Temnocyon percussor Cook, 1909

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 : 42-45

Temnocyon percussor Cook, 1909 Figure 15

Temnocyon percussor Cook, 1909: 266 , 271, fig. 3.

TYPE: AMNH 81005, associated right and left partial mandibles, left p3–m2, right p1, p4, m2, roots or alveoli of p2–3, m1, and a partial m3 alveolus, a doubtfully associated right upper canine, comprising Cook’s (1909) type of Temnocyon percussor , found ‘‘about one-half mile west of Agate, Nebraska, in the Lower Harrison beds ….’’, Sioux County, Nebraska (see Discussion for additional information on site of collection). Cook’s figure of the type is a composite based on both mandibles, although this is not plainly stated in his publication.

DISTRIBUTION: Late Arikareean, upper Arikaree Group, Sioux County, Nebraska.

DIAGNOSIS: Differs from T. altigenis and T. subferox in larger size and from T. macrogenys in much smaller size. Distinguished from T. ferox by taller differently proportioned p3–4 and by dimensions of m1– 2 and M1 that together exceed the size attributable to a T. ferox population (tables 2, 3). T. percussor differs from T. fingeruti in having a taller more robust, somewhat longer p3–4, shorter m1–2, and no m2 metaconid. M1 lingual cingulum better developed in T. percussor relative to T. fingeruti .

REFERRED SPECIMENS: (1) AMNH 81054 , right m1, American Museum–Cook Quarry ( Hunt, 1972), 2 mi north of the Agate Spring Quarries, basal Anderson Ranch Fm., Sioux County, Nebraska, H. Cook, August 1909; (2) AMNH 81047 , right M1, American Museum–Cook Quarry, basal Anderson Ranch Fm., Sioux County, Nebraska, H. Cook, July 1908 .

DESCRIPTION: AMNH 81005— Cook (1909: 266) only briefly described this individual but presented an accurate illustration of the dentition. This illustration posterior to p3 is based on the left mandibular fragment, and anterior to p3 on the anterior part of the right mandible. It is not possible to estimate jaw depth since the lower border of the mandible has been broken away along the length of the horizontal ramus. However, an estimate of mandibular length, based on length of toothrow (c–m 2, 110.9 mm), compares with the mandibular length of Eyerman’s type of T. ferox (c–m2,, 102 mm), an observation complemented by the similar premolar spacing in both animals.

The p1 measures 8.4 mm in length, 5.0 mm in width. It is a small, single-rooted peglike tooth situated on the jaw margin anterointernal to the anterior root of p2. The main cusp is anteriorly inclined, and there are no accessory cusps.

The p2 is not preserved in either mandible; the alveolar measurements of length and width are 14.8 and 7.2 mm. The anterior root is more labially placed than the posterior root, the tooth being directed outward, a trait also seen in several other temnocyonines.

The p3 measures 15.9 mm in length, 7.8 mm in width. There are no posterior accessory or basal cingulum cusps. The tooth in lateral view is nearly an equilateral triangle, the posterior slope being only slightly longer than the anterior slope. Weak enamel ridges run down the anterior and posterior slopes; there is slight development of a lingual and posterior cingulum that is even less well defined labially.

The p4 measures 19.7 mm in length, 9.0 mm in width. It has the same form as p3, only larger and taller, with the addition of a prominent posterior accessory cusp and a shelflike heel at the rear of the tooth. The posterior accessory cusp is situated about half the distance down the posterior slope and is labial in position. A weak cingulum is present on the lingual and posterolabial faces of p4. A small cingular cusp also occurs on the shelflike heel of p4 where the posterior enamel ridge descends to meet it.

The m1 measures 23.5 mm in length, 10.5 mm in width. Trigonid length, 16.2 mm; talonid length, 7.3 mm. The trigonid appears somewhat inflated: paraconid and metaconid are of about equal height, the taller protoconid elevated only, 1.5 mm above the main p4 cusp. The metaconid is present, reduced, and situated on the posterolingual corner of the massive protoconid about half the distance to its base. The hypoconid is the only talonid cusp: it is centrally positioned, and occupies nearly all the talonid. A cingulum is developed on labial and lingual margins of the talonid. On the posterior slope of the hypoconid there is a tiny cusp apparently derived from the cingulum.

The m2 measures 13.9 mm in length, 9.0 mm in width. The form of the tooth is most like m 2 in YPM-PU 10787 ( T. ferox ) and differs from the elongate m2 of Mammacyon. The prominent m2 protoconid is the same height as the m1 hypoconid. These two cusps, aligned in series with the much smaller hypoconid of m2, are low, blunt, crushing instruments. A low paraconid occupies the anterolingual corner of m2. A low ridge descends the protoconid to the lingual margin of m2. The m2 talonid is occupied solely by the low hypoconid placed directly behind the much larger protoconid. There is no entoconid.

The isolated upper canine measures 12.5 mm in labiolingual width at the enamel base; anteroposterior length, 17.2 mm; height, 61.2 mm. The tooth is recurved and exhibits a flat wear surface,, 25 mm in length, 6 mm in width, on the anterointernal face near the tip. The tooth is badly cracked, probably from postmortem drying prior to burial.

AMNH 81054—This isolated right m1 compares well with Cook’s type m1 (AMNH 81005) but exhibits more wear. Although a vertical shear facet is evident on the labial face of the protoconid-paraconid, the tips of the protoconid, paraconid, and hypoconid are blunted by apical wear, suggesting mastication of hard material. This tooth measures 24.2 mm in length, 10.8 mm in width. The m1 talonid of AMNH 81054 is nearly identical to that of the holotype m1, including the cingular swelling on the posterior slope of the hypoconid.

Because they retain the m1 metaconid, T. percussor carnassials are easily confused with those of Daphoenodon superbus , the amphicyonid that occurs with T. percussor in the American Museum–Cook Quarry. In T. percussor , however, the hypoconid fills almost the entire talonid, but in Daphoenodon the hypoconid occupies only the labial talonid surface, and does not encroach on a narrow lingual talonid shelf. Isolated teeth in the American Museum–Cook Quarry demonstrate the presence of three beardogs ( Temnocyon percussor , Daphoenodon superbus , Ysengrinia americana ) at the site.

AMNH 81047—This isolated M1 was also found in the American Museum–Cook Quarry. Although not associated with other material of T. percussor , it has the form and predicted cusp morphology that correspond to mandibular teeth of the holotype. The tooth is an enlarged version of the M1 of T. altigenis and T. ferox , measuring 20.9 mm in length, 27.9 mm in width; hence it lacks the extremely swollen lingual cingulum found in Mammacyon and Delotrochanter . The paracone and metacone are conspicuously elevated above the lingual half of the tooth, forming a vertical shear surface. The paracone is somewhat larger than the metacone; both cusps are bordered by a sharply defined labial cingulum continuous with a prominent parastyle. The protocone is centrally situated on a flat enamel platform, and is less knoblike, more crescentic in form than in Mammacyon . A long preprotocrista runs from the protocone to the anterior cingulum, and a short low postprotocrista extends from protocone to posterior cingulum, similar in this respect to M1 of T. ferox .

The plesiomorphic form of the T. percussor M1 (AMNH 81047) is unique in the late Arikareean; the older Mammacyon lineage and contemporary Delotrochanter have more derived upper molars in which the protocone region forming the lingual half of M1 is more expanded, primarily due to the more swollen lingual cingulum. In AMNH 81047 the anteroposterior length of the protocone region is quite narrow relative to the length of the labial margin of the tooth (table 5), and its anterior cingulum is plesiomorphic (as in T. altigenis ) in contrast to the enlarged cingulum of Mammacyon . The narrow M1 protocone region and the lack of a swollen anterior cingulum indicate the presence of a metaconid on the mandibular carnassial, and so AMNH 81047 cannot be referred to Mammacyon or Delotrochanter in which the m1 lacks a metaconid.

DISCUSSION: Temnocyon percussor is restricted to upper Arikaree Group sediments in western Nebraska, and has never been found outside of the Niobrara River valley near Agate in Sioux County. The species is more advanced in dental traits than the John Day holotype of T. ferox : T. percussor has broader, more robust teeth, taller premolars (p3–4), and a much larger yet still plesiomorphic M1. Although the two species are similar in dental features, the Great Plains T. percussor has a slightly longer m2 than found in the John Day T. ferox (ratio E/F is, 1.7 in AMNH 81005,, 1.8 in YPM-PU 10787, table 6). Eyerman’s type (YPM-PU 10787) of T. ferox represents a smaller individual than T. percussor and a less evolved dental stage, suggesting an older age for the John Day carnivore.

Cook (1909) was aware that the holotype, found by him in 1906, was dentally similar to the John Day T. ferox yet represented a larger carnivore with different p3–4 proportions. The M1 later found by Cook in 1908 is reasonably referred to T. percussor , its large size and form in agreement with the m1–2 of the holotype mandible.

If Cook (1909) correctly attributed the holotype of Temnocyon percussor (AMNH 81005) to the Syndyoceras level or ‘‘quarry’’ one-half mile west of Agate, then the holotype material comes from the Harrison Formation about 40–50 ft (12–15 m) below the upper contact of the formation with the Anderson Ranch beds. The isolated molars (AMNH 81054, 81047) come from the American Museum–Cook Quarry in the basal Anderson Ranch Fm., about 50 ft stratigraphically higher than Cook’s AMNH 81005. The Syndyoceras level is late Arikareean in age and occurs in proximity to the Agate Ash, initially 21.3 Ma (KA 481, Evernden et al., 1964), now more recently dated by the 40Ar/39Ar method at,22.9 Ma ( Izett and Obradovich, 2001). The similarity of the teeth of T. percussor and T. ferox suggests a relationship: although the age of T. ferox is uncertain (see Age and Correlation), a date older than 22.9 Ma is supported by its less advanced dentition relative to the more robust teeth of T. percussor .

The presence of T. percussor (with m1 metaconid) in western Nebraska in both the Harrison Formation and basal Anderson Ranch beds in the Agate area, and the occurrence of large Delotrochanter oryktes (without m1 metaconid) in these same two formations, also in the Agate area, demonstrates that the temnocyonine dentition was evolving at different rates in these two contemporary lineages. This had been difficult to prove given the fact that two or more temnocyonine species rarely occur in proximity in the same rock unit. However, based on fossils in this study, two temnocyonine genera had already lost the m1 metaconid prior to the late Arikareean ( Mammacyon, LACM 9194; Delotrochanter, CM 1603 ); hence these represent more derived dentitions relative to a geologically younger Temnocyon percussor that retains its m1 metaconid and plesiomorphic M1 into the late Arikareean interval. If T. percussor directly precedes T. macrogenys , which is probable, then Temnocyon maintains this plesiomorphic dental pattern in the Great Plains until its extinction at the end of the Arikareean.

Figures

Fig. 15. Temnocyon percussor Cook, Niobrara River valley, Sioux Co., Nebraska. Holotype (AMNH 81005) left partial mandible with p3–m2 (A, B) and right mandibular fragment with p1, p4, and m2 (C, D) from 0.5 mi west of Agate Ranch headquarters, Harrison Formation, Sioux Co., Nebraska. A, D, labial and B, C, lingual views. Referred right m1 (E, AMNH 81054) and right M1 (F, AMNH 81047) from AMNH–Cook Quarry, basal Anderson Ranch Formation. Cook’s 1909 drawing of the holotype is a composite of the two mandibular fragments (A to D) illustrated here, both from the same individual.