Apternodus mediaevus Matthew, 1903

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 : 19-33

Apternodus mediaevus Matthew, 1903

TYPE SPECIMEN: AMNH 9601 (left dentary with m1­m2; see fig. 2) .

REFERRED SPECIMENS: AMNH 9607, left dentary with p3 (see Schlaikjer, 1933: 6, footnote 2); AMNH 76745, posterior braincase with most of left lambdoid plate and petrosal, associated right partial maxilla with I2, C, P2, broken I1, left dentary fragment with p2–3, broken p4, isolated right and left canines, left i3, several other isolated tooth fragments, isolated?lumbar and caudal vertebrae, right and left proximal humeri with partial right shaft, right distal humerus (fig. 4), metapodial fragments; AMNH 97255, left dentary with p2­m3 (fig. 5); AMNH 97256, right dentary with m1­m3; AMNH 97258, left dentary with p3­m2; USNM, 18966, right dentary with p3­m3; MPUM 0576, right dentary with p4­m2, roots of m3; MPUM 3799, palate with left I1­P3, right C­ M1 and alveoli for I1–2, associated with left lambdoid plate (fig. 6); MPUM 6767, left dentary with p3­m3; MPUM 6768, right dentary with p3­m1; MPUM 6855, skull missing anterior rostrum (fig. 7), associated right dentary with p4­m3 (fig. 8); MPUM 6856, left maxillary fragment with canine; MPUM 7820, right dentary with p3­m3; CM 13676, rostrum with left I1­M3, left I2­P2 (original lost, known only from photographs; see fig. 9); CM 31323, right maxilla with C and P2; CM 37455, fragmentary rostrum with right I1­C, left I1, P2, maxillary fragments with partial right M1–2, left P4­M1, associated right and left dentaries with fragmentary right m1­m3, left i1, and roots for p4­m2, isolated lower right i1, c, p2, p3, lower left i3, c, and other fragmentary upper and lower teeth, many disarticulated, fragmentary bits from right and left maxillae and dentaries, isolated cervical and lumbar vertebrae, distal tibia and ulna, partial calcaneus, and phalanx (fig. 10); CM 71569, right maxilla with P2­ M2; and CM 71570, right dentary with p3­ m2.

TEMPORAL AND GEOGRAPHIC DISTRIBUTION: Middle to late Chadronian (Late Eocene) of southwestern Montana (Pipestone Springs 29, Canyon Ferry 6, West Easter Lily 13, Highway 10N 22, and Little Pipestone Creek 25). In this section and those that follow, numbers given after localities correspond to those depicted on the map shown in figure 1.

(top), lateral (middle), and lingual (bottom) views.

DIAGNOSIS: The upper and lower canines have elongate, bulbous crowns. The upper canine has two broad roots, anterior and posterior; some specimens (e.g., MPUM 3799; fig. 6) show a small, lingually situated third root. The P2 is slightly enlarged and elongate, with a distinct crest running distally from the main cusp and a slender, lingual, third root extending from the midpoint of the tooth. The i2 is diminutive with a small, poorly defined alveolus that abuts against the much larger i1. The i3, canine, p2, and p3 are also enlarged and bulbous. The p3 shows a distinct posterior cusp and a slight buccal cingulum. The upper molars have distinct protocones. Buccal cingulids are present on the lower molars. Basicranially, the rostral tympanic process of the petrosal exhibits a medial ridge. The posttympanic process extends well ventral to the external auditory meatus.

REMARKS: Particularly noteworthy among the Apternodus mediaevus specimens not previously discussed in the literature are MPUM 3799, an associated rostrum and partial posterior braincase (fig. 6); AMNH 76745, a fragmentary skull with associated postcranial fragments (fig. 4); MPUM 6855, a partial skull (fig. 7) and associated mandible (fig. 8); CM 13676, a rostrum and complete upper dentition (fig. 9; original lost); CM 37455, a fragmentary skull and mandibles associated with several broken postcranial elements (fig. 10); and AMNH 97255, a well­preserved dentary (fig. 5). These specimens provide a fairly complete picture of the skull of A. mediaevus , as well as some information about its skeleton. With the exception of the more gracile upper P2 and I2, the more ventrally pronounced lambdoid plate, the diminutive i2, and the posterior cusp and cingulid on p3, A. mediaevus re­ sembles A. baladontus sp. nov., described below.

The Canyon Ferry specimen preserving associated cranial and postcranial fragments (CM 37455; White, 1954) is included in Apternodus mediaevus based on the morphology of its I2, P2, p3, and enlarged, bulbous upper and lower canines. However, this specimen is larger than others in A. mediaevus ; its inclusion increases the size range in this species (see later section on metric variation). The only other Apternodus specimen known from Canyon Ferry (USNM 18914, a maxillary fragment with M1 and alveoli for P3­P4, M2­M3) is smaller and well within the range of variation for A. mediaevus . Nevertheless, considerable variation in size does occur in some modern taxa (see below); more importantly, CM 37455 is anatomically cohesive with specimens in the A. mediaevus hypodigm, and for the present is retained in this species.

One mandible in the AMNH collection from Pipestone Springs (AMNH 76747, a left dentary with an erupting p3 and broken m1­m3) does not fit into existing specific diagnoses, and may represent yet another species from Montana. This specimen differs from A. mediaevus in showing a very gracile coronoid process and dentary. The coronoid does not anteriorly overlap with m3, and the mandibular corpus is thinner and more gracile than those of any other Pipestone Springs specimen, including one with erupting permanent premolars (CM 71570). Some breakage and postmortem damage has made the mandibular condyle of AMNH 76747 shorter mediolaterally than it was in life. Although the trigonids of all three molars of AMNH 76747 are severely damaged and the intact p3 is not yet fully erupted, remnants of its teeth closely resemble those of other Apternodus specimens from Pipestone Springs. In particular, the p3 is bulbous, with a distinct posterior cusp and weak buccal cingulum. Because of these similarities and because AMNH 76747 represents a juvenile and possibly pathologic individual, we refrain from recognizing it as a new species.

Apternodus baladontus , new species

TYPE SPECIMEN: FMNH UM1690, complete skull (fig. 11) with both mandibles (fig.

12). Damage to FMNH UM1690 is confined mainly to the occiput, the anterior upper incisors, and the coronoid process of the left mandible. Part of the cerebellar cast is exposed by damage to the posterior parts of the parietals. Otherwise, the specimen is very well preserved. The frontispiece of this monograph shows a reconstruction of Apternodus baladontus as it may have appeared in life, based on this specimen.

REFERRED SPECIMENS: MPUM 0450, left dentary with broken p3­m2; MPUM 2620, rostrum with left I2­P3, right C­P2; MPUM 2634, rostrum (fig. 13) and mandibles (fig. 14) with nearly complete upper and lower dentition, missing anterior incisors and crown of left m1; MPUM 2645, right maxilla with canine; CM 9552, right dentary with p4­m2, erupting p3; CM 71563, right dentary with p3­m3.

TEMPORAL AND GEOGRAPHIC DISTRIBUTION: Late Duchesnean (late middle Eocene) to middle Chadronian (late Eocene) of southwestern Montana (McCarty’s Mountain 27, Diamond O Ranch 9, and Little Pipestone Creek 25).

ETYMOLOGY: Named for the sphere­shaped anterior dentition, based on the Greek words for ‘‘ball’’ and ‘‘tooth’’.

DIAGNOSIS: In addition to large, bulbous up­ per canines, bulbous i3­p3, and prominent molar protocones, A. baladontus shows a large, spherical upper P2 and I2, with P2 showing at least three and sometimes four roots (e.g., MPUM 2645). The upper canine has a prominent, buccally situated third root. The p3 lacks a prominent posterior cusp. A. baladontus is further distinguished from A. mediaevus by the small but distinct i2 which has an alveolus separate from that of i1, small cheek teeth with molars that show weak or absent buccal cingulids, the presence of a bony torus defining the ventrolateral margin of the external auditory meatus, and the ventrally short posttympanic process (fig. 11).

REMARKS: In addition to the type, the other exceptional element of the A. baladontus hypodigm is MPUM 2634, a rostrum (fig. 13) with complete mandibles (fig. 14). This specimen is unusual in that it preserves a suture between the maxilla, palatine, and frontal in the orbital mosaic, and demonstrates that Apternodus possessed a maxilla that extended far into the orbit. Remnants of the orbital sutures of the maxilla are also evident in a specimen of A. mediaevus (MPUM 6855; fig. 7). In other specimens (e.g., AMNH 22466 and MCZ 17685) these sutures are fused, and do not permit estimates of the maxillary contribution to the orbital mosaic (contra Mc­ Dowell, 1958). Although many of its cranial sutures are untraceable, the A. baladontus type (FMNH UM1690) is otherwise complete enough to warrant particular attention, as follows.

ROSTRUM: The nasals, maxillaries, and premaxillaries have completely fused. Above the enlarged upper incisors, a shallow pit can be seen on each premaxillary, just posterior to the anterior end of the bony snout. The dorsomedial boundary of a large muscle scar extends from the medial edge of these pits back to a point above the lacrimal foramen. This scar covers most of the side of the snout and evidently supported powerful musculature for the movement of some sort of proboscis. Similarity to the snouts of Solenodon , Nesophontes , and Centetodon ( Lillegraven et al., 1981) is marked. The anterior rim of the orbit is likewise similar to that of Nesophontes , Solenodon , and Centetodon . There is a large lacrimal foramen, from which the nasolacrimal canal runs anteromedially across the roof of the infraorbital canal. The latter is relatively short. A short nubbin over M1 is all that remains of the zygomatic wing of the maxillary bone. In other insectivorans (e.g., most tenrecs and Solenodon ), this structure is usually larger and more posteriorly located. Several nutritive foramina open into the infraorbital canal. The sphenopalatine and greater palatine foramina open separately into the lower part of the orbit and are even with the posterior end of the palate.

The palatines extend forward to a point opposite the posterior half of M1. Their combined anterior border takes a straight course across more than half of the palate, although the squamous nature of the contact results in an irregular series of about eight forwardprojecting lobes of bone. The posterior bor­ der of the palate terminates relatively farther behind the level of M3 than in A. brevirostris , in agreement with the condition in A. gregoryi . The border is swollen at the internal nares and projects somewhat ventrally, but a true torus is not present, nor is there any posterodorsal shelf behind the swelling. The border is instead notched at the midline in this specimen. There are no palatal fenestrae.

At least seven pairs of palatal foramina are present posterior to the level of P2. The most posterior pair is the largest, situated posteromedial to M3, and presumably carried the greater palatine artery. Dorsally, the opening of the greater palatine foramen is ventrolateral to that of the sphenopalatine foramen; the two are not coincident. The greater palatine artery apparently divided into a number of branches that emerged onto the palate at intervals. A small, paired foramen can be seen anteromedial to the opening for the minor palatine artery at the level of M3. Anterolabial to this foramen lies another paired foramen in the maxillopalatine suture opposite the interdental embrasure between M2 and M3; this gives rise to a faint groove that runs forward on the surface of the maxillary close to the lingual roots of the molars. This may be related to a feature called the third supply tract of the palate in Centetodon (Mc­ Kenna 1960: 139). The next pair of foramina is located at the anterior limits of the palatine bones. Approximately 1 mm from each side of the midline, a large vascular foramen is present. The maxillary is grooved from there to a point opposite P3. The groove from each Fig. 11 . Continued. foramen of the pair divides into two slightly divergent grooves. Between the lingual roots of P2 and P3 are two small foramina on each side. Anteriorly, no additional paired foramina are present except for the large incisive foramina between the incisors.

ORBITOTEMPORAL REGION AND BASICRANI­ UM: The ventrally projecting pterygoids are similar to those of several insectivorans, including Microgale and Solenodon . Only a single pair of ventral projections is present; each of these hooks posteriorly opposite the posterior opening of the alisphenoid canal as in Microgale cowani and probably supported a membrane separating the trachea from the rear of the oral cavity, in addition to musculature of the soft palate and auditory tube. Lateral to the ventral border of the pterygoid is a flat surface for attachment of the internal pterygoid muscle.

The sphenorbital fissure and associated foramina in FMNH UM1690 are quite distinctive and well preserved. A massive overhanging wall of bone extends forward as a continuation of the lateral wall of the sphenorbital fissure, terminating at a point approximately even with the posterior border of the palate. In the roof of the resulting vault are several foramina, all of which are hidden from lateral view. The most anterior of these is the ethmoidal foramen, which leads into the anterior cranial fossa and posterior ethmoidal region. Immediately posterior to the ethmoidal foramen are two tiny foramina of uncertain significance. Posterior to these is the large anterior opening of the sinus canal, which in life transmitted the superior ramus of the stapedial artery along the interior of the braincase. Medial to this is a small optic foramen. Immediately posterior to the optic and sinus canal foramina is the much larger opening for the ophthalmic and maxillary divisions of the trigeminal nerve. No separate foramen rotundum is present. Other specimens that preserve the sphenorbital fissure in good condition include some with breakage that permits examination of these foramina from both dorsal and ventral perspectives (e.g., A. brevirostris AMNH 74951). These specimens share the morphology exhibited by FMNH UM1690.

Behind the posterior base of the sphenorbital fissure, just anterior to foramen ovale, is the small posterior opening of the alisphenoid canal. It is about 0.5 mm in diameter and opens directly into the floor of the sphenorbital fissure. The foramen ovale is about 0.7 mm in diameter and faces anteroventrally, medial to the glenoid fossa.

Apternodus baladontus possesses a piriform fenestra, as the squamosal and alisphenoid bones fail to contact the anterolateral edge of the petrosal. The rostral tympanic process of the petrosal ( MacPhee, 1981) projects ventrally along the area postulated (erroneously) by McDowell (1958: 169) to contain a shallow groove for the internal carotid artery in A. brevirostris .

The anterior carotid foramen, through which the internal carotid artery presumably entered the braincase, lies in the petrosalsphenoid suture at the anteromedial corner of the tympanic roof, just medial to the point at which the suture separates to produce the piriform fenestra. Further expansion of the piriform fenestra would engulf the anterior carotid foramen, resulting in passage of the internal carotid artery through the enlarged fenestra, much as in soricids. Slightly anteromedial to the anterior carotid foramen, a small opening in the alisphenoid is evident; this presumably transmitted a vidian branch of the internal carotid artery.

JAW JOINT: Figure 25 of McDowell (1958: 170) is an excellent illustration of the differences between the entoglenoid and postglenoid processes. McDowell recognized that for posterior support of the mandibular condyle, Solenodon and Nesophontes (along with soricids, moles, and tenrecs) possess a modified entoglenoid process located medial to the postglenoid foramen and anterior to the promontory of the petrosal. These taxa lack a true postglenoid process (see also Mac­ Phee, 1981: 220). Strangely, McDowell did not recognize that this was also the case for Apternodus . As exemplified by the type of A. baladontus , and similar to the condition in Tenrec figured by McDowell (1958: fig. 25C), the mandibular condyle of Apternodus is supported posteriorly by a flange of squamosal that is coincident with the anterior bor­ der of the middle ear, grooved by the chorda tympani, and located anteromedial to the postglenoid foramen. Some variation exists in the contribution of the alisphenoid to the entoglenoid process; in Tenrec it is considerable, while in Solenodon and Apternodus it is small.

This contrasts with a true postglenoid process (as illustrated in Leptictis by McDowell 1958: fig. 25A), which is located anterolateral to the postglenoid foramen, and well lateral to the promontory of the petrosal and path of the chorda tympani. McDowell’s figure 24 (p. 169) accurately shows the morphology of the entoglenoid process in Apternodus , which can be seen to differ as noted above from the postglenoid process of Leptictis .

BRAINCASE: The parietals of FMNH UM1690 are quite rugose, a feature in keeping with powerful temporal muscles. The ventral margin of the lambdoid plate does not reach as far ventrally as that of other members of the genus Apternodus . In contrast, other species show a ventral mastoid rim of the lambdoid plate well below the level of the jaw joint.

HYOID APPARATUS: In the course of prep­ aration of the skull, two hyoid elements were found. These appear to be (1) an ankylosed complex made up of the basihyal and both thyrohyals and (2) either a right ceratohyal (of Sprague, 1944, or epihyal of Allen, 1910) or a right hypohyal (also of Sprague, 1944, or ceratohyal of Allen). If the latter really is a ceratohyal, as seems probable because of its large size relative to the basihyal­thyrohyal complex, then the hypohyal must have been very small, a special similarity to Solenodon among insectivorans in which the hyoid region is known ( Sprague, 1944: table l). If the bone is a large hypohyal, then there is little similarity to that of any other insectivoran.

DENTITION: The dental formula of A. baladontus is the same as that of other Apter­ nodus species: 2.1.3.3/3.1.3.3. The few Apternodus specimens that preserve erupting teeth (e.g., AMNH 76747, CM 9552, CM 71570) have three fully erupted molars posteriorly, and are thus consistent with the replacement pattern observed in most other placental mammals. Based on this pattern, we assume that the premaxillary teeth of taxa discussed here are incisors, the anteriormost maxillary tooth is a canine, and the anterior three lower teeth are incisors, followed by a canine.

Accepting these assumptions, there are only two upper incisors in FMNH UM1690, identified here as I1 and I2 based on the observation that in some other insectivorangrade placentals, I3 is small or absent (e.g., Solenodon and Tenrec ). The crown of I1 is not preserved on either side of the skull. The root of the left I1 is 2.0 mm in anteroposterior diameter just within its alveolus. The right and left I1 pitch toward each other and may have met medially, depending on the length of the crown. The tips of the enlarged anterior lower incisors pass forward between the right and left I1 and are accommodated by a gutter in the bone below the nares when the mouth is closed. Lateral motion of the anterior end of the occluded mandible is thereby restricted. I2 has a bulbous crown, and is smaller and more spherical than the elongate, semipremolariform I2 of A. mediaevus . The root is straighter than in CM 13676 or A. brevirostris , but the tooth is tilt­ ed medially in the same manner. Wear is confined to a flat facet at the apex of the tooth.

The upper canine is a massive crushing tooth with a bulbous conical crown, supported by three roots. The anterior and posterior roots are massive and diverge slightly when seen from the side. The third, medial root supports the anterolabial corner of the tooth and projects anterodorsally in the wall of the snout, approximately parallel with the anterior root. The crown overhangs the root medially. Wear is confined to a flat surface at the apex. The wear surface tilts slightly medial from the horizontal.

As in most other placentals with three premolars, we consider P1/p1 (or dP1/dp1) of Apternodus to be absent. P2 possesses two main roots and has a third, posterolingual root that is also present in A. mediaevus . The crown is circular and bulbous, larger than P2 of A. brevirostris , but less elongate than P2 of A. mediaevus . Wear is confined to a flat surface at the apex of the crown. P3 is small and triangular, with a weak posterolabial shearing crest. The cingula meet lingually, but a protocone is only faintly indicated. The molars are more gracile, but are otherwise similar to those of A. mediaevus from Pipestone Springs.

The anteriormost lower incisor (i1) is greatly enlarged and procumbent, and contacts the opposite anterior incisor for about half the length of its crown. The root is oval in cross section and, in contrast to Solenodon , is not hollowed out medially. No serrations are present along the upper edge of the labial enamel surface. The second lower incisor is small, but not as diminutive in FMNH UM1690 as it is in A. brevirostris ; unlike the latter, the i2 of A. baladontus has a distinct alveolus. As in the upper dentition, i3 to p2 present a series of bulbous, procumbent crowns, presumably adapted for crushing. This curious morphology somewhat resembles an overlapping series of inverted spoons. All are single­rooted.

The lower canine and p2 are similar in appearance and possess bulbous crowns. This contrasts with species of Apternodus common outside of Montana, in which both teeth are premolariform. The ventral side of the anterior part of the canine crown of FMNH UM1690 is hollowed to fit the rear of the crown of i3. No trace of a central high cusp or ridge remains. The second lower premolar of A. brevirostris possesses a crested crown, whereas p2 of A. baladontus does not. The third lower premolar is the first double­root­ ed lower cheek tooth, and is more premolariform than anterior teeth. Its crown forms a low cone with a very weak posterior crest that runs from the apex to a tiny ridge that lies directly ventral to the paraconid of p4. The first and second lower molars are approximately equal in size; m3 is slightly smaller. Buccal cingulids on the molars of the type (fig. 12) and most referred specimens (e.g., fig. 14) are very slight.

The teeth of FMNH UM1690 seem to retain traces of pigment once present in life. Pigment is present in various soricid teeth as far back as the Oligocene ( Patterson and McGrew, 1937: 247) and can also be seen in Solenodon (especially under ultraviolet illumination). The pigment is visible as an opacity blocking the normal yellow or orange radiation emanating from the enamel near the apices of cusps. In the preserved upper den­ tition of FMNH UM1690, only the left I2 is fully pigmented, but traces of pigment can be seen within the enamel of all teeth, especially M2, distributed near cusp apices. The lower anterior incisors are heavily pigmented on the labial and medial faces, but not on the preserved upper face. Similar pigmentation is present in the type specimen of A. brevirostris , in the upper dentition of the type specimen of A. iliffensis , and in some specimens from Pipestone Springs (e.g., AMNH 97255). It is of course possible that in some cases ‘‘pigment’’ may be an artifact of fossilization, and is in any event obscured in specimens (e.g., MPUM 3799, AMNH 76745, USNM 18966, CM 71569) with jetblack teeth. Although the distribution of pigment in Apternodus teeth resembles that of soricids and probably had a biological cause, we note that many vagaries of preservation and wear heavily influence whether or not pigmentation is observable in any given specimen. We therefore refrain at present from attributing much systematic importance to this character.

Figures

Fig. 11. FMNH UM1690, Apternodus baladontus sp. nov. type specimen from McCarty’s Mountain, Montana. Stereo view of ventral skull, (p. 27) anatomical guide to ventral view (see text for abbreviations), (p. 28) dorsal (top) and lateral (middle and bottom) views of skull, (p. 29) detail of left ear region. See text for abbreviations. Dorsal and lateral photos of skull by Chester Tarka.

Fig. 25. Intertrochanter width (top) and acetabular width (bottom) divided by anteroposterior skull length in selected taxa (in millimeters). See figure 52 for measurements taken. Dots indicate individual specimens measured. Diamonds indicate 95% confidence intervals of the mean; nonoverlapping diamonds indicate a significant difference at alpha = 0.05, assuming normality.