Mammacyon, Loomis, 1936

Hunt, Robert M., 2011, Evolution Of Large Carnivores During The Mid-Cenozoic Of North America: The Temnocyonine Radiation (Mammalia, Amphicyonidae), Bulletin of the American Museum of Natural History 2011 (358), pp. 1-153 : 72-74

cf. Mammacyon

Figure 32

Sunkahetanka pahinsintewakpa (in part): Macdonald, 1970: 60 (LACM 15908 was initially placed in the canid genus Sunkahetanka ).

REFERRED SPECIMEN: Fragment of mandible with left m1–2, posterior part of p4, and alveolus for m3 (LACM 15908), Sharps Formation, LACM loc. 1872, south of Wolf Ranch, NE1/4, sec. 15, T40N, R42W, Shannon County, South Dakota, collected by J. Harksen, July 1963.

DISTRIBUTION: Early Arikareean, Sharps Formation, Wounded Knee area, South Dakota.

COMMENTS: A small temnocyonine slightly larger than T. altigenis and with more derived m1–2; LACM 15908 has a more robust ml (length, 20.5 mm; width, 9.9 mm) relative to the T. altigenis hypodigm (m1 length, 17.4–19.5 mm; m1 width, 7.7–9.3 mm); m1 metaconid more reduced relative to T. altigenis (metaconid is a small vestigial cusp on posterolingual slope of protoconid); m2 not elongate (m1/m2 length ratio,,1.7) thus differs from M. obtusidens–M. ferocior (m1/ m2 length ratios, 1.57, 1.6); m2 protoconid and hypoconid in anteroposterior alignment and more centrally situated than these cusps in T. altigenis ; posterior base of p4 only slightly broadened, and posterior accessory cusp labially (not centrally) placed; small m3 present; thick, robust dentary below molars. Lacks an enamel swelling at base of m1 posterolingual to protoconid (present in Delotrochanter petersoni , a carnivore of about the same size).

DESCRIPTION: The fragmentary mandible of LACM 15908 is about the same depth (28 mm) below the m1 as Cope’s type of Temnocyon altigenis (29 mm), however, the jaw is thicker and the teeth are larger and more robust (mandibular depth below m2 is similar,, 31 mm in both species). Delotrochanter petersoni , smallest species of its genus, has a depth below m1 of, 33 mm; although of about the same size as LACM 15908, its m1 lacks the metaconid and differs in the form of the principal cusps and cingulum. Only the posterior part of p4 is preserved in LACM 15908, anchored in the jaw by the posterior root. This tooth has both a basal cusp and a labially placed posterior accessory cusp. The posterior border of p4 fits tightly against the base of the m1 paraconid. The posterior part of p4 is only somewhat widened, and could be transitional between the plesiomorphic laterally compressed p4 of T. altigenis and the posteriorly broad p4 of M. obtusidens .

The m1 is a small, well-worn carnassial. The placement of the three trigonid cusps is similar to their position in AMNH 6855 , Cope’s holotype of T. altigenis , however the metaconid is more reduced in LACM 15908 . The cusps are more worn than in AMNH 6855 , and apical wear is pronounced on all m1 cusps but not present on m2. The broadened talonid of m1 bears a large, rounded hypoconid, nearly centrally situated ; lingual to the hypoconid the enamel surface slopes downward to a narrow shelf lacking an entoconid. The labial m1 cingulum is sinuous, not straight as in D. petersoni .

The m2 is not elongated as in M. obtusi-dens–M. ferocior . The centrally placed, low, blunt protoconid is the largest cusp; a vestigial paraconid is indicated only by the slightly elevated anterolingual corner of the tooth; there is no metaconid. The hypoconid is lower than the protoconid, is placed directly behind it, and is the sole talonid cusp. The m2 paraconid is not as pronounced as in M. obtusidens–M. ferocior , hence LACM 15908 lacks the anteriorly extended m2 trigonid of the latter two species, formed by the paraconid and a protrusion of the anterolabial corner of the tooth. However, the m2 of LACM 15908 could structurally precede the more elongate m2 of M. obtusidens . A single alveolus for m3 is present.

The masseteric fossa terminates anteriorly under the posterior part of m2, and the forward inclination of m2–3 on the edge of the ascending ramus of the mandible is evident in LACM 15908 exactly as in Mammacyon obtusidens .

DISCUSSION: This mandibular fragment, originally identified as a canid, is the earliest evidence of the Temnocyoninae described from the Cenozoic of the North American midcontinent. LACM Locality 1872 occurs in the Sharps Formation with an estimated age of between 28.8 and 29.4 Ma ( Tedford et al., 1996, 2004; Macdonald, 1970). The mandible represents a temnocyonine more specialized in its dentition than fossils of Temnocyon altigenis from the John Day beds of Oregon. T. altigenis (UCMP 9999) from Logan Butte is certainly older than,28.9 Ma, the date obtained on an ash bed near the top of the John Day Formation at the butte, and more likely falls near 29.2–29.3 Ma if collected near dated tuffs at the base of Logan Butte. UCMP 9999 and LACM 15908 could be approximately contemporaneous species, although a somewhat younger age for LACM 15908 seems more probable.

The teeth of LACM 15908, despite small size, have a robust quality much like the larger durophagous temnocyonines of the later Arikareean. The absence of its anterior premolars makes referral to a genus difficult because the short, wide premolars of Delotrochanter are diagnostic relative to more plesiomorphic premolars of Mammacyon and Temnocyon . Lack of the elongate m2 that distinguishes the Mammacyon lineage prevents certain referral to that genus but the shorter m2 of LACM 15908 could represent the ancestral state. The labial m1 cingulum appears to be slightly sinuous, a trait present in Mammacyon , differing from the straight labial cingulum of Delotrochanter . Referral of LACM 15908 to Temnocyon altigenis is not appropriate because of the strong reduction of the m1 metaconid and the deep mandible and broader teeth, indicating a more derived carnivore. More complete material of the Sharps temnocyonine will be necessary to establish its identity.

Figures

Fig. 32. Tentatively referred mandibular fragment of Mammacyon (LACM 15908) with left m1–2 and partial p4, LACM loc. 1872, Sharps Formation, Shannon Co., South Dakota. A, labial view; B, occlusal stereopair. Note apical wear on m1 cusps.