Bolodon hydei, Cifelli & Davis & Sames, 2014

Cifelli, Richard L., Davis, Brian M. & Sames, Benjamin, 2014, Earliest Cretaceous mammals from the western United States, Acta Palaeontologica Polonica 59 (1), pp. 31-52 : 37-38

Bolodon hydei sp. nov.

Fig. 5B .

Etymology: In honor of Dayton Hyde, founder of the Institute of Range and the American Mustang, for his support and cooperation on the part of the Institute.

Holotype: OMNH 62670 , a right M2.

Type locality: OMNH V1243 , Fall River County, South Dakota, USA. Type horizon: Chilson Member , Lakota Formation ( Lower Cretaceous : upper Berriasian–Valanginian) .

Diagnosis.—Small multituberculate most comparable to Bolodon osborni but differing in that the second cusp of the lingual row is larger than the other two, the buccal ridge is less developed, mesiolingual corner of the crown extends farther mesially, and the distal margin of the crown is more squared-off (less pointed).

Description.—OMNH 62670 is complete and in a good state of preservation, except that it is heavily worn (L = 2.04 mm, W = 1.74 mm). Much of the enamel is missing, and the original extent of ornamentation is unclear. The presence of some ornamentation is suggested by two ridges that ascend mesially from the apex of cusp 1 in the buccal row, enclosing a small basin on the mesial flank of that cusp ( Fig. 5B ); a second small basin is present mesiobuccal to the apex of buccal cusp 1. The buccal cusp row bears two large, subequal cusps, while the lingual cusp row appears to have had three cusps (abrasion of the mesial part of the lingual cusp row leaves interpretation open). The buccal ridge is heavily worn; it appears to have been buccolingually widest at the level of cusp 1 in the buccal row. The ridge extends distally to cusp 2 of the buccal cusp row as only a narrow ridge. The crown is therefore broadest at the level of cusp 1 of the buccal row. The crown tapers mesiolingually, with cusp 1 of the lingual row placed farther mesially than cusp 1 of the buccal row. The remaining cusps are relatively in-line transversely (as they are in Iberica hahni Badiola, Canudo, and Cuenca-Bescós, 2011 ). Buccal and lingual cusp rows are subequal in height but the buccal cusps are significantly broader-based. Adjacent faces of the buccal cusps (distal face of cusp 1, mesial face of cusp 2) are flat; both cusps have a semi-pyramidal appearance (buccal and lingual faces have flat wear facets). The lingual cusps have been substantially reduced by wear, but it is clear that they were joined to form a ridge (not developed as separate, conical cusps), with cusps separated by narrow, shallow grooves rather than deep valleys. Cusp 2 of the lingual row appears to have been the largest. Cusps 1 and 3 of the lingual row were probably subequal, but breakage of cusp 1 makes absolute size difficult to determine. The central valley is straight-sided and is worn into a broad V. Substantial wear is also present on the buccal face of the buccal cusp row, extending to and nearly obliterating the buccal ridge.

Remarks.— Individual molar characteristics often have perplexing distributions among the “Plagiaulacida” and basal Cimolodonta, and character polarities are not well understood in some cases; the following comments are based on the recent treatment of Cifelli et al. (2013). Bolodon hydei sp. nov. is probably plesiomorphic in a number of features, such as the presence of only two buccal and three lingual cusps, lack of separation between the mesial cingulum and first cusp of the buccal row, slight development of (and lack of cusps on) the mesiobuccal ridge, length relative to width, proportions of the cusps in the buccal row, and mesiodistal alignment of all cusps in the lingual row ( Fig. 5B ). The mesial margin of OMNH 62670 is sinuous, with a pronounced, mesially-projecting “spout” at the central valley. Polarity of this feature is uncertain; the condition occurs within various groups of “Plagiaulacida” and basal Cimolodonta. Derived conditions seen in Bolodon hydei include the probable presence of enamel ornamentation, sub-pyramidal outline of cusps in the buccal row, and joining of lingual cusps into a ridge, with narrow grooves (rather than well developed valleys) between cusps.

We compared Bolodon hydei with all “plagiaulacidans” and basal cimolodontans (“ Paracimexomys group”; Kielan-Jaworowska and Hurum 2001) for which M2 is known. The foregoing combination of characters, together with apomorphies of individual taxa, distinguishes B. hydei from basal Cimolodonta, Allodontoidea , Arginbaataroidea, Paulchoffatoidea, and Eobaataridae . On the other hand, we found the comparison with B. osborni Simpson, 1929 ( Plagiaulacidae ) so compelling that we consider the species congeneric. The holotype and referred M2 (see Kielan-Jaworowska and Ensom 1992) demonstrate some intraspecific variability in B. osborni , notably in the fact that enamel ornamentation (in the form of ribbing and pitting) is better developed on the referred specimen. Both differ from OMNH 62670 in having less difference in size between cusps of the lingual row, in having a more prominent mesiobuccal rim, and in having a distal margin that is more rounded, with the lingual side projecting farther distally.

Stratigraphic and geographic range.— Lower Cretaceous (upper Berriasian–Valanginian), South Dakota, USA. Currently known only from the type locality and horizon .

Figures

Fig. 5. “Plagiaulacidan” multituberculate mammals from OMNH locality V1243, Lakota Formation (Lower Cretaceous: upper Berriasian–Valanginian), South Dakota, USA. A. The allodontoid Passumys angelli gen. et sp. nov., OMNH 64191 (holotype), right m1 in occlusal (A 1, stereopair), buccal (A 2), and lingual (A 3) views. B. The plagiaulacoid Bolodon hydei sp. nov., OMNH 62670 (holotype), right M2 in occlusal (B 1, stereopair) and oblique lingual (B ) views.