Temnocyon macrogenys, 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 : 51-54

Temnocyon macrogenys , new species Figures 20 , 21

TYPE: F: AM 54139 , left mandible with canine alveolus, p1–2, broken roots of pathologic p3, complete p4–m3, with most of the ascending ramus, articular condyle, and angular process preserved ; also two partial metapodials and a fragmentary astragalus, from the Anderson Ranch Fm., ‘‘ High Daemonelix zone,’’ 2- Mile District of Charles Falkenbach, near Guernsey, Platte County, Wyoming, collected by C. Falkenbach, 1939 .

DISTRIBUTION: Latest Arikareean, Anderson Ranch Fm., Platte County, Wyoming.

ETYMOLOGY: From the Greek, macro, ‘‘long,’’ and genys, ‘‘jaw,’’ in reference to the large, elongate mandible of the terminal species of the Temnocyon lineage.

DIAGNOSIS: Largest species of Temnocyon (basilar length,, 30 cm, table 7), and the only large temnocyonine in which the plesiomorphic cusp pattern of the lower teeth is retained: m1 shear maintained; metaconids present on m1–2; m1/m2 ratio, 1.62, the lowest among Temnocyon species (table 6). Depth of mandible below m1,, 50 mm; below p2, 46 mm. Mandibular dental measurements highest values among species of Temnocyon : length of toothrow, p1–m 3, 135 mm; lengths in mm of p2, 16.6; p4, 22.1; m1, 28.2; m2, 17.4 (table 2).

REFERRED SPECIMENS: None.

DESCRIPTION: The nearly complete mandible lacks only part of the ventral border of the horizontal ramus and small sections from the labial and lingual sides. From the anterior limit of the jaw to the posterior face of the articular condyle is, 26 cm; this is a large carnivore with estimated basilar skull length of 30 cm, thus the largest known temnocyonine. The articular condyle and angular process are in close proximity: from the center of the condyle to the process is 24 mm. The distance from the tip of the coronoid process to the articular condyle is 77 mm; from tip of coronoid to base of angular process is 106 mm. The coronoid process is gently recurved posteriorly, with a shallow depression on its lingual face, and a moderately deep masseteric fossa on the buccal face. The articular condyle, mandibular foramen, and coronoid and angular processes are configured as in Canis lupus . Except for the much greater size and mandibular depth of the beardog, the form of the jaw is similar to that of the wolf. As in other temnocyonines, the anterior border of the ascending ramus is gently inclined so that m2–3 are slightly elevated and tilted forward on the edge of the ramus. Depth of the mandible below p2 is 46 mm, beneath m1,50 mm, thus differing from Delotrochanter oryktes (ACM 4804) for which the same measurements, respectively, are 46 and 42 mm, and for Mammacyon obtusidens (LACM 9194), 38 and 35 mm. The mandibular depth beneath m1 for T. macrogenys is nearly twice that of an average wolf.

The mandibular symphysis was united by strong binding ligaments; there is no evidence of symphyseal fusion. The symphysis is widest near its posterior edge. The greatest anteroposterior length is, 56 mm; greatest dorsoventral height, 37 mm. The upper part of the symphysis (a rectangular area 22 mm in length, 11 mm in width) is smoother and less rugose than the ventral portion and represents the attachment of a compressible fibrocartilage pad like that described by Scapino (1981; Hunt, 2009). This indicates a flexible mandibular symphysis in T. macrogenys that facilitates close registration of the carnassials during the bite, as in the wolf (fig. 50F).

None of the lower incisors are preserved, however the extremely narrow space between the symphysis and canine alveolus shows that they would have been very small teeth.

The large canine alveolus measures 26.5 mm in length, 20.7 mm in width, with the long axis of the ellipse directed anteriorly and somewhat laterally. There is no intervening space between the canine and p1, their alveolar borders nearly in contact.

Premolars (p2–4) are not crowded and are separated by 2–3 mm diastemata; they are not shortened or posteriorly widened as in Delotrochanter but are relatively narrow, tall yet robust, thus representing a scaled-up version of the premolars of T. percussor .

The single-rooted p1 measures 10.6 mm in length, 7.3 mm in width. It has the form of an oblate cone, anteriorly inclined, whose posterior face has been slightly depressed. There are no accessory or basal cusps. A fine enamel ridge runs from the principal cusp down the posterior slope to the cingulum.

The p2 measures 16.6 mm in length, 8.5 mm in width. In lateral view it approximates an equilateral triangle; there are thin anterior and posterior enamel ridges but no posterior accessory cusp. There is a moderately developed heel with a very small, low basal cusp.

The p3 is represented only by the posterior basal part of the tooth. The tooth was either abnormally triple-rooted, or there was a supernumerary single-rooted premolar inserted in front of a normal p3, this latter alternative more probable. The three alveoli occur in anteroposterior sequence, with a total alveolar length of 24.9 mm. This is longer than p4. If a normal p3 was present, its length based on alveoli would be, 18 mm.

The p4 measures 22.1 mm in length, 10.4 mm in posterior width, 8.5 mm in anterior width. There is a prominent, labially placed posterior accessory cusp. The posterior border of the tooth is squared and produced as a shelf with a small basal cusp situated below the posterior accessory cusp. Height of p4 is much greater than the m1 paraconid, and I suspect that were p3 present, it also would exceed the paraconid in height.

Length of the premolar series (p1–4) is, 82 mm. However, because this measurement includes an abnormality (p3), it probably represents a maximum value for the species.

The m1 measures 28.2 mm in length and a talonid width of 13 mm. Length of the trigonid is, 20 mm, the talonid, 9.5 mm, emphasizing that the trigonid dominates the tooth and the talonid is not enlarged. The metaconid is well developed; this is the only large temnocyonine in which the metaconid survives. The short talonid is dominated by a large, labially situated hypoconid, directly behind the protoconid. The labial face of the asymmetric hypoconid descends abruptly to the cingulum but the lingual face is more gradually inclined, sloping to the medial edge of the talonid (there is a small lingual talonid shelf). The apex of the hypoconid is an anteroposteriorly aligned ridge that curves posterolingually to become the posterior edge of the talonid. There is weak swelling on the lingual talonid shelf that appears to be a vestigial entoconid.

The m2 measures 17.4 mm in length, with a talonid width of 10.0 mm, and a trigonid width of 10.4 mm. The trigonid is low but higher than the talonid. There is an anterolabial cingulum; the large protoconid is the most prominent cusp and is connected by a smoothly curving ridge to the low, broad paraconid. The small knoblike metaconid lies lingual to the protoconid and is separated from it by a slight constriction. The hypoconid is the only talonid cusp and is labial in position. The talonid surface descends gradually from the hypoconid to the lingual margin.

The m3 measures 10.7 mm in length,, 8.5 mm in width. The only prominent cusp is a conical protoconid, somewhat anterior to the center of the tooth, flanked lingually by weak enamel ridges descending to barely discernible vestigial paraconid and metaconid swellings. The talonid is much reduced and shows only a vestige of the hypoconid as a swelling on the posterior face of the protoconid.

The toothrow measures as follows: anterior border of canine alveolus to m 3, 160.9 mm; p1–m3 length, 135.0 mm; p1–m2 length, 125.4 mm; m1–3 length, 55.5 mm.

DISCUSSION: Temnocyon macrogenys is the largest North American species of the genus and it retains the metaconid on m1–2. The m1 remains a shearing carnassial (i.e., a tall, shearing trigonid and low talonid), and the premolars, although robust, are tall, relatively narrow teeth. These dental features indicate a large predatory carnivore with a more sectorial dentition than found in Mammacyon and Delotrochanter . When compared to the mandibular dentition of the wolf, the canine, premolars, and m2–3 of T. macrogenys are much larger whereas the carnassial is the same length, although a broader tooth. The beardog has incorporated the carnassial as an integral part of a crushing dental battery made up of premolars and posterior molars, while still retaining some shearing ability.

The dentition of T. macrogenys can be derived from that of T. percussor , and there is a particularly strong similarity in cusp pattern to AMNH 81005, Cook’s type of the species. This opinion is supported by m1–2 structure and similar m1/m2 length ratios of 1.62 and 1.69 in AMNH 81005 and F:AM 54139 (table 6). Temnocyon macrogenys seems the lineal descendant of T. percussor from the Niobrara River valley at Agate National Monument.

Temnocyon macrogenys (F:AM 54139) comes from the Anderson Ranch beds about 2 mi south of Guernsey, Wyoming; in this district and throughout northwest Nebraska and southeast Wyoming the formation includes widespread flat geomorphic surfaces representing level, early Miocene grasslands ( Hunt, 1990). These surfaces are densely packed with fine root casts (rhizoliths) and are cemented by silica derived from diagenesis of volcanic glass shards and unstable minerals in these fine-grained tuffaceous sandstones. Charles Falkenbach, who collected the mandible of T. macrogenys , used the term ‘‘High Daemonelix zone’’ for these silica-cemented geomorphic surfaces, although they have no relationship to rodent burrows first termed ‘‘Daimonelix’’ many years previously by E.H. Barbour (1892). Evidence that Falkenbach in fact collected the mandible from one of the geomorphic surfaces of the Anderson Ranch Formation can be seen in the sediment adhering to the mandible that includes the small siliceous rootlets and pale reddish-brown sand typical of these grassland surfaces. Temnocyon macrogenys was found within one of these siliceous paleosols. Fossil mammals in the Childs Frick collection from the ‘‘High Daemonelix’’ level of the Guernsey collecting district are similar to mammals of the Niobrara Canyon local fauna from the stratotype area of the Anderson Ranch Formation (Niobrara Canyon, Sioux County, Nebraska) and establish the age of T. macrogenys as latest Arikareean.

Figures

Fig. 20. Holotype mandible of Temnocyon macrogenys (F:AM 54139), with left p1–2, p4–m3 (pathologic p3), Anderson Ranch Formation, 2-Mile District, near Guernsey, Platte Co., Wyoming. This is the largest and youngest species of Temnocyon. A, labial and B, lingual views.

Fig. 21. Occlusal steropair of the holotype dentition (p4–m3) of Temnocyon macrogenys (F:AM 54139). Note presence of m1 and m2 metaconids.