Ptilodus gnomus, S.Scott & C.Fox & P.Youzwyshyn, 2002

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 : 692-694

Ptilodus gnomus sp. nov.

Fig. 1A–C ; Table 1.

cf. Ectypodus hazeni ( Jepsen, 1940) ; Gazin 1956a: 13, pl. 1: 2.

Holotype: UALVP 45145 , isolated right P4, Fig. 1A 1 –A 3 .

Etymology: Gnomus, New Latin, meaning “diminutive fabled being” or “dwarf”, in reference to the diminutive size of the species.

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

Ageanddistribution: Earliest Tiffanian of Alberta (type locality); early Tiffanian of Wyoming ( Saddle locality, Fort Union Formation , Bison Basin , Fremont County [ Gazin 1956a]) .

Material.—UALVP 18742, 18743, 18746, 18748, 18749, 18751, 18763, 18765, 18770, 18771, 18776, 18779, 45133, 45134, P1s (total: 14); UALVP 18512, 18745, 18747, 18753, 18755, 18756, 18761, 18762, 18769, 18772, 18783, P2s (total: 11); UALVP 18740, 18741, 18744, 18750, 18752, 18754, 18757–18759, 18764, 18766–18768, 18773, 18774, 18780, 18782, 18784, P3s (total: 18); UALVP 18413, 18502, 18669, 18686, 18688, 18689, 18690–18697, 18699, 18700–18702, 18704, 18705, 45135–45144, P4s (total: 30); UALVP 18425, 18514, 18666, 18731, 18734, 18736, M1s (total: 6); UALVP 426, 18416, 18653, 18723–18730, M2s (total: 11); UALVP 130, 18670, 18671, 18673–18675, 18677–18681, 18683, 45146–45149, p4s (total: 16); UALVP 18707–18713, 18717, m1s (total: 8); UALVP 18484, 18486, 18714–18716, 18718– 18720, m2s (total: 8).

Diagnosis.—Smallest known species of Ptilodus , with p4 length approximately 51 percent less than p4, Ptilodus mediaevus Cope, 1881a , 28 percent less than p4, Ptilodus kummae Krause, 1977 , 15 percent less than p4, Ptilodus tsosiensis Sloan, 1981 , and 5 percent less than p4, Ptilodus fractus Dorr, 1952 .Differs further from P. fractus in p4 having at least one fewer serration.Differs from all other species of Ptilodus in having a teardrop−shaped P4, with the posterior margin narrowing abruptly to a point, and a higher−crowned, more evenly arcuate p4.

Description.—P1, P2, and P3: Except for their smaller size, P1–3 are virtually identical to teeth at these positions in other species of Ptilodus , particularly P. mediaevus .P3 bears four or five cusps, resembling the condition in P3 of P.mediaevus (mean = 4.39 cusps [ Krause 1982]).

P4: Cusp formula, (1–2)4–5:6–8:0.P4 of P. gnomus is of characteristic ptilodontid morphology, being low−crowned and bearing large, coarse cusps.In occlusal aspect, it is tear − drop−shaped: the anterior margin is wide and rounded and the posterior portion narrows to a point.The anterolabial lobe is poorly to moderately developed; occasionally, one or two tiny cuspules occur on the labial ridges of the first or second external cusp.The enamel covering the cusps of the medial and external row is wrinkled.The external row of cusps bends weakly towards the medial row anteriorly and posteriorly.Among all P4s at hand, the second external cusp is the largest in either row.The medial row of cusps runs anterolingualy−posterolabially and is slightly concave lingually. The cusps of the medial row are nearly equal to one another in size and height, except the first cusp, which is smaller on some specimens.

Upper molars: M1 cusp formula, 9:9:4–5; M2 cusp formula, 1:3:3.These teeth differ from M1 and M2 of other species of Ptilodus only in their smaller size (see Krause 1977, 1982).

i1: i1 is elongate, slender, and of characteristic ptilodontid morphology (see Krause 1977, 1982).

p3: p3 is small, peg−like, and has a bulbous, featureless, enamel−covered crown.

p4: Coronal morphology of p 4 in P. gnomus resembles that in other species of Ptilodus : the blade is high and in profile forms a symmetrical arc; its cutting edge has 10–11 serrations.The anterobasal concavity is conspicuous and nar − rows dorsally; by contrast, in P. fractus the anterobasal concavity is flat or rounded dorsally.Just dorsal to the postero − external shelf, the enamel is marked by several irregular, dorsoventrally directed ridges.The exodaenodont lobe is ro − bustly developed: its base is over two−thirds the length of the crown, and deep.

Lower molars: m1 cusp formula, 5–6:4; m2 cusp formula, 4:2.Except for their small size, the lower molars of P.gnomus are of characteristic Ptilodus morphology (see Krause 1977, 1982).

Remarks.—Although more similar in size to homologous teeth referable to P. tsosiensis and P. fractus , the upper and lower ultimate premolars of this new species most closely resemble those of P. mediaevus in overall morphology.The crown of p 4 in both taxa is high with a low serration number, and both have similar length: height ratios (2.6 in P.gnomus , 2.5 in P.mediaevus ).Additionally, the anterior premolars are more similar to those of P.mediaevus than those of other species of Ptilodus , differing only in their smaller size.

An isolated left p4 (UALVP 130) collected by R.L. Rutherford from Cochrane 2 in 1926 and identified by Russell as “ Ectypodus sp. ” pertains to P.gnomus , as does a single left M2 (UALVP 426) identified by Russell as “ Ectypodus cochranensis ?” in 1929.Neither specimen has been described nor previously reported in the literature.An isolated p4 (USNM 20878) identified by Gazin (1956a) as “cf. Ectypodushazeni ”, from the earliest Tiffanian Saddle locality of Wyoming, appears to be referable to P. gnomus , both in respect to lateral profile and serration number (GPY personal observation).The specimen is, however, slightly less (L = 5.0) than the mean length of p4s from Cochrane 2.

The recognition of P.gnomus sp.nov. at Cochrane 2 dem − onstrates the presence of three species of Ptilodus at the site ( Youzwyshyn 1988; Fox 1990).Two new and undescribed species of Ptilodus (P.“T” and P.“C”) are represented by well−preserved material at Cochrane 2; these were first recognized by Krause (1982) from fossils occurring elsewhere, and await naming and description by him.Three sympatric species of Ptilodus from a single locality also occur at Douglass Quarry (Fort Union Formation), Montana, and possibly at Shotgun locality (Fort Union Formation), Wyoming, both of which are considered earliest Tiffanian in age ( Archibald et al. 1987).

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.

Fig.3. A–C. Pararyctes rutherfordi sp.nov., Cochrane 2 locality, Alberta. A.Holotype, incomplete right maxilla with P4, M2–3, UALVP 45094 in occlusal view (A1, A2). B.Left M2, UALVP 25180 in occlusal view. C.Left m1 or m2, UALVP 45095 in labial (C 1), lingual (C2), and occlusal (C3) views. D–I. Bessoecetor septentrionalis comb.nov., Cochrane 2 locality, Alberta. D.Right P4, UALVP 25119 in occlusal view. E.Left M1, UALVP 28423 in occlusal view. F.Left M2, UALVP 28412 in occlusal view. G.Right p4, UALVP 28438 in labial (G 1), lingual (G2), and occlusal (G3) views. H. Left m2, UALVP 28414 in labial (H1), lingual (H2), and occlusal (H3) views. I.Holotype, left m3, UALVP 126 in labial (I 1), lingual (I2), and occlusal (I3) views.Scale bars 1 mm.