Phenacodus, Cope, 1873

Lofgren, Donald, Mckenna, Malcolm, Honey, James, Nydam, Randall, Wheaton, Christine, Yokote, Bryan, Henn, Lexington, Hanlon, Whitney, Manning, Stephen & Mcgee, Carter, 2014, New records of eutherian mammals from the Goler Formation (Tiffanian, Paleocene) of California and their biostratigraphic and paleobiogeographic implications, American Museum Novitates 2014 (3797), pp. 1-60 : 30-35

publication ID	https://doi.org/ 10.1206/3797.1
persistent identifier	https://treatment.plazi.org/id/587387C9-1425-514F-FE38-1803FBE1F901
treatment provided by	Carolina
scientific name	Phenacodus
status

Treatment

Phenacodus cf. P. bisonensis Gazin, 1956

HOLOTYPE: USNM 20564 View Materials , right maxilla with P4–M2, and a left maxilla with M1 and M2 thought to be from the same individual from the Bison Basin, Fort Union Formation , Wyoming.

REFERRED SPECIMENS: UCMP 69122, parts of both dentaries with right m2 and left p4–m2 from UCMP locality V65710 View Materials , member 4b(?); RAM 9659, RM1 from RAM locality V98012 View Materials , member 4b ( fig. 11C View FIGURE 11 ); RAM 6722, right M1 ( fig. 11D View FIGURE 11 ), RAM 7205, right dP4 ( fig. 11I View FIGURE 11 ), RAM 7245, maxilla fragment with left dP3–4 ( fig. 11H View FIGURE 11 ), RAM 7208, left dP4, RAM 9725, right dP4, RAM 6721, damaged right m2, RAM 6723, right dentary fragment with m2–3 ( fig. 9E View FIGURE 9 ) from RAM locality V200001, member 4a; RAM 7248, left maxilla fragment with P4–M1 and damaged M2 from RAM locality V200304, member 4a ( fig. 11B View FIGURE 11 ); RAM 9019, right m3 from RAM locality

V200508, member 4a; RAM 9022, left dentary with damaged dp4 and erupting m1, RAM 9672, left maxillary with P3-4 ( fig. 11A View FIGURE 11 ) from RAM locality V200510, member 4a; RAM 9025, right m1, RAM 9021, right M2 ( fig. 11F View FIGURE 11 ), RAM 9046, left dentary fragment m1–3 ( fig. 9D View FIGURE 9 ) from RAM locality V200603, member 4a; RAM 9020, associated left M2 and M3 ( fig. 11G View FIGURE 11 ); RAM 9024, left dentary with m2–3 ( fig. 9F View FIGURE 9 ), RAM 9023, right dentary with p3–m2 and anterior portion of m3 trigonid ( fig. 9A–C View FIGURE 9 ) from RAM locality V200612, member 4a; RAM 9045, left dp4 from RAM locality V200704, member 4b ( fig. 9G View FIGURE 9 ); RAM 10290, right dentary fragment with m1 and m2 trigonid and partial talonid, RAM 10291, left m3, RAM 10292 left dentary with c–m3 and right dentary with i1–2, and c–m1 from RAM locality V200802, member 4a ( fig. 10A–C View FIGURE 10 ); RAM 15000 right maxilla fragment with dP4–M2 from RAM locality V201122, member 4a ( fig. 11E View FIGURE 11 ).

DESCRIPTION AND DISCUSSION: This medium-sized phenacodontid, referred to Phenacodus cf. P. bisonensis , is the most common mammalian species found by surface collecting in the Goler Formation. Included in P. cf. P. bisonensis is UCMP 69122, a concretion with parts of both dentaries previously referred to Tetraclaenodon puercanensis ( West, 1970, 1976: pl. 120, 1–2). Found by a hiker in 1954, exact provenance of UCMP 69122 is uncertain, but based on general location information ( West, 1970), it likely came from member 4b. In the past two decades, multiple specimens of the upper dentition of Phenacodus were recovered from member 4a and member 4b and these specimens have well-developed molar mesostyles and large P4 metacones, features that distinguish Phenacodus from Tetraclaenodon ( Thewissen, 1990) . Lower dentitions of Phenacodus and Tetraclaenodon are often difficult to distinguish ( Thewissen, 1990), but if all upper dentition specimens from member 4a and member 4b are Phenacodus, UCMP 69122 almost certainly represents Phenacodus as well.

Features of the lower dentition of Phenacodus cf. P. bisonensis are shown in RAM 9023 ( fig. 9A–C View FIGURE 9 ), RAM 9046 ( fig. 9D View FIGURE 9 ), RAM 6723 ( fig. 9E View FIGURE 9 ), RAM 9024 ( fig. 9F View FIGURE 9 ) and RAM 10292, an extraordinary complete specimen for the sparsely fossiliferous Goler Formation, with the left and right dentaries of a single individual within a concretion ( fig. 10A–C View FIGURE 10 ). Based on RAM 10292, crowns of the canines and incisors are missing due to breakage, but these teeth were single rooted. Smaller than i3, i2 probably had a circular-shaped crown based on its root, while the crown of i3 was anteroposteriorly elongate, based on the same line of evidence. Small diastemas separate i2 and i3, and i3 from the much larger canine, which has a circular root in cross section. The p1 is single rooted and its crown is elongate anteroposteriorly, exhibiting a single tall cusp and a miniscule posterior basal cusp on a weakly developed posterior cingulid. In contrast, the posterior basal cusp and posterior cingulid are well developed on the tworooted p2. Diastemas are small between c1 and p1, and p1 and p2, and minute between p3 and p4. The p3 lacks a metaconid, but has a large protoconid and small paraconid positioned low and directly anterior to the protoconid, and a very small talonid basin. The p4 metaconid is smaller than the protoconid, and a paralophid connects the small and low paraconid to the protoconid. The talonid of p4 has a relatively narrow basin and a large hypoconid and small entoconid. The m1–m3 of P. cf. P. bisonensis are of basic phenacodontid morphology, as they lack paraconids, but have well-developed metaconids, protoconids, and hypoconids, and smaller entoconids and hypoconulids, and wide talonid basins. The m3 of P. cf. P. bisonensis is significantly smaller than the m2, but it has a larger hypoconulid.

The upper dentition of Phenacodus cf. P. bisonensis consists of two M1s (RAM 9659, RAM 6722), an M2 (RAM 9021), maxillary fragments with P4–M2 (RAM 7248) and dP4–M2 (RAM 15000), an associated M2–3 (RAM 9020), and a maxillary fragment with heavily worn P3–4 (RAM 6972). The P3 of P. cf. P. bisonensis has a large paracone and protocone and smaller metacone and the P4 has a large paracone, metacone, and protocone, and a small parastyle ( fig. 11A–B View FIGURE 11 ). All upper molars of P. cf. P. bisonensis have well-developed mesostyles and parastyles ( fig. 11B–G View FIGURE 11 ). The single M3 of P. cf. P. bisonensis (RAM 9020) has a small hypocone ( fig. 11G View FIGURE 11 ).

The dentition of this midsized species of Phenacodus from the Goler Formation is too small to be P. grangeri , P. magnus , or P. intermedius , and too large to be P. matthewi or Ectocion ( tables 10–11) (see Thewissen, 1990: tables A4–A 14, A18–A20, A26–A29), and Ectocion lacks a hypocone on M3 ( Thewissen 1990). Direct comparison of specimens from member 4a and member 4b to the Bison Basin sample of Phenacodus bisonensi s at the USNM, indicates they are more similar to P. bisonensis than any other phenacodontid. Phenacodus vortmani and P. bisonensis are both similar in size to the Goler specimens ( table 10) ( Thewissen, 1990: tables A15–A17, A21–A25), but the p4 of the Goler sample is more aligned with P. bisonensis . The entoconid is present in 59% of p4s of P. bisonensis , but is absent in all Tiffanian and early Clarforkian p4s of P.vortmani , and P. bisonensis has a relatively narrower talonid basin than P.vortmani ( Thewissen, 1990) . Goler p4s from member 4a and member 4b have entoconids and relatively narrow talonid basins ( figs. 9A–C View FIGURE 9 , 10A–C View FIGURE 10 ), which indicates they are not P. vortmani . Goler lower molars are also very similar to those of P. bisonensis from the Bison Basin.

One notable difference between Goler p4s and those from the Bison Basin is paraconids of P. bisonensis are slightly larger and more anteriorly positioned relative to the protoconid in comparison to Goler p4s. The only consistent difference between Goler upper molars and those of P. bisonensis is the mesostyle is usually shifted posteriorly, so part of it is positioned labial to the metacone in P. cf. P. bisonensis ( fig. 11B–G View FIGURE 11 ), and in P. bisonensis the mesostyle is positioned close to the metacone, but not partly labial to it. Also, the postparacrista and premetacrista usually intersect with the mesostyle at its lingual base in P. cf. P. bisonensis ( fig. 11C–G View FIGURE 11 ), while the postparacrista and premetacrista intersect with the mesostyle at its apex in P. bisonensis . Because of these differences, the Goler specimens are referred to Phenacodus cf. P. bisonensis .

Seven specimens of the deciduous dentition of Phenacodus were also recovered from the Goler Formation and RAM 15000, a maxilla fragment with dP4–M2 ( fig. 11E View FIGURE 11 ), is particularly informative as it preserves part of the permanent and deciduous dentition. The M1–M2 of RAM 15000 are identical to M1s and M2s referred to P. cf. P. bisonensis , indicating that the dP4 of RAM 15000 must also be this taxon, which facilitated identification of four isolated dP4s (RAM 7245; fig. 11H View FIGURE 11 , RAM 7205; fig. 11I View FIGURE 11 , RAM 7208, and RAM 9725) as representing P. cf. P. bisonensis . Two dp4s (RAM 9022 and RAM 9045) also represent P. cf. P. bisonensis as the specimens are closely comparable to dp4s of P. bisonensis described by West (1971) and RAM 9022 is from a site that yielded a specimen of the permanent dentition (RAM 9672).

West (1971) described two specimens of the deciduous dentition of P. bisonensis , a dp4 from Bison Basin (USNM 26325; Fort Union Formation, Wyoming) and a dP4 from Douglas Quarry (PU 14634b; Lebo Formation, Montana). Estimates of their dimensions based on graph plots by West (1971: figs. 14, 15) are comparable in size to deciduous teeth of P. cf. P. bisonensis ( tables 10–11). RAM 9022 is a partial dentary with dp4 that is missing its crown, so only its approximate size is known ( table 10). RAM 9045 is an unworn dp4 that is anteroposteriorly elongate, with a metaconid and protoconid of equal size, a lingually positioned and smaller paraconid, a miniscule cusp labial to the paraconid, and a well-developed and cuspidate cristid obliqua ( fig. 9G View FIGURE 9 ). The talonid basin of RAM 9045 is large and the talonid has a large entoconid and hypoconid and smaller hypoconulid. RAM 9045 is most similar in morphology and size to USNM 26325 ( table 10), the single dp4 referred to P. bisonensis by West (1971).

RAM 7245, a maxillary fragment with dP3–dP4, represents the only known dP3 of P. cf. P. bisonensis ( fig. 11H View FIGURE 11 ). Triangular in shape, dP3 has a large paracone and protocone, smaller metacone, and a parastyle, paraconule and metastyle, but lacks a metaconule. A dP3 of P.bisonensis was not identified ( West, 1971), so the dP3 of RAM 7245 serves as a close proxy. Five dP4s of P. cf. P. bisonensis all have well-developed paraconules and hypocones ( fig. 11E, H–I View FIGURE 11 ). The single dP4 (PU 14634b) referred to P. bisonensis does not have a paraconule and hypocone, but these cusps are present on deciduous dP4s of the similar sized phenacodontid, P.vortmani ( West, 1971) . Goler dP4s are similar in approximate dimensions to dP4s of P. bisonensis and P. vortmani , but Goler dP4s are wider than long, unlike both P. bisonensis and P.vortmani ( table 11). Also, the mesostyle, and postmetacrista and premetacrista in Goler dP4s are positioned as they are in molars of P. cf. P. bisonensis , while these features in the only known dP4 of P.bisonensis (PU 14634b) have the same orientation as molars of P. bisonensis . These differences are further evidence that this mid-sized Goler phenacodontid referred to Phenacodus cf. P. bisonensis is not conspecific with P. bisonensis , although it resembles it in many ways.