Passumys angelli, 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 : 36-37

Passumys angelli sp. nov.

Fig. 5A .

Etymology: In honor of Jim Angell of Edgemont, South Dakota, in recognition of his scientific, logistic, and diverse other help to OMNH field parties through the years.

Holotype: OMNH 64191 , right m1.

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

Diagnosis.— Small multituberculate with m1 ovoid to subrectangular in occlusal view, having mesial and distal margins of the crown gently rounded and symmetrical instead of beveled; central valley broad buccolingually; cusps of lingual and buccal rows mesiodistally aligned, conical, and well separated, with valleys between cusps extending to the level of the central valley. Most closely similar to Ctenacodon ,? Psalodon (we follow Simpson [1926] in questionably referring to Psalodon specimens preserving the lower dentition, as the genus was established on upper teeth only), and Glirodon , but differs in having well-separated first two cusps of the lingual row (with cusp 1 the largest), and a larger cusp 1 of the buccal row that is also subequal to the other cusps of that row.

Description.—The crown of OMNH 64191 is ovoid or subrectangular and nearly symmetrical in occlusal outline (L = 1.41 mm, W = 1.12 mm). It bears two rows of three cusps; the lingual cusp row is slightly higher than the buccal row. The cusp rows are offset, with the cusps of the buccal row lying slightly mesial to their counterparts of the lingual row (most apparent in occlusal view; Fig. 5A 1 ). The cusps of each row are well spaced; cusp 1 of each row has a flat distal face and cusp 2 has flat mesial and distal faces, but each cusp is subconical. The distal cusp of each row is entirely conical. No enamel ornamentation or crenulation is observed. The central valley is straight and broad, bearing light wear only. Light wear is also present on the internally-facing sides of the cusp rows. Very faint wear is present on the buccal faces of cusps 1 and 2 of the buccal cusp row. The tallest cusp is the mesial-most of the lingual row; it is twice as high as its counterpart on the buccal row. From the apex of this dominant cusp, a crest descends mesially and then buccomesially, terminating at the mesial end of the central valley. Weak crests descend from the apices of the distal cusps of each row, rimming the distal end of the central valley.

Remarks.—The m1 of Passumys is clearly distinct from that of cimolodontans in being proportionately short, in bearing only three cusps in each row, in having well separated, conical cusps, and in lacking any sort of enamel ornamentation ( Fig. 5A ). Among “plagiaulacidans” known by m1, Passumys differs from the Paulchoffatoidea in lacking cingulids (mesially and distally) and enlargement of the second cusp in the buccal row; from the Plagiaulacoidea in having better separated, more conical cusps, in lacking enamel ornamentation, in being symmetrical in occlusal view (no beveling of mesial or distal borders); and in having only three cusps in the buccal row ( Eobaatar clemensi Sweetman, 2009 is described as bearing three cusps on the buccal row; however, the distal-most of these is comprised of two cuspules separat- ed by a pit, giving the appearance of four cusps in this row); and from the Arginbaataroidea in having three (rather than two) cusps in the lingual row and in being symmetrical (not beveled mesially and distally) in occlusal view.

Among the Allodontoidea , Passumys is dissimilar to Zofiabaatar pulcher Bakker and Carpenter, 1990 , which has an m1 that is sub-quadrate and that appears to have only two main cusps in each row, with additional, mesially-placed cusps united by a strong mesial cingulid ( Kielan-Jaworowska et al. 2004: fig. 8.25G 2). Passumys compares most favorably with m1s referred to species of Ctenacodon Marsh, 1879b ,? Psalodon Simpson, 1926 , and Glirodon Engelmann and Callison, 1999 . As noted in the diagnosis, Passumys differs in its more symmetrical occlusal outline, better separation of cusps in the lingual row (particularly cusps 1 and 2), and in having relatively larger first cusps of both rows. Like the lower dentition in general ( Simpson 1929), m1s assigned to the various species of? Psalodon and Ctenacodon are rather similar, differing in minor details. All have a distinctly small first cusp in the buccal row; this cusp tends to be rapidly reduced or obliterated by wear. These species are also characterized by significant union (to about half their heights) and close appression of the first two cusps in the lingual row. Judged by the published illustration ( Engelmann and Callison 1999: fig. 25), the m1 of Glirodon grandis Engelmann and Callison, 1999 appears to be more similar to that of Passumys than are those of? Psalodon and Ctenacodon , though it appears to differ in that the cusp rows converge mesially.

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.

Fig. 1. Map of exposures of the Lower Cretaceous Inyan Kara Group in southwestern South Dakota and northeastern Wyoming (USA), which includes the Lakota Formation (Berriasian–Barremian).Stars indicate OMNH localities which yielded specimens described in this study. Modified from Sames (2011a), as based on Waagé (1959) and Sohn (1979).