Delotrochanter major, Hunt, 2011

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 : 85-88

Delotrochanter major , new species Figures 36 , 40–42

TYPE: F: AM 27561 , right and left lower canines ; partial p2–3, complete p4–m1, m2 trigonid, all of the left side; right p3–m1; an upper canine, right P2–4, maxilla fragment with right I1–3, left I1–2, and right P 1 in matrix, from the Anderson Ranch Formation, collected by Paul Miller in 1927 from the vicinity of Van Tassell, Niobrara County, Wyoming .

DISTRIBUTION: Latest Arikareean, Anderson Ranch Formation, near Van Tassell, Niobrara County, Wyoming.

ETYMOLOGY: From the Latin, major , ‘‘greater,’’ in reference to the large size of the terminal species of this genus.

DIAGNOSIS: Distinguished from D. petersoni by larger size (m1 length: D. petersoni , 22.3 mm; D. major , 28.0 mm). All D. major teeth are larger and more robust than the equivalent teeth of its predecessor D. oryktes (tables 2, 3). Differs from Temnocyon by loss of the m1 metaconid and by short, posteriorly wide p2–3 and P2–3; from Mammacyon by a posterior accessory cusp on p3, a large centrally placed posterior accessory cusp on p4, and by a smaller P4 with lingually abbreviated protocone region; from Rudiocyon by the form of p4.

REFERRED SPECIMENS: None.

DESCRIPTION: Neither mandible of D. major (F:AM 27561) survived. The dentition is embedded in series in sediment without mandibular bone supporting the teeth. Thus only D. oryktes (ACM 4804) and D. petersoni demonstrate the form of the mandible in Delotrochanter .

In F:AM 27561, the left lower canine measures 36.6 mm in height from the slightly worn apex to the labial enamel-dentine junction (estimated unworn height,, 40 mm); length and width at the base of the enamel are respectively 18.4 mm and 12.8 mm. The posterolabial surface is grooved by the upper canine; a second wear facet made by I3 occurs at the base of the anterolingual face. In other respects the tooth is similar to ACM 4804.

The p1 is not preserved, and only the main cusp of the left p2 remains; this indicates a tall principal cusp—there are no evident differences from p2 of ACM 4804.

The p3 measures 17.7 mm in length, 10.2 mm in greatest width, and is nearly identical to ACM 4804 except in its larger size and greater posterior width. A centrally placed posterior accessory cusp is present.

The p4 measures 21.6 mm in length, 9.2 mm in anterior width, 12.3 mm in posterior width; its posterior accessory cusp is very large, centrally placed, and surrounded by a welldefined cingulum, which becomes less conspicuous anteriorly on both lingual and labial sides. The heel of p4 is much wider than in ACM 4804 as is true for p3.

The m1 is similar in all respects to that of ACM 4804 but is more massive and robust, measuring 28.0 mm in length, 14.1 mm in greatest width. Although these lower carnassials were undoubtedly capable of limited shear (vertical wear facets occur on the paraconid-protoconid blade), the premolars, m2–3, and m1 functioned primarily as crushing instruments. The m1 hypoconid and the protoconid of m2 are blunt, low cusps, and in this species could have served no role other than crushing.

The m2 is represented only by the trigonid. It is similar to the m2 trigonid of D. oryktes but is more massive, having a very large, blunt protoconid and small vestigial paraconid with a strong labial cingulum; there is no metaconid. Trigonid width is 10.6 mm. Based on similarity of the cheek teeth in F:AM 27561 and ACM 4804, the complete m2 of D. major was a larger version of the m 2 in D. oryktes .

The m3 was not preserved but would have been present.

I1–I2 are of equal size and similar cusp form. Both have a vertical anterior face and a curvilinear posterior surface, and both possess prominent auxiliary cusps (Nebenzacken) on their lateral margins as in D. oryktes . I2 measures 9.9 mm in length, 5.6 mm in width; I1 measures 9.1 mm in length, 5 mm in width. The large I3 measures 22.8 mm in height, 13.3 mm in length, and about 10 mm in width at the enamel base. There is a wear groove on the posterolabial face of I3 made by the upper canine.

P1 has the same subconical form as in ACM 4804 and measures 9.4 mm in length by 6.9 mm in width.

P2 is much larger than P1; it is tall, subtriangular in labial view, with nearvertical anterior and more inclined posterior faces, and measures 17.0 by 9.0 mm. The heel of P2 is broad; the thin enamel ridge on the posterior slope ends in a minute cingular cuspule at the posterolingual corner.

P3 is tall, triangular in lateral view, and posteriorly broad, measuring 18.7 mm in length, 11.9 mm in posterior width. The posterior face has a small, labially situated, posterior accessory cusp. P3 has fine enamel ridges on its anterior and posterior faces; the anterior ridge terminates at the anterolingual margin of these teeth; the posterior ridge at the posterolabial corner. The expansion of the heel of both P2 and P3 is more pronounced than in D. oryktes .

P4 measures 23.5 mm in length, 19.8 mm in width across the protocone, and is similar in form to the P4 of ACM 4804 but is larger and slightly more robust. The metastylar blade is blunt as is the protocone; both are essentially unworn, and the slightly worn paracone shows only a flat apical wear facet. Vertical shear, although possible, appears to have been secondary to a crushing action.

The upper molars of D. major were not recovered, but based on the remaining teeth they would be similar in form yet somewhat larger than M1–2 of D. oryktes .

DISCUSSION: F:AM 27561 differs from D. oryktes in larger size and in the greater accentuation of many defining features of the cheek teeth. Not only are its teeth larger but cingula are more pronounced, the cusps more massive, and p3–4 and P2–3 are posteriorly broadened. The defining dental traits of Delotrochanter are evident and indicate that D. major is clearly derived from D. oryktes . Paul Miller, the collector, reported that F:AM 27561 came from the ‘‘vicinity of Van Tassell,’’ Wyoming. Near the town of Van Tassell only the Harrison Formation and the Anderson Ranch beds outcrop in the area where Miller worked. Fortunately, sediment adhering to F:AM 27561 shows the pale reddish-brown patina of the Anderson Ranch sandstones. The advanced features of its dentition indicate that Miller may have obtained F:AM 27561 stratigraphically high in the Anderson Ranch beds in the stratotype area of the formation in the Niobrara Canyon (see Hunt, 2002b: 35–39), which is in proximity to the town of Van Tassell. F:AM 27561 is of latest Arikareean age and represents the youngest occurrence of Delotrochanter . More than 80 years of exploration of Anderson Ranch Formation outcrops in northwest Nebraska and Wyoming have failed to yield additional remains of this rare temnocyonine since Miller’s fortunate discovery.

The stratigraphic levels that produced D. oryktes are well documented, and the horizon that yielded D. major can be reliably estimated. The stage of evolution of these two species of Delotrochanter , which occur in late to latest Arikareeean time, can be contrasted with that of Mammacyon . The occurrence of the terminal and largest species, M. ferocior , preceded the earliest occurrence of D. oryktes in the Harrison Formation at Stenomylus Quarry. Thus , the Mammacyon lineage reached its apex at a time before 23 Ma, when the coeval species of Delotrochanter must have been a much smaller carnivore in the same geographic region.

Figures

Fig. 36. Occusal stereopairs of the holotype mandibles of (A, C) Delotrochanter oryktes (ACM 4804) and (B, D) Delotrochanter major (F:AM 27561). D. oryktes, right p4–m2, alveoli of p1–3; D. major, left p2– m2. Note absence of m1 metaconids and relative anterior and posterior alveolar diameters of p2 in D. oryktes. The p4–m2 are crushing teeth in both species.

Fig. 40. Holotype dentition of Delotrochanter major (F:AM 27561), left p2–m2 (p2, p3, and m2 damaged), Anderson Ranch Formation, near Van Tassell, Niobrara Co., Wyoming. A, labial and B, lingual views.

Fig. 41. Holotype dentition of Delotrochanter major (F:AM 27561), right p3–m1, Anderson Ranch Formation, near Van Tassell, Niobrara Co., Wyoming. A, labial and B, lingual views. Note large p4 relative to the carnassial in this terminal species of Delotrochanter.

Fig. 42. Holotype maxillary teeth of Delotrochanter major (F:AM 27561). A, lingual view of right P2–4. B, stereopair of right P3–4; enamel was etched by plant roots.