Apternodus brevirostris Schlaikjer, 1934

ASHER, ROBERT J., McKENNA, MALCOLM C., EMRY, ROBERT J., TABRUM, ALAN R. & KRON, DONALD G., 2002, Morphology And Relationships Of Apternodus And Other Extinct, Zalambdodont, Placental Mammals, Bulletin of the American Museum of Natural History 2002 (273), pp. 1-117 : 34-37

Apternodus brevirostris Schlaikjer, 1934

TYPE SPECIMEN: AMNH 22466, slightly damaged complete skull (fig. 15) with associated lower left dentary, complete except for the crown of i1 and coronoid process (fig. 16). This specimen was originally accessioned into the UW collections ( Schlaikjer, 1933: 15) and is probably associated with UW 26, a right dentary with p4­m3.

REFERRED SPECIMENS: AMNH 74941, broken skull with left P2­M2; AMNH 74942, broken skull, missing anterior rostrum, associated with left dentary with m2­m3 and unprepared right dentary with p4­m3; AMNH 74946, rostrum with left P3­P4 and broken M1; AMNH 74948, rostrum with right P2­M2, broken M3, left C, P3­M3; AMNH 74949, rostrum with right P2­M1, broken M2, left P2­M2; AMNH 74950, half rostrum with left P3­M3; AMNH 74951, broken skull preserving orbitotemporal regions, left petrosal, fragments of left and right lambdoid plates, and rostrum with left P2, broken P4­M3, right canine, broken P3­ M1, M3, associated left dentary with p4­m3 and right dentary with m1­m3, also associ­ ated with fragmentary left distal humerus; AMNH 76692, left maxillary fragment with P2­M3; USNM 437460, rostrum (fig. 17) with right and left C­M3, left I2, alveoli and roots for both I1s and right I2, missing buccal margins of right P3­M3; associated with left (fig. 18) and right dentaries with i2­m3 and broken i1s. (For purposes of exhibition, and long before this writing, USNM 437460 was attached to the basicranium of another individual from a different part of the Flagstaff Rim section.)

TEMPORAL AND GEOGRAPHIC DISTRIBUTION: Chadronian (late Eocene) of central Wyoming (Flagstaff Rim 18, Bates’s Hole 3).

DIAGNOSIS: As the name implies, the rostrum is short, holding a closely packed dentition anterior to P4, usually with overlap between the posterior root of P2 and the anterior root of P3 and without diastemata on either side of the upper canine. The anterior dentition is gracile, with premolariform upper and lower I2­P3. The upper molars have lingually situated protocones that are similar in size to the molar parastyles. The i2 is very small ( Schlaikjer, 1934), with an alveolus continuous with that of the larger i1. Both the upper and lower second premolars are much smaller than adjacent teeth; and the p3 shows a prominent buccal cingulum and posterior cusp. The rostral tympanic process of the petrosal is medially flat. The external auditory meatus is ventrally concave. The lacrimal foramen is offset posteriorly from the orbit by the superior margin of the infraorbital canal.

REMARKS: Several rostra with complete or nearly complete dentitions are now known for A. brevirostris (AMNH 74941, 76692, 74950, 74948, 74949, and USNM 437460), most of which were collected by Morris Skinner and colleagues from the Flagstaff Rim area, Wyoming during the 1950s. The type (AMNH 22466) remains the most complete specimen known. Surprisingly, very few specimens have been recovered with associated mandibles. One of these is AMNH 74951, which preserves not only a rostrum and posterior mandibles, but is also associated with a distal humerus, petrosal, and fragmentary lambdoid plate. Several braincases and basicrania are known from Flagstaff Rim, including AMNH 74940, one the few specimens known to preserve an ectotympanic (fig. 19). These are presumably referable to A. brevirostris , but without the dentition it is impossible to be certain. Frustratingly, no specimen unambiguously referable to A. brevirostris preserves the ventral margins of the lambdoid plates. These can nevertheless be inferred to have extended well below the level of the glenoid fossa, based on the size of their breakage scars, which are quite large posterior to the external auditory meatus and presumably extended for several millimeters farther ventrally (as reconstructed by Schlaikjer, 1934).

Although a closely packed I2­P4 toothrow characterizes this species, some variation is present in the size of the P2­P3 diastema. For example, in AMNH 74948 (identifiable as A. brevirostris by virtue of its upper molar protocones, gracile anterior dentition, small canine diastemata, and ridge separating the lacrimal foramen from the orbit) the space between P2 and P3 is small, but the roots of the two teeth do not overlap. The ratio of palate length to width in A. brevirostris is smaller than that of other Apternodus species , but not always with decisive statistical significance (fig. 20). Within the A. brevirostris hypodigm, the rostrum of the type specimen (AMNH 22466) is one of the shortest.

McDowell (1958) published a discussion and four illustrations of Apternodus that contained several mistakes. He based these illustrations primarily on the type of A. brevirostris (AMNH 22466), with missing parts restored from the type of A. gregoryi (MCZ 17685) and with the inferred morphology of one side reconstructed from a mirror image of the other. Both skulls are imperfectly preserved and do not show a number of characters depicted in McDowell’s reconstruction.

First, McDowell (1958: fig. 23A) repeated Schlaikjer’s error (1933: fig. 3, top; see also section below on A. gregoryi ) of running a suture between the parietal and occipital (Schlaikjer’s mastoid) bones diagonally across the posterolateral corner of the skull roof. There is no evidence for a suture at this site.

Similarly, there is no evidence to support McDowell’s (1958: fig. 23) placement of the squamosal­parietal, fronto­parietal, and fron­ to­nasal sutures. The dorsal exposures of most dermal braincase elements are completely fused, and leave no indication of any sutures.

McDowell (1958: 170) argued that ‘‘from the American Museum skull (AMNH 22476 [sic, actually 22466]), it can be determined that the maxilla forms little of the orbital wall, an important contrast to the Lipotyphla .’’ In fact, the sutures of the maxilla in the orbital mosaic are completely fused in both AMNH 22466 (fig. 15) and MCZ 17685 (fig. 21), as they are in the majority of other Apternodus skulls, with the two exceptions mentioned above: MPUM 2634 ( A. baladontus ) and MPUM 6855 ( A. mediaevus ). In the former, a suture is clearly visible on both sides curving around the anterior border of the sphenopalatine foramen and posterior to the lacrimal region (fig. 13). Similarly, both sides of the skull in MPUM 6855 (fig. 7) show distinct, wavy lines representing the maxillary­frontal suture and indicating that the maxillary incursion into the orbital mosaic of Apternodus is considerable, similar to that of Erinaceus ( MacPhee and Novacek, 1993: fig. 3.5).

McDowell’s reconstructed cribriform plate (1958: fig. 23) is located too far posteriorly, and is oriented too vertically. A partial, broken skull of A. brevirostris (AMNH 74951) exposes most of the cribriform plate intact, and shows its posteriormost point just anterior to the sella turcica, as in most other mammals. Externally, this corresponds with a point just anterior to the glenoid fossa of the mandible, close to the posteriormost external opening of the sphenorbital fissure. Relative to this point, the cribriform plate in Apternodus extends both anteriorly and dorsally toward the anterior end of the sagittal crest, to a point coincident with a coronal plane shared with the posterior margin of the hard palate.

McDowell’s figure 23C incorrectly shows a completely solid tympanic roof, with no piriform fenestra on either side. In fact, the region immediately anterior to both petrosals in AMNH 22466 is perforate, although the large size of these ‘‘fenestrae’’, particularly on the left side, is due to postmortem damage. Nevertheless, most specimens of Apternodus that preserve the relevant anatomy (e.g., FMNH UM1690, AMNH 74941, USNM 455680, UW 11046) clearly show prominent piriform fenestrae anterior to the petrosal. This was shown in AMNH 22466 by Matthew (1910), who included photographs in his original description of the specimen. The type of A. gregoryi (MCZ 17685; see below) was also incorrectly reconstructed as lacking piriform fenestrae by Schlaikjer (1933: fig. 3) and Scott and Jepsen (1936: plate 1). In some individuals (MPUM 6855 and AMNH 74940) the fenestrae are indeed small or absent; AMNH 74942 appears to show a piriform fenestra on the left side but not on the right, although this is undoubtedly the result of postmortem damage. In general, the tympanic roof anterior to the petrosal is naturally perforate in most specimens of Apternodus , including the type of A. brevirostris .

McDowell (1958: figs. 23C, 24, and p. 168) argued that the internal carotid artery of A. brevirostris runs medial to the middle ear after giving off a stapedial branch that oc­ cupies a groove on the promontorium ventral to the vestibular foramen. This pattern, he wrote, was ‘‘very different... from that of the Lipotyphla , [and is] more like that of creodonts... ’’ (p. 168). In fact, as discussed elsewhere (Asher, 2000, 2001; Whidden and Asher, 2001) McDowell overestimated the conservatism of cranial blood supply among insectivoran mammals, and drew a misleading contrast between an alleged ‘‘lipotyphlan pattern’’ and a speculative model of arterial circulation presented for Apternodus based on AMNH 22466 and MCZ 17685. In short, although there is a faint arterial groove associated with the proximal stapedial artery in the immediate vicinity of the vestibular foramen in the petrosal of AMNH 22466, there is no indication of a groove for the internal carotid artery. Neither the medial aspect of the petrosal of AMNH 22466, nor its ventral apex, is marked by subtle depressions that in other specimens define the course of the internal carotid artery along the ventrum of the tympanic roof. As noted elsewhere ( Whidden and Asher, 2001), other specimens of Apternodus (e.g., AMNH 74942, 76745; USNM 455680) do preserve shallow grooves running between the anterior carotid foramen and ventral apex of the petrosal in a position well lateral to the petrosal­basisphenoid suture. These specimens indicate either that arterial circulation in Apternodus was polymorphic, or more likely, that McDowell’s vascular reconstruction (1958: fig. 24) was incorrect. In sum, it is doubtful that the course of the internal carotid artery in Apternodus was located farther medially than that of many insectivorans, such as Microgale ( MacPhee, 1981; Asher, 2001).