these adult crania.

DISCUSSION: As developed further below, C. skinneri is most similar to C. occidentale , with which it is contemporaneous, but differs in the features indicated in the Diagnosis. The Texan occurrence of C. skinneri relative to the more northern distribution of C. occidentale suggests that C. skinneri may have pertained to a population characteristic of a southern province.

Figures

Fig. 10. Cormohipparion occidentale, referred. A, F:AM 71800, XMas-Kat Quarry, Merritt Dam Member, Ash Hollow Formation, Cherry County, Nebraska, lateral view of cranium. After MacFadden (1984: fig. 131). See figure 7C for upper cheek tooth dentition. B–E, F:AM 71801. B, lateral view of cranium, after MacFadden (1984: fig. 132). C–E, cross-section and occlusal pattern of left upper cheek tooth dentition. C, near base of tooth. D, approximate middle of tooth. E, occlusal surface of tooth. After MacFadden (1984: fig. 18).

Fig. 13. Cormohipparion occidentale. Log-ratio diagrams of cranial dimensions of samples from XMas- Kat, Machaerodus, and Hans Johnson quarries, Merritt Dam Member, Ash Hollow Formation, Nebraska.