Sinclairella dakotensis Jepsen, 1934

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 : 10-12

publication ID	https://doi.org/ 10.26879/732
persistent identifier	https://treatment.plazi.org/id/5B47878A-FFEF-CE5C-373C-FA8BFA3DF936
treatment provided by	Felipe
scientific name	Sinclairella dakotensis Jepsen, 1934
status

Treatment

Figure 6 View FIGURE 6 , Tables 1 and 2

Referred specimens. Left M1, UMPC 13937; right M1, UMPC 14860; left M2, UMPC 14862; right M3, UMPC 14863, 14864; left m1, UMPC 14270; right m1, UMPC 14861, 13938; left m2, UMPC 13869, 14866; right m2, UMPC 13601, 14865.

Horizon and locality. UMPC Loc # MV 6335 “Whitehead Creek”: UMPC 14270, 14860, 14866; UMPC Loc # MV 6324 “Orella Road”: UMPC 14861, 14865; UMPC Loc # MV 6316 “Sand Creek”: UMPC 13601, 13869, 13937, 13938, 14862, 14863, 14864. All localities are located within the middle Chadronian Peanut Peak Member, Chadron Formation.

Description. Two M1s are known from this sample, one from the Whitehead Creek locality and one from the Sand Creek locality. Both of these teeth are consistent with published descriptions of the M1 of Sinclairella ( Jepsen, 1934; Clemens, 1964; West, 1973), although the two specimens differ slightly from each other. The right M1 from Whitehead Creek (UMPC 14860; Figure 6.1 View FIGURE 6 ) shows little wear but exhibits moderate enamel damage on the cusp tips, the anterior border of the parastyle, and the postero-buccal corner of the tooth. The protocone is slightly larger than the two buccal trigon cusps, which are subequal in size and connected by a high centrocrista. A short preprotocrista runs antero-bucally from a point about halfway up the protocone to the anterior border of the tooth, where it joins with a small but prominent paraconule. The paraconule sits equidistant from the protocone and paracone and is separated from the paracone by a shallow groove. Directly anterior to the paracone is a well-developed parastylar shelf containing a small, worn parastyle. The parastylar shelf fades into the antero-buccal corner of the paracone, so that there is no stylar shelf along the antero-buccal margin of that cusp. Toward the postero-buccal corner of the paracone, the stylar shelf again appears and continues along the buccal edge of the tooth crown to its postero-buccal corner where it joins with the postmetacrista. Whereas the posterobuccal corner of the tooth is damaged, it is clear that the metastylar shelf is considerably smaller than the parastylar shelf. On the postero-lingual corner of the tooth, the talon holds a large cuspate hypocone. This cusp is approximately one-third of the size of the protocone. The hypocone is separated from the protocone by a prominent groove.

The Sand Creek M1 (UMPC 13937; Figure 6.4 View FIGURE 6 ) generally agrees morphologically with the specimen from Whitehead Creek, except that the tooth displays several less accentuated features. The bases of the talon cusps are less bulbous basally, giving the cusps a less conical appearance. This is particularly true of the protocone, which is somewhat bucco-lingually compressed. Distal to the protocone, the hypocone of the Sand Creek specimen, while prominent, is relatively smaller and more widely separated from the protocone. The paraconule is smaller in the Sand Creek specimen, and it is confluent with the paracone. Although the condition of the parastylar development cannot be evaluated due to damage to the antero-buccal corner of the tooth crown, the stylar development is not as well-developed in the Sand Creek specimen, judging from the buccal development of the stylar shelf. Whereas some of the difference between the specimens may result from the slightly more worn condition of the specimen from Sand Creek, we do not think that wear can fully account for these differences.

UMPC 14862 ( Figure 6.5 View FIGURE 6 ) is a left M2 from the Sand Creek anthills. Unlike the condition in M1, the three trigon cusps are subequal in size. There is a week preprotocrista running antero-bucally from the protocone and fading into the anterior edge of the tooth crown at about the midline of the tooth, before reaching a modest paraconule. The paraconule is adjoined to the antero-lingual border of the paracone. The paracone and metacone are connected by a stout centrocrista that forms a shallow “u” along the midline of the tooth. A short preparacrista angles antero-bucally from the paracone, and joins the anterior margin of the tooth crown. There is minor enamel damage to the postero-buccal margin of the tooth, but it appears that the postmetacrista is slightly longer than the preparacrista and terminates at the postero-buccal corner of the tooth. The buccal margin of the tooth contains a broad stylar shelf. This shelf follows the contours of the paracone and metacone, and thus reaches further buccally along these cusps, creating a shallow ectoflexus along the center of the buccal margin. Whereas the stylar shelf is well-developed, there are no clear styles projecting from it.

The postero-lingual corner of the M2 consists of a large talon. This shelf slopes toward the posterior margin of the tooth crown and dips cervically. The result is an undulating posterior crown border with the greatest cervical projection about midway between the lingual and buccal border of the talon and the greatest occlusal projections along the postero-lingual border of the tooth and directly posterior to the metacone. There is a small hypocone along the lingual margin of the talon.

There are two M 3 specimens from the Sand Creek locality. UMPC 14863 ( Figure 6.6 View FIGURE 6 ) is complete but exhibits a weathered groove running occluso-cervically on the lingual surface of the protocone and minor enamel damage on the hypocone. The antero-buccal corner of UMPC 14864 ( Figure 6.2 View FIGURE 6 ) is missing. These teeth exhibit the same general pattern of the M3 of Sinclairella discussed by Jepsen (1934) with a reduced metacone, hypocone, and metastylar shelf compared with the other upper molars. The result is an M3, which is more triangular in outline than either of the other upper molars .

Although similar, the two M3s differ from each other in overall size, the relative development of the talon, and the relative size of the metacone. UMPC 14863 is approximately 20–30% larger than UMPC 14864 ( Table 2). Lingually, the larger specimen exhibits a broader talon with a small hypocone; the smaller specimen lacks a hypocone. Buccally, the larger specimen has a larger, more bulbous metacone. Whereas these features suggest that UMPC 14863 may be an M2, the lack of a metastylar lobe results in the characteristic postero-lingual taper of the buccal crown surface from parastyle to metacone and suggests that this tooth is an M3. While not a perfect indicator of tooth position, the lack of an interproximal wear facet posterior to the metacone adds additional evidence that this specimen is an M3.

One m1 was recovered from each of the three study areas. Of these three teeth, the crown of the Orella Road specimen ( UMPC 14861 ; Figure 6.8 View FIGURE 6 ) is in excellent condition, showing minimal wear and only minor enamel damage on the underside of the anterior-most point of the trigonid. The crown morphology of this tooth is consistent with that described for the type specimen of Sinclairella ( Jepsen, 1934) . The trigonid is antero-posteriorly elongate and narrows anteriorly. The protoconid and metaconid are subequal, and each cusp rises well above the height of the protolophid. The paraconid is virtually absent, existing as a small swelling on the tooth anterior and slightly buccal to the metaconid. A ridge of enamel runs anteriorly and slightly lingually from the protoconid where it broadens into a small cuspule at the anterior edge of the tooth crown. The talonid is broader than the trigonid and is deeply basined. The cristid obliqua originates at the postero-buccal corner of the tooth from a modest hypoconid. This crest borders the buccal margin of the talonid in a antero-posterior direction before curving slightly linguad to join the postvallid low along the postero-lingual margin of a high protoconid. Buccal to this juncture between the cristid obliqua and the protoconid is a strong hypoflexid, accentuated by a substantial postero-buccal extension of the protoconid. This extension increases down the cusp as it approaches the cervical margin of the tooth crown. The posterior talonid margin consists of a sharp, arcuate postcristid. The hypoconulid and metaconulid are not clearly present .

The other two m1s in this sample show varying degrees of wear and enamel damage. UMPC 14270 , from the Whitehead Creek Locality , is heavily worn and exhibits enamel damage on the antero-lingual border of the trigonid. This tooth agrees with the Orella Road specimen in the majority of features. It differs from this specimen in possessing a trigonid and talonid that are equal in height, a less marked hypoflexid, and an interruption in the postcristid at the midline of the tooth. Each of these differences could be attributed to the advanced state of wear of the Whitehead Creek specimen. The specimen from Sand Creek ( UMPC 13938 ; Figure 6.11 View FIGURE 6 ) shows modest wear and enamel damage to the anterior and posterior crown borders. The Sand Creek specimen is consistent with the other two specimens but exhibits a slightly more prominent paraconid .

Sinclairella dakotensis m2 is represented in this sample by two specimens from the Sand Creek Locality (see UMPC 13869, Figure 6.12 View FIGURE 6 ), one specimen from the Whitehead Creek locality (UMPC 14866; Figure 6.9 View FIGURE 6 ), and one partial specimen from the Orella Road locality. These four specimens agree with each other in crown morphology and all are consistent with other descriptions of S. dakotensis ( Jepsen, 1934; Scott and Jepsen, 1936; West, 1973). Of the four specimens, the left m2 from Whitehead Creek (UMPC 14866; Figure 6.9 View FIGURE 6 ) is in the best condition and provides the basis of this description. The trigonid consists of four cusps, the metaconid being about the same size as the protoconid. Directly anterior to the metaconid is a small paraconid. As is the case with m1 there is a crest running from the protoconid to a small cuspule. However, on m2 this cuspule sits at the antero-buccal corner of the tooth, further mesiad than the paraconid and further buccad than the protoconid, thus giving the trigonid the characteristic “quadrilateral” appearance described by Jepsen (1934).

The talonid is broad and deeply basined. Of the talonid cusps, only the hypocone is clearly visible as a modest swelling at the postero-buccal bor- der of the tooth. The cristid obliqua runs in an antero-lingual direction from the hypoconid to the postero-lingual corner of the protocone, where the crest joins the postvallid. The hypoflexid is marked. The posterior and lingual margin of the talonid is bordered by a continuous crest (postcristid, preentoconid cristid, postmetaconid cristid).

Remarks. First described from a crushed skull from the Chadron Formation of southwestern South Dakota ( Jepsen, 1934), Sinclairella dakotensis is known from the Chadronian of Wyoming ( West, 1973), Nebraska ( Hough and Alf, 1956; Ostrander, 1987; this study), North Dakota ( Pearson and Hoganson, 1995), and Saskatchewan ( Storer, 1996; Meyer, 2007), the Orellan of South Dakota ( West, 1973) and Colorado ( Clemens, 1964), and the Arikareean of South Dakota ( Simpson, 1985), Nebraska ( Gunnell et al., 2008), and Oregon ( Cavin and Samuels, 2012). The genus is also known from the Duchesnean of Saskatchewan ( Sinclairella sp. , Storer, 1996) and the Arikareean of Florida ( Sinclairella simplicidens, Czaplewski and Morgan, 2015 ). Whereas there is variation in the expression of traits, such as the degree of complexity in M1, cusp development, and overall size of M3, the Sinclairella specimens collected from each of these three sampling localities are consistent with previous descriptions of S. dakotensis (e.g., Jepsen, 1934; Clemens, 1964).