Mammacyon obtusidens Loomis, 1936

Hunt, Robert M., 2011, Evolution Of Large Carnivores During The Mid-Cenozoic Of North America: The Temnocyonine Radiation (Mammalia, Amphicyonidae), Bulletin of the American Museum of Natural History 2011 (358), pp. 1-153 : 58-64

Mammacyon obtusidens Loomis, 1936 Figures 24–27 , 67

Mammacyon obtusidens Loomis, 1936: 44–47 , fig. 1. Mammacyon obtusidens: Macdonald, 1963: 219 . Mammacyon obtusidens: Macdonald, 1970: 63 . Temnocyon percussor: Macdonald, 1970: 61–62 .

TYPE: As given in Loomis (1936: 44), but modified as follows: skull with well-preserved basicranium and dentition, axis vertebra, humerus, radius, both ulnae, four metapodials, tibia, partial scapula, two isolated canines, and an astragalus ( ACM 34–41), found ‘‘in the summer of 1934 in the lower Miocene Rosebud formation on Porcupine Creek, South Dakota, just above the concretionary layer, or about 100 feet above the base of the beds.’’ Additional information is found in Macdonald (1963: 157, 219) as to the type locality, ‘‘Amherst College Museum Rosebud 2, Monroe Creek Formation or Harrison Formation.’’ The locality producing the holotype is in the lower Arikaree Group and is of Oligocene age.

DISTRIBUTION: Early to mid-Arikareean: lower Arikaree Group, Wounded Knee area , ACM Rosebud 2, Shannon County, South Dakota ; lower Arikaree Group, LACM loc. 1964, Wounded Knee area , Shannon County, South Dakota ; John Day Formation , CIT loc. 29, Wheeler County, Oregon .

DIAGNOSIS: Distinguished from M. ferocior by smaller size of cranium (basilar length: M. obtusidens , 25.5 cm; M. ferocior , 28 cm, table 7), postcrania, and mandibular dentition (toothrow length,,98 vs. 120 mm, table 2), including m1–m2 lengths ( M. obtusidens , 24.4, 15.5 mm; M. ferocior , 27.7, 17.3 mm) and by a much narrower p3–4 width in M. obtusidens . The two species differ in P4 and M1 dental ratios A/B and C/D (table 6).

REFERRED SPECIMENS: (1) LACM 9194 , left mandible with canine, p1 alveolus, p2– m2, m3 alveolus, isolated right lower canine, right mandibular fragment with m1 talonid and m2 roots, right mandibular fragment with anterior root of p4 and its posterior alveolus, from LACM loc. 1964, Wounded Knee area, ‘‘ Monroe Creek Formation ,’’ NW4, NW4, sec. 3, T39N, R42W, Shannon County, South Dakota, collected by H. Garbani, June 30, 1964; (2) LACM 5386 , partial maxilla with P4–M1, from the John Day Formation, CIT loc. 29, ‘‘from a large exposure of green beds on north side of Haystack Valley , Wheeler Co., Oregon.’’

DESCRIPTION: ACM 34–41— Loomis (1936) only briefly described the upper dentition of this fine skull, and did not mention the wellpreserved basicranium. It is one of only three temnocyonine skulls in which some part of the auditory bulla is preserved, in this case a crescentic anterior portion formed largely or entirely by the ectotympanic (see Basicranial Anatomy).

The left maxilla retains the canine, P2–M2; on the right, I3, P2–M2. An alveolus for a single-rooted P1 is present on the right, but other alveoli are distorted.

The upper canine, blunted at the tip from wear, measures, 45 mm in length from the tip to the enamel base on the labial face. There is no evident wear groove produced by the lower canine. At the enamel base the canine measures 22.1 mm in anteroposterior length and 14.1 mm in width.

The alveolus of P1 measures 8.7 mm in length, 5.4 in width, and has been slightly distorted by crushing.

P2 measures 18.5 mm in length, the width anterior to the main cusp is 6.8 mm, and the width posterior to this cusp is 7.7 mm. A long posterior slope descends from the main cusp in contrast to the shorter anterior face, both devoid of accessory or cingular cusps. On all upper teeth the cingula are slightly developed basal swellings without sharp or distinct edges. On both P2 and P3 a weak, thin, enamel ridge runs from the main cusp anterolinguad to the cingulum on the anterior slope, and a similar ridge on the posterior slope runs to the posterolabial cingulum.

P3 is an enlarged version of P2, measuring 20.3 mm in length; width anterior to main cusp, 8.2 mm; width posterior to this cusp, 10.6 mm. P3 is somewhat more robust than P2 and much more expanded at its base posterior to the main cusp. There are no accessory or cingular cusps. The anterior and posterior faces of P3 are more nearly equal in length, and the tip of the main cusp shows flat apical wear.

P4 is a robust, massive tooth; M. obtusidens is the first of the larger temnocyonines to develop this crushing carnassial. P4 has a short metastylar blade, an anteroposteriorly elongate stout paracone, and a greatly enlarged protocone that crushes against the p4 heel. Shear is still accomplished to some degree by the lingual surfaces of the paracone and metastylar blade. The metastylar blade is directed outward at a 50 ° angle from the anteroposterior axis of the paracone. The blade measures 10.5 mm in length; the paracone measures 18.4 mm from the carnassial notch to its anterior base. A weak enamel ridge runs down the anterointernal face of the paracone to end in a tiny cingular cusp. Directly lingual to the paracone is the blunt, knoblike protocone: the protocone is separated from the paracone by a broad valley. Anteroposterior length of the protocone is 11 mm; greatest width of P4 at the level of the protocone is 21.5 mm. Only a narrow embrasure (, 7 mm in length) remains between P4 and M1 to receive the trigonid of m1, and a similarly reduced space also occurs in M. ferocior .

M1 measures 20.4 mm in length, 28.2 mm in width. In M. obtusidens the tooth comprises a labial part dominated by the paraand metacone, and an enlarged lingual half created by swelling of the lingual cingulum around the broad protocone region. Between the lingual and labial parts of the tooth is a prominent constriction, giving the tooth a ‘‘waist’’ at the level of the protocone basin. The paracone is somewhat larger than the metacone; the parastyle, swollen labially and ventrally downturned, is more pronounced than the metastyle. Here, as in other large temnocyonines, M1 has a ‘‘folded’’ appearance as if the entire labial half were inwardly rotated toward the lingual half about an anteroposterior axis through the protocone basin. The protocone is centrally situated in a flat expanse of enamel. A preprotocrista runs from the protocone to the anterior cingulum but there is no postprotocrista nor para- and metaconules. A heavy swollen lingual cingulum, about equally developed throughout its extent, surrounds the protocone region.

M2 measures 10.5 mm in length, 17.9 mm in width. It is much smaller than M1 and fits into the concavity on the posterior margin of the larger molar. The paracone is much larger than the abbreviated metacone, but only slightly taller. The stylar shelf and labial cingulum external to the paracone are better developed than these features labial to the metacone as a result of the reduction of the metacone. The protocone basin occurs at the constricted ‘‘waist’’ of the tooth, and here also, as for M1, the labial half of M2 is rotated or ‘‘folded’’ inward. The protocone is a small, low blunt cusp protruding from the enamel flat forming the lingual half of the tooth. It is surrounded by a swollen cingulum, which is considerably worn. M3 was absent; there is no alveolus and there is no place on the maxilla posterior to M2 for an M3.

The associated limb and vertebral elements are similar in form to skeletal elements of Mammacyon ferocior (F:AM 27562) from north of Keeline, Wyoming. Comparisons of the ulna, humerus, astragalus, and scapula show no differences other than size. The limb elements of M. ferocior , in association with its mandible, indicate a species larger than M. obtusidens (ACM 34–41).

LACM 9194—Only the complete left mandible is described here; the two isolated jaw fragments and canine only duplicate structure observable in the more complete lower jaw. LACM 9194 was initially described as Temnocyon percussor by Macdonald (1970: 61–62).

The jaw is short and massive. Depth under the main cusp of p2 is 38.4 mm, and 35.9 mm below the m1 protoconid. Premolars and molars are large, robust teeth; the molars, especially m2–3, are conspicuously tilted forward on the curvilinear margin of the ascending ramus. The inclined m2–3 are characteristic of temnocyonines, consequently M2 is often dorsally elevated relative to M1 to accommodate this configuration of the lower molars. Such an M2 is present in the holotype (ACM 34-41). The unworn teeth indicate a mature young adult.

The canine is a large, recurved tooth, 38.9 mm in height from tip to enamel base, measured on the labial face. Its anteroposterior width is 19.1 mm measured at the base of the enamel; its labiolingual width is 12.6 mm. A wear groove produced by the upper canine begins near the tip on the posterior surface, continues down the posterior slope, trending slightly labiad, eventually terminating about 9.5 mm above the enamel base on the posterolabial face. Two fine enamel ridges traverse the tooth from tip to enamel base: the first courses down the center of the lingual face, the second traverses the posterolingual face.

The single alveolus for p1, although crushed, measures, 8 mm in length, 6.2 mm in width. Elongation of the large alveolus suggests that the tooth was directed anterolabially.

The p2 measures 14.4 mm in length, 5.8 mm in anterior width, and 6.4 mm in posterior width. There are no accessory or cingular cusps. A fine enamel ridge descends the somewhat steeper anterior face to the anterolingual corner of the tooth, and a similar ridge occupies the posterior face, ending at the posterolabial corner.

The p3 measures 17.7 mm in length, 6.4 mm in anterior width, and 7.8 mm in posterior width. There are no accessory cusps, but there is a small basal cusp situated at the center of the posterior cingulum. A thin enamel ridge descends the anterior face to the anterolingual corner; a second ridge traverses the posterior face to the basal cingulum cusp.

The p4 measures 20.4 mm in length, 7.5 mm in anterior width, and 9.7 mm in posterior width. A large posterior accessory cusp is labially situated half the distance down the posterior slope. A small basal cusp on the posterior cingulum lies below the posterior accessory cusp. A fine enamel ridge follows the anterior slope and ends in the anterolingual corner as in p2–3, but a posterior ridge is largely obscured by development of the large posterior accessory cusp.

The bases of P3 and p3–4 are posteriorly widened but this is less pronounced relative to M. ferocior or D. oryktes teeth where such widening of the posterior base is strongly developed.

The m1 measures 24.4 mm in length, 11.4 mm in posterior trigonid width, and 10.9 mm in greatest talonid width. Trigonid length is 17.4 mm. This is a robust, short, blunt-cusped carnassial that has lost the metaconid, although a small wear facet on the labial face of the protoconid shows that paraconid-protoconid shear is retained. The short, swollen paraconid blade is angled inward and is much lower in height than the principal cusp of p4. The protoconid is compressed to form a blade running forward and downward to form the incisure with the paraconid. A short enamel ridge 4 mm in length descends the posterior surface of the protoconid to form the incisure with the hypoconid, a massive blunt cusp placed directly behind the protoconid and centrally situated on the talonid. There is also a low enamel swelling on the posterolingual wall of the protoconid that represents the vestige of the incisure once shared with the metaconid. Its presence indicates that the metaconid has probably been lost relatively recently in this lineage. There is no entoconid. The m1 has a swollen, weakly differentiated cingulum, most evident on the labial side where it is sinuous: it rises below the protoconid, descends at the protoconid-hypoconid incisure, and rises again beneath the hypoconid (in contrast, the labial m1 cingulum is straight in Delotrochanter ). At the posterolingual corner of the m1 talonid, the cingulum is slightly enlarged, creating a marked indentation of the cingulum anterior to this swelling (also found in M. ferocior ).

The elongate m2 is diagnostic of the Mammacyon lineage in the Great Plains. Its length is 15.5 mm; greatest trigonid width, 9.6 mm; greatest talonid width, 8.7 mm: m2 has a rectangular occlusal outline. The labially placed protoconid is the largest cusp. There is no metaconid but a small, distinct paraconid occupies the anterolingual corner of m2. The only talonid cusp is a low, blunt, somewhat labially situated hypoconid. There is no entoconid. The pronounced inclination of m2 (and m3) on the edge of the ascending ramus of the mandible, tilted forward nearly 30 ° from the horizontal, further emphasizes the short toothrow and massive quality of the teeth.

The m3 is represented by a single alveolus, length 4.7 mm, width 4.1 mm, placed close against the posterior edge of m2.

Length of the toothrow from p2–m2 is 86.2 mm; from the base of the enamel on the posterior face of the canine to the anterior rim of m3 alveolus is 98.3 mm; from anterior rim of p1 alveolus to posterior rim of m3 alveolus is, 99 mm.

LACM 5386—I refer to M. obtusidens a partial maxilla with right P4–M1 and the alveolus for the posterior root of P3, from CIT Locality 29, Haystack Valley, Wheeler County, Oregon. It was found among unidentified material in the collection of the Los Angeles County Museum and has not been previously described. The teeth are most similar in form to ACM 34-41 ( M. obtusidens ) and F:AM 54134 ( M. ferocior ). This is the only specimen from the John Day beds assigned to Mammacyon ; it represents a smaller individual than the holotype of M. obtusidens and is closer in size to LACM 9194.

The alveolus (greatest width, 8.2 mm) for the posterior root of P3 is very large, indicating a posteriorly broad premolar at least 9 mm in width. This alveolus indicates that P3 was closely set against the base of P4 as in the holotype.

P4 measures 21.8 mm in length, 18.2 mm in greatest width, and is characterized by the short yet robust metastylar blade relative to the large paracone and protocone. Metastylar blade length is 8.1 mm; width of the blade is 7.4 mm. The upper carnassial is nearly as wide as long, and shows blunted cusps with pronounced apical wear (there is only a weak vertical shear facet on the lingual face of the paracone-metastylar blade). The enormous protocone is responsible for the width of P4: it extends as far toward the midline of the palate as does the M1. An enlarged protocone is characteristic of Mammacyon ; in Delotrochanter the P4 protocone does not extend as far toward the midline. A slightly swollen cingulum surrounds P4, and is somewhat more defined around the base of the metastylar blade than elsewhere on the tooth. A conspicuous valley separates the large blunted paracone from the much lower knoblike protocone.

M1 measures 17.8 mm in length, 24.8 mm estimated transverse width (the anterolingual cingulum is broken), and 13.1 mm in anteroposterior width of the protocone region. The form of the tooth is comparable to M1 of ACM 34-41. The embrasure for m1 between P4 and M1 is much reduced as in the holotype, thus there was probably no m1 metaconid. In addition, the M1 of LACM 5386 shows the ‘‘folded’’ appearance in which the labial half of the tooth is rotated inward toward the lingual half as in ACM 34-41. The knoblike protocone of M1 was isolated within the broadened protocone region and surrounded by a swollen lingual cingulum; a weak preprotocrista runs from protocone toward the anterior cingulum but no postprotocrista is apparent. Paracone, metacone, and protocone of M1 are blunt cusps with flat apical wear surfaces. The paracone and metacone were once tall cusps but wear has reduced their height so that evidence of vertical shear on their lingual faces is equivocal. In mature individuals mastication emphasized the processing of hard food items, also suggested by the blunt, knoblike protocones of both M1 and P4.

A vestige of the labial M2 alveolus indicates that the tooth was small and fitted tightly against the posterior margin of M 1 in the same location as the holotype.

DISCUSSION: Mammacyon obtusidens is the oldest species that can be confidently attributed to the genus. The maxilla (LACM 5386) from the John Day Formation of Haystack Valley, Oregon, can be conservatively dated at younger than 28.7 Ma based on its probable site of collection close above the Picture Gorge ignimbrite; an age estimate for the locality falls in the interval from,28 to 28.7 Ma. The first appearance of M. obtusidens in the Great Plains is based on the referred mandible (LACM 9194) from the ‘‘Monroe Creek Formation’’ of the Wounded Knee area in southwestern South Dakota. The holotype cranium and associated postcranials (ACM 34-41) were attributed to the ‘‘Monroe Creek or Harrison Formation’’ and are also part of the Wounded Knee fauna of Macdonald (1963, 1970). However, a reliable correlation of these rock units with the type Monroe Creek and Harrison formations of Hatcher (1902a) north of the town of Harrison in northwest Nebraska has not been established, and the mammalian fauna from the ‘‘Monroe Creek’’ and ‘‘Harrison’’ of the Wounded Knee area lacks agediagnostic species found in the stratotype Harrison Formation in Sioux County. The Wounded Knee fauna from the ‘‘Monroe Creek-Harrison’’ interval in South Dakota is an early to mid-Arikareean fauna (see Age and Correlation), hence the holotype of M. obtusidens is assigned to the late Oligocene — the species had attained the specialized durophagous dental traits typical of the genus at least by the earlier late Oligocene, and this dental trend continued in the Great Plains with M. ferocior .

The M. obtusidens mandible (LACM 9194) is from the same Wounded Knee locality as the canid Enhydrocyon pahinsintewakpa , the most plesiomorphic species of its genus ( Wang, 1994: 90); this locality (LACM 1964) occurs near the base of the ‘‘Monroe Creek’’ unit and in close proximity to the Sharps Formation, which includes leptauchenine oreodonts in the same area ( Macdonald, 1963: 162). These mammals document an early Arikareean age for the stratigraphic level that yielded the mandible of M. obtusidens .

This conclusion is supported by the stratigraphic position of the descendant of M. obtusidens , the larger terminal species of the lineage, M. ferocior . Mammacyon ferocior from the Pine Ridge of Niobrara County, north of Keeline, Wyoming, is a species of larger size in which derived dental features of the M. obtusidens holotype become further accentuated. Thus, in M. ferocior the teeth are similar in form but larger, more massive, with even greater emphasis on the crushing function of the cheek teeth. The fauna from north of Keeline is considered here to be intermediate in age between the Wounded Knee ‘‘Monroe Creek’’ fauna and the fauna of the stratotype Harrison Formation in northwest Nebraska.

The mandibular fragment (LACM 15908) attributed in this study to cf. Mammacyon from the Sharps Formation of South Dakota was collected stratigraphically below and, 4 mi (6.4 km) north of the locality that produced the referred M. obtusidens mandible (LACM 9194). It possibly represents the earliest record of Mammacyon in the Great Plains. It has reduced the m1 metaconid to a vestigial remnant and shows the robust teeth and thick mandible that would be expected in the ancestor of M. obtusidens . However, LACM 15908 lacks the elongate m2 relative to m1 (m1/m2 length ratio,,1.7) common to M. obtusidens and M. ferocior and is too incomplete to permit confident referral to the genus. The m2 protoconid and hypoconid are centrally situated, a character of Mammacyon and Delotrochanter ; these cusps are labially placed in Temnocyon .

Figures

Fig. 24. Holotype rostrum of Mammacyon obtusidens (ACM 34-41) with right I3, P2–M2, left C, P2– M2, from the lower Arikaree Group, Porcupine Creek, Shannon Co., South Dakota. This is one of the earliest durophagous temnocyonines. Focal point of the crushing dentition is centered on the P4–M1 pair. See figs. 67, 69.

Fig. 25. Referred mandible of Mammacyon obtusidens (LACM 9194) with left canine, p2–m2, LACM loc. 1964, lower Arikaree Group, Wounded Knee area, Shannon Co., South Dakota. A, labial and B, lingual views.

Fig. 26. Occlusal stereopair of the mandibular dentition (p4–m2) of Mammacyon obtusidens (LACM 9194). Note absence of m1 and m2 metaconids.

Fig. 27. Referred maxilla of Mammacyon obtusidens (LACM 5386) with right P4–M1, Turtle Cove Member, John Day Formation, Haystack Valley, Wheeler Co., Oregon. Note apical wear on all cusps, indicating crushing action of P4–M1.

Fig. 67. Holotype basicranium of Mammacyon obtusidens (ACM 34-41) with the remnant of the ectotympanic auditory bulla (T) in life position. The posterior portion of the bulla was lost but contacted the medial margin of the promontorium (P), evidenced at that location by a shallow depression (indicated by dashed line on inset). For abbreviations, see page 5.