Apatemys sp.

Tornow, MA & Arbor, TC, 2017, Apatemyids (Mammalia, Apatotheria) from the middle Chadronian (late Eocene) of Sioux County, Nebraska, US, Palaeontologia Electronica 20 (1), pp. 1-16 : 5-9

Apatemys sp. aff. A. downsi Gazin, 1958

Figures 2 and 3.3 , Tables 1 and 2

Referred specimens. Left M2, UMPC 14190; left M3, UMPC 14191; right m1, SDSM 10242.

Horizon and locality. UMPC Locality # MV 6335 “Whitehead Creek”, UMPC 14190, 14191; SDSM Locality # V 772-A “Raben Ranch”, SDSM 10242. Middle Chadronian Peanut Peak Member, Chadron Formation.

Description. UMPC 14190 is a left M2 that exhibits a crack extending across the midline of the tooth from the anterior to the posterior margin ( Figure 2.1 ). Otherwise, the tooth is in excellent condition. It displays minimal wear and all crown features are clearly visible. Of the three cusps that comprise the trigon, the protocone is slightly larger than the two, subequal, buccal cusps. The protocone is conical, although the cusp appears slightly bucco-lingually compressed given the presence of a short, stout postprotocrista that decreases in height as it is traced posteriorly from the apex of the protocone. The paracone and metacone are conical. The anterior margin of the trigon is closed by a strong preprotocrista that runs antero-buccally along the anterior edge of the tooth crown from the tip of the protocone to a point directly anterior to the paracone, where it joins with a preparacrista that runs in an anterior and slightly buccal direction from the paracone. A weak paraconule arises from a modest swelling of the preprotocrista lingual and slightly anterior to the paracone. Running posteriorly from the paracone is a well-formed postparacrista. This crest joins with an equally well-developed premetacrista on the midline of the tooth to form a deep “v.” The postmetacrista travels postero-buccally from the tip of the metacone to the posterior margin of the tooth. The buccal margin of the tooth is characterized by strong parastylar and metastylar lobes. These stylar developments are separated by a deep ectoflexus.

aApproximate measurement based on damaged specimen. *Measurement not available. Clemens (1964) provided only length and maximum width. aApproximate measurement based on damaged specimen.

There is a large talon heel extending off of the postero-lingual margin of the M2 crown, extending slightly more lingually and posteriorly than the protocone and metacone, respectively, to give the posterior margin of the tooth a waisted appearance. A small, but prominent hypocone arises from the postero-lingual corner of the talon. The hypocone is slightly elongated, running diagonally across the talon in an antero-lingual to postero-buccal direction.

On the M3 (UMPC 14191, Figure 2.2 ) the anteriorly-seated protocone is the largest of the trigon cusps in volume, and it is subequal in height to the paracone. The paracone is slightly larger than the metacone in both volume and height. As is the case with M2 the preprotocrista of M3 runs antero-buccally from the tip of the protocone along the anterior edge of the tooth crown. However, on M3 this crest stops short of the preparacrista, leaving the trigon basin open along its antero-buccal corner between the lingual margin of the paracone and the anterior border of the tooth crown. The preparacrista runs anteriorly from the paracone to the anterior edge of the tooth where, with the paracone, it forms the lingual margin of a large, antero-buccally extending parastylar lobe. The postparacrista is somewhat longer than the premetacrista, and the two crests meet to form a broad, shallow “v” posterior to the midline of the tooth. The postmetacrista runs from the metacone to the posterior margin of the tooth. The metastylar lobe is considerably smaller than is seen on M2, and, subsequently, the anterior border of the ectoflexus is longer than the posterior border.

As is the case with M2, there is a prominent talon lobe extending postero-lingually from the protocone on M3. Although less prominent on M3, the posterior extension of this lobe creates a waisted appearance of the posterior margin of the tooth similar to the condition expressed on M2. A small rise, suggestive of a hypocone, occupies the postero-lingual corner of the talon.

SDSM 10242 is a complete right m1 with minor enamel damage on the lingual margin of the tooth between the metaconid and entoconid ( Figure 3.3 ). The trigonid is antero-posteriorly elongate and narrows anteriorly. The protoconid and metaconid are subequal and conical. The metaconid sits postero-lingual to the protoconid, thus giving the posterior trigonid wall (i.e., postvallid) an oblique orientation. The postmetacristid joins with the postprotocristid at the midpoint of the tooth to form a u-shaped notch that runs approximately one-third of the depth of the postvallid. Directly anterior to the metaconid is a small, but discernible paraconid. A prominent crest runs anteriorly from the apex of the protoconid to the antero-buccal corner of the tooth where it swells into a minute antero-buccal cusp.

The m1 talonid is broader than the trigonid. The postero-buccal corner of the tooth is marked by a conical hypoconid. The cristid obliqua runs antero-lingually in a straight line from the hypoconid to the posterior margin of the protoconid. Buccal to the junction of the cristid obliqua and the protoconid is a marked hypoflexid. The talonid is closed posteriorly by an arcuate postcristid. The hypoconulid and entoconid are suggested by two swellings on this posterior crest, along the midline of the tooth and at the postero-lingual corner of the tooth, respectively.

Remarks. The genus Apatemys is known from the Wasatchian (e.g., Simpson, 1954; Bown and Shankler, 1982), Bridgerian (e.g., West, 1973; Gunnell, 1998), and Uintan (e.g., Prothero, 1996b) of the Rocky Mountains Faunal Province. Beyond the Rocky Mountains, the genus is known in North America from the Wasatchian of Mississippi ( Beard and Dawson, 2001, 2009), the Uintan of California ( Gazin, 1958; Golz and Lillegraven, 1977; Walsh, 1991), and the Uintan ( Storer, 1984), Duchesnean ( Storer, 1987), and Chadronian ( Storer, 1996; Meyer, 2007) of Saskatchewan. In Europe, Apatemys is known from the Sparnacian of France ( Russell et al., 1979; Marandat, 1989), Belgium ( Russell et al., 1979), and Portugal ( Estravis, 2000). Although widely known, Apatemys is a rare component of the faunas from which it is known. Diagnoses of Apatemys species (e.g., McKenna, 1963; West, 1973; Bown and Shankler, 1982) overwhelmingly consist of characteristics of the lower premolars (e.g., McKenna, 1963), overall size (e.g., Robinson, 1966; West, 1973), and mandibular characteristics (e.g., McKenna, 1963). Few discussions of the genus (e.g., Beard and Dawson, 2009) assign upper teeth to the species level, and none provide detailed descriptions of the m1. The addition of these two middle Chadronian upper molars of Apatemys from the Whitehead Creek Local Fauna of northwestern Nebraska and attribution of a lower molar from Raben Ranch, extends the geographic distribution of this genus during the late Eocene and adds to the range of known intrageneric variation within the genus Apatemys .

Whereas the genus Sinclairella is known from the research area, the two upper molar specimens from Whitehead Creek are attributed to the genus Apatemys on the basis of a suite of characters outlined by West and Atkins (1970). Both specimens exhibit more well-developed, parastylar and, in the case of M2, metastylar lobes than are seen in the corresponding teeth of Sinclairella , and the ectoflexus is more pronounced on each when compared with Sinclairella . The specimens of Apatemys exhibit reduced hypocones (particularly on M2) when compared with Sinclairella , thus resulting in a more triangular occlusal outline. The teeth here attributed to the genus Apatemys are also considerably smaller than the corresponding teeth in Sinclairella dakotensis ( Table 1), the only other apatemyid taxon known from these deposits. Instead, the teeth being assigned to Apatemys are closer in size to Sinclairella simplicidens from the Arikareean of Florida (Czeplewski and Morgan, 2015) than to S. dakotensis . Morphologically, however, in their possession of marked stylar developments and the accompanying pronounced ectoflexus, the Whitehead Creek specimens diverge even further from the simplified teeth of S. simplicidens than they do from the older species, S. dakotensis .

The Whitehead Creek M2 exhibits an intermediate morphology between a Bridgerian apatemyid specimen (AMNH 56046) from Tabernacle Butte, Wyoming, described as Apatemys sp. by West and Atkins (1970) and an M2 from Chadronian-age deposits of the Cypress Hills Formation, Saskatchewan (RSM P2374.006) attributed to Apatemys sp. ( Storer, 1996; described by Meyer, 2007). It is similar to both of the aforementioned specimens in possessing well-developed, large parastylar and metastylar lobes that are separated by a pronounced ectoflexus. However, the Whitehead creek specimen differs from the Tabernacle Butte, Wyoming, specimen in possessing more equally proportioned stylar lobes, the parastyle being relatively wider with less buccal extension. In this respect, the specimen described herein appears to be more similar to the M2 from Saskatchewan ( Meyer, 2007, figure 5.8). The talon lobe is well-developed compared to Bridger specimens, albeit less so than that of the Chadronian specimen from Saskatchewan in which the talon lobe exhibits greater lingual extension. The hypocone is not as well developed in the Whitehead Creek M2 as it is in the specimen from Saskatchewan. In this feature, the small crestiform hypocone is more similar to the condition found in Bridgerian specimens attributed to Apatemys ( West and Atkins, 1970) . The Nebraska specimen is also intermediate in size between early and middle Eocene Apatemys bellus and the larger, Chadronian-age specimen from Cypress Hills ( Figure 4 ).

The M3 (UCMP 14191) is considerably smaller than the M2. Whereas the antero-posteriorly short dimension of the M3 results in a tooth that is relatively more transverse than is the case for M2, the tooth is absolutely narrower than M2 and thus significantly reduced when compared with associated Apatemys M2-3 ( West, 1973). Whereas the size discrepancy between the Nebraska specimens does fall within the range of size differences (largest M2s, smallest M3s) for isolated Apatemys bellus specimens identified by West (1973), the taxonomic allocation of these specimens is disputed (Bown and Shankler, 1982). Thus, the size discrepancy between the Whitehead Creek M2 and M3 is likely greater than that for known species of Apatemys .

Ostrander’s (1980, 1987) discussion of SDSM 10242, an m1 from the Raben Ranch local fauna, did not provide a detailed specimen description. He noted the small size of this specimen but emphasized the small sample size and resulting lack of knowledge of intraspecific variation for Sinclairella dakotensis in assigning this tooth to that species. Figure 5 illustrates that SDSM 10242 is considerably smaller than known S. dakotensis m1s from Sioux County, and that the Raben Ranch m1 is consistent with an apatemyid of the size represented by the M2 from Whitehead Creek (UMPC 14190), here attributed to Apatemys .

Whereas the general structure of the Raben Ranch m1 is similar to that of Sinclairella , it differs in features that more closely align with the morphology of Apatemys . In the trigonid, SDSM 10242 has a larger paraconid and a smaller antero-buccal cusp than is seen in Sinclairella specimens from Sioux County, Nebraska (see Figures 3.1-2 , 6.8 , and 6.11). Taken in conjunction with a relatively shorter crest running anteriorly from the protoconid, the difference in cusp proportions results in a trigonid that is relatively shorter antero-posteriorly and broader bucco-lingually and gives the trigonid a more rhomboidal appearance than is seen in the m1 of Sinclairella . Distally, the talonid cusps of SDSM 10242 are better developed than they are in specimens of Sinclairella . The hypoconid of SDSM 10242 is more conical, and, when combined with a straighter cristid obliqua, results in a more angular postero-buccal corner of the tooth. Although tiny, the hypoconulid and entoconid are visible in SDSM 10242, while these cusps are not clearly discernible in specimens of Sinclairella from Nebraska. The presence of comparably-sized upper molars of Apatemys from a locality that is close to Raben Ranch, both geographically and temporally, combined with the suite of Apatemys -like characteristics, demonstrate that SDSM 10242 is best assigned to the genus Apatemys .

Assigning the Nebraska Apatemys specimens to any species is hindered by the lack of diagnostic features of M2, M3, and m1 for larger species of Apatemys . Whereas the M2 from Whitehead Creek differs from described Apatemys specimens (e.g., AMNH 56046) in features such as the size of the hypocone, and stylar development, it is considerably larger than upper molars that have been assigned to species. The Raben Ranch m1 (SDSM 10242) is of comparable size to the type and only known specimen of the late Uintan Apatemys downsi (LACM [CIT] 5202; m1 length = 2.5 mm; Gazin, 1958), and, although no upper teeth are known for A. downsi , there is nothing to suggest that the Whitehead Creek M2 (UMPC 14190) would be incompatible with the m2 of that species. Whereas the Raben Ranch m1 is morphologically similar to the A. downsi m1, the Raben Ranch specimen exhibits a somewhat more elongate anterobuccal crest running from the protoconid and a reduced paraconid when compared with A. downsi ( Gazin, 1958, plate 13.9). However, without some understanding of intraspecific variation for A. downsi , the significance of this variation is dubious. Whether the Nebraska specimens represent a geographic and temporal range extension for A. downsi or represent a new species of Apatemys is presently unclear and must await further sampling.

Figures

FIGURE 2. Apatemys specimens from the Whitehead Creek locality. 1, left M2 (UMPC 14190) and 2, left M3 (UMPC 14191). Teeth are shown in occlusal view. Scale is approximately 1 mm.

FIGURE 3. Apatemyid first molars from Raben Ranch. 1 and 2, right m1s of Sinclairella dakotensis (SDSM 10241) and 3, right m1 of Apatemys sp. (SDSM 10242). Teeth are shown in occlusal view. Scale is approximately 1 mm.

FIGURE 4. Plot of apatemyid M2 width x length. Circles indicate Sinclairella dakotensis specimens. Squares indicate Apatemys specimens. The open square indicates the Whitehead Creek specimen described in this study. Data for AMNH 56046 and YPM 15284 are from West and Atkins (1970). Data for (RSM) P2374.006 are from Meyer (2007). Sinclairella data come from Clemens (1964), Cavin and Samuels (2012), and this study.

FIGURE 5. Plot of natural log width x length of M2, open symbols above diagonal, and m1, solid symbols below diagonal from different apatemyid species, the M2 from Whitehead Creek (UMPC 14190), and the m1 from Raben Ranch (SDSM 10242). Note the compatible size relationship between the specimens from the Chadronian of Sioux County, Nebraska (circles). Data are compiled from the following sources: Apatemys chardini (Gingerich, 1982; Ma, 1998), A. bellulus (Gazin, 1958; Ma, 1998), A. bellus (Gazin, 1958; West, 1973; Ma, 1998), Sinclairella dakotensis (Clemens, 1964; Ostrander, 1987; Cavin and Samuels, 2012; this study).

FIGURE 6. Teeth of Sinclairella dakotensis from Sioux County, Nebraska. 1, right M1 (UMPC 14860); 2, right M3 (UMPC 14864); 3, left P4 (SDSM 10392); 4, left M1 (UMPC 13937); 5, left M2 (UMPC 14862); 6, right M3 (UMPC 14863) (reversed); 7, left p2 (SDSM 10244) (reversed); 8, right m1 (UMPC 14861); 9, left m2 (UMPC 14866) (reversed); 10, left m3 (SDSM 10245) (reversed); 11, right m1 (UMPC 13938) and 12, left m2 (UMPC 13869). Teeth are shown in occlusal view. Divisions in scale are approximately 1 mm.