Thryptacodon Matthew and Granger, 1915 Thryptacodonorthogonius

S. Scott, Craig, C. Fox, Richard & P. Youzwyshyn, Gordon, 2002, New earliest Tiffanian (late Paleocene) mammals from Cochrane 2, southwestern Alberta, Canada, Acta Palaeontologica Polonica 47 (4), pp. 691-704 : 695

Genus Thryptacodon Matthew and Granger, 1915 Thryptacodonorthogonius comb.nov. ( Russell, 1929)

Fig. 1E–H .

Chriacus orthogonius Russell, 1929: 175–177 , fig. 3.

Thryptacodon demari Gazin, 1956a: 28 , pl. 6: 2, 3.

Thryptacodon belli Gazin, 1956a: 29 , pl. 6: 1–4.

Holotype: UALVP 124 , isolated left M2, Fig. 1F .

Type horizon and locality: Cochrane 2, Paskapoo Formation, southwestern Alberta.

Age and distribution: Earliest Tiffanian of Alberta (type locality [ Russell 1929]) and Montana (Douglass Quarry, Melville Formation, Sweetgrass County [ Krause and Gingerich 1983]); early Tiffanian of Wyoming (Saddle locality, Fort Union Formation, Bison Basin, Fremont County [ Gazin 1956a]); middle Tiffanian of Wyoming (Ledge locality, Fort Union Formation, Bison Basin, Fremont County [ Gazin 1956a]); late Tiffanian of Wyoming (Rock Springs localities V−77006, V−77007, and V−78052, Fort Union Formation, eastern Rock Springs Uplift, Sweetwater County [ Winterfeld 1982]).

Material.— UALVP 24991, 45112, DP4; UALVP 45099, M1; UALVP 24990, 24993, 24994, 45103, 45107, 45109, 45110, M3s (total: 7); UALVP 24996, 45101, p4s; UALVP 24998, 24999, 42872, 42874, 45102, 45105, m1s (total: 6); UALVP 24997, 42873, 45108, 45111, m2s (total: 4); UALVP 24995, 42875, 45100, 45104, 45106, m3s (total: 5).

Revised diagnosis.—Smaller than Thryptacodon australis Simpson, 1935b and Thryptacodon antiquus Matthew and Granger, 1915 , with upper molars more transverse and rectangular in occlusal outline, with weaker hypocones; paracone and metacone subequal in size.

Remarks.—The lower molars from Cochrane 2 that we refer to T. orthogonius closely resemble those of T. “ demari ” Gazin, 1956a from the Ledge locality, and T. “ belli ” Gazin, 1956a from the Saddle locality, Bison Basin, Wyoming ( Gazin 1956a) differing only in m1 having a more prominent hypoconulid.Moreover, UALVP 24997, an m2, and the type specimen of “ Chriacus ” orthogonius ( Russell, 1929) from Cochrane 2 fit tightly when manually occluded. Van Valen (1978) considered T. “ belli ” and T. “ demari ” to be synonymous, and recognized “ C. ” orthogonius as the senior synonym of “ Metachriacus ” punitor Simpson, 1935a.

“ C.” orthogonius was founded on a single tooth from Cochrane 2, UALVP 124, a left M2 that Russell (1929) described. Sloan (1987) concluded that “ C.” orthogonius was descended from C. baldwini ( Cope, 1882) (= “ Metachriacus provocator ” Simpson, 1935a and “ C. truncatus ” Cope, 1884b [ Van Valen 1978]), yet C. baldwini differs significantly from “ C.” orthogonius in the structure of its upper molars.The holotype of “ C.” orthogonius can be readily distinguished from M2, C. baldwini , in its more symmetrical and rounded occlusal outline, weakly convex anterior and posterior sides, shallower ectoflexus, larger, more anterolabial hypocone, concave postprotocrista, and in being smaller overall.The upper molar morphology of “ C.” orthogonius is otherwise known only from Thryptacodon , differing significantly from the upper molar features of Chriacus , which include a robust, posterolingually−projecting hypocone, a rectangular occlusal outline with concave anterior and posterior sides, and a weakly convex postprotocrista that joins with a straight, posterolabially directed postmetacrista; they differ further from those of C.baldwini in M1 having a more salient parastylar area and larger paracone, and M3 having a better−developed lingual cingulum.

Until the recent UALVP collections were made from Cochrane 2, the holotype was the only specimen from the locality referable to “ C.” orthogonius , and the affinities of the species have been uncertain (see, for example, Krause and Gingerich 1983).However, UALVP 124 is virtually identical to M2s of T. “ demari ” from the Ledge locality, and of T. “ belli ” from the Saddle locality (GPY personal observation), supporting referral of the Cochrane 2 specimen to Thryptacodon .From these comparisons, T. “ demari ” (including T. “ belli ”) is herein considered a junior synonym of T. orthogonius .

Figures

Fig.1. A–C. Ptilodus gnomus sp.nov., Cochrane 2 locality, Alberta. A.Holotype, right P4, UALVP 45145 in labial (A 1), lingual (A2), and occlusal (A3) views. B.Left p4, UALVP 45148 in labial (B 1), lingual (B2), and anterior (B3) views. C.Incomplete left dentary having i1, p3–4, UALVP 18670 in labial (C1) and lingual (C2) views. D. Baiotomeusrusselli sp.nov., Cochrane 2 locality, Alberta.Holotype, left P4, UALVP 18787 in labial (D 1), lingual (D2), and occlusal (D3) views. E–H. Thryptacodonorthogonius, comb.nov., Cochrane 2 locality, Alberta. E.Left M1, UALVP 45099 in occlusal view. F.Holotype, left M2, UALVP 124 in occlusal view. G.Left M3, UALVP 24993 in occlusal view. H.Right m1, UALVP 42872 in labial (H 1), lingual (H2), and occlusal (H3) views. Scale bars 1 mm.