Delotrochanter oryktes, 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 : 77-85

Delotrochanter oryktes , new species Figures 34–39

‘‘The major part of a skeleton of Daphoenodon superbus Peterson ….’’: Loomis, 1910: 298.

TYPE: ACM 4804 , partial skull with right I2–3, C, P1–2, P4–M2, and left I2–3, C (partial), P1–2, P4–M1, labial half of M2 ; right mandible with c, p4–m2; left mandible with c, p2–4, m1–2; and much of the postcranial skeleton, from the Harrison Formation, Stenomylus Quarry, Agate Fossil Beds National Monument , Sioux County, Nebraska, collected by F. B. Loomis, 1908 .

DISTRIBUTION: Late Arikareean, Harrison Formation and basal Anderson Ranch Formation, Agate Fossil Beds National Monument, Sioux County, Nebraska.

ETYMOLOGY: From the Greek, oryktes , for ‘‘digger’’ or ‘‘excavator,’’ in allusion to the discovery of one of the individuals of the species in a burrow.

DIAGNOSIS: Distinguished from D. petersoni by larger size ( D. petersoni m1 length, 22.3 mm; D. oryktes m1 length, 27.1 mm). Dentition clearly antecedent to that of the much larger D. major (tables 2, 3), but not as massive and robust. 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; and from Rudiocyon by the form of p4.

REFERRED SPECIMENS: (1) UNSM 47800 (field no. 8-81), partial skull with left P2–M2, lingual half of right M1, right I2–3, and petrosal fragment; associated left calcaneum, left metatarsals 2–4, left metacarpals 3–5 with proximal phalanx and 2 sesamoids, left magnum; found in place within a burrow, Carnegie Quarry 3, Beardog Hill den site, Agate Fossil Beds National Monument , Sioux County, Nebraska, collected September 14, 1981, by J. Kaufman and R.M. Hunt ( UNSM field no. 9-81 was given to the fifth metacarpal, found near the skull, presumably from the same individual); (1a) CM 1589 b, right metatarsal 5, Carnegie Quarry 3, Beardog Hill den site, collected by O.A. Peterson, 1904–1905, probably the same individual as UNSM 47800; (2) YPM-PU 24872, left calcaneum, removed from Princeton University quarry block (now YPM-PU 12213 ), Princeton Expedition of 1914, loc. 1002A, Carnegie Hill waterhole bonebed, Agate Fossil Beds National Monument , Sioux County, Nebraska .

DESCRIPTION: Both individuals referred to this species (ACM 4804, UNSM 47800) come from Agate Fossil Beds National Monument, Sioux County, Nebraska, and were collected 73 years apart. ACM 4804, designated here as the holotype, includes a nearly complete skeleton discovered by Loomis at Stenomylus Quarry in 1908. It was originally referred to Daphoenodon superbus: Loomis (1910) did not recognize it as a temnocyonine, a group almost unknown at that time. A second individual (UNSM 47800) is represented by a skull (fig. 37) and bones of the fore- and hind feet, found within a carnivore burrow (front cover) during the reopening of Carnegie Quarry 3 by the University of Nebraska in 1981 ( Hunt et al., 1983; Hunt, 1990: 106– 107).

The mandible and lower dentition are known in ACM 4804 but were not recovered with UNSM 47800. The following description is based on ACM 4804 ; it is selected here as the holotype of the species because of the association of maxillary and mandibular dentition with the postcranial skeleton (see Postcranial Osteology). Appropriate comparisons are made with the larger derivative species, D. major , known only from teeth.

The mandible is deep yet thin, with a distinctive recurved coronoid process and a wide (35.8 mm) articular condyle. The anterior margin of the ascending ramus curves gradually downward beneath the rear molars so that m2–3 are forwardly inclined as in several other temnocyonines. The nearly uniform depth of the horizontal ramus is demonstrated by similar measurements beneath m1 (45.7 mm) and p2 (41 mm). Length of the mandible measured from the symphysis (between the first incisors) and the posterior surface of the articular condyle is 205 mm.

Lower incisors did not survive in either individual referred to this species. Alveoli for i1–3 are present in the left mandible of ACM 4804: the single alveolus for i3 (width, 4.5 mm) shows that it was the largest of the lower incisors. A smaller i2 alveolus is situated medial to i3, and a very small i1 alveolus is placed slightly internal and ventral to i2.

The left lower canine of ACM 4804 is not as strongly recurved as in some species of Temnocyon (UCMP 9999, YPM-PU 10787). Canine height from the base of the enamel to the unworn tip on the labial side is 36.8 mm. Length and width measured at the base of the enamel are 18.5 mm and 12.5 mm, respectively. A pronounced enamel ridge runs from the tip to the enamel base on the posterior edge, and a second ridge travels down the anterolingual face. Although the m1 of a wolf is about the same size as the D. oryktes m1, the beardog canine is much larger.

The p1 is not preserved in ACM 4804; the single large alveolus measures 8.2 mm in length, 6 mm in width, and is crowded between the canine and p2 alveoli, and is in contact with both. Closely spaced alveoli are common to the entire lower toothrow. The slight crowding of premolar alveoli is due to the youth of this carnivore (all teeth are only in early wear).

The p2 is a tall, yet anteroposteriorly short, double-rooted tooth, 14.9 mm in length, 8.2 mm in greatest width. There is no posterior accessory cusp. The anterior face is steeply inclined and is traversed by a fine enamel ridge that extends from tip to anterolingual cingulum. The posterior face is more gradually inclined with an enamel ridge running from tip to posterolabial cingulum. The anterior root of p2 is much smaller (length, 4.9 mm) than the robust posterior root (length, 8.5 mm), a distinguishing trait of the species.

The p3 measures 16.6 mm in length, 9.4 mm in posterior width, and 6.9 mm in anterior width. The tooth is short and posteriorly wide, which is evident when compared with the respective dimensions of p 3 in Mammacyon ferocior (19.3 mm, 9.0 mm, 6.8 mm). There is a posterior accessory cusp, somewhat labially placed. A slightly swollen basal cingulum is more pronounced on the lingual and posterior sides. Just as for p2, the anterior root is smaller than the posterior root.

The p4 is robust, with a wide heel, measuring 21.0 mm in length, 10.8 mm in posterior width, and is much larger than p3. Its form is diagnostic of the genus, notably the large posterior accessory cusp centrally placed on an expanded heel behind the principal cusp. Delotrochanter is the only temnocyonine that has evolved a centrally placed posterior accessory cusp on p4. In Temnocyon and Mammacyon the accessory cusp is labially placed.

The height of the principal cusps of p2–4 is pronounced, so much so that the linear serial alignment of the premolar cusps is continued along the toothrow by the principal cusps of m1 and m2, and this cusp alignment is further emphasized by the elevation of m2– 3 on the rising margin of the ascending ramus.

The m1 is a crushing carnassial, 27.1 mm in length, 12.7 mm in greatest width, formed by three aligned bunodont cusps, the paraconid-protoconid-hypoconid. There is no metaconid or entoconid. The thickened labial cingulum is essentially straight, not sinuous as in Temnocyon and Mammacyon . The hypoconid is centrally placed on the talonid and in labial view is as tall as the paraconid (as in Mammacyon but not Temnocyon ). The nearly equal development of hypoconid and paraconid flanking the protoconid creates a profile typical of large species of Mammacyon and Delotrochanter , but a somewhat larger hypoconid in Delotrochanter emphasizes the cusp alignment adapted for crushing.

The m2 is rectangular in occlusal view, measuring 16.0 mm in length, 10.0 mm in greatest width (m1/m2 length ratio,,1.7, table 6). The protoconid and hypoconid are placed in direct anteroposterior alignment behind the m1 hypoconid. These three cusps are about the same height and form a crushing battery posterior to the carnassial blade. There is a low vestigial paraconid. The m2 is not as elongate (relative to m1 length) as in Mammacyon , but the cusps of m2 are centrally placed as in that genus. Hence, one of the hallmarks of Delotrochanter is the central placement of all cusps from p4 to m3 (with slight lingual diversion of the m1 paraconid).

The m3 is not preserved but is represented in ACM 4804 by a single large alveolus, length 5.3 mm, width 5 mm.

In ACM 4804, p1–m3 length is 104.5 mm; p2–m2 length, 89.6 mm; p1–4 length, 59.7 mm; m1–3 length, 48.2 mm.

The skull of ACM 4804 was damaged during preparation and mounting of the specimen at Amherst College shortly after it was collected in 1908. With the aid of Drs. Margery and Walter Coombs, we were able to recover the skull and restore part of the rostrum and palate and a few basicranial fragments, including the right petrosal and a partial basioccipital.

The central incisors (I1–2) of ACM 4804 are much smaller than I3. I1 is not preserved in ACM 4804, however the alveolus measures 9.3 mm in length, 4.1 mm in width. I2 is present with a vertical anterior face, a sloping posterior surface, and a prominent auxiliary cusp (Nebenzacke) on its lateral margin. I2 is 9.8 mm in length, 5.7 mm in width at the enamel base. I3 is a large, recurved caniniform tooth measuring 23.2 mm in height from tip to posterolabial enamel border, 13.1 mm in length, 10.1 mm in width at the base of the enamel. I3 has a thin enamel ridge from tip to posterolabial base and another on the lingual face from tip to enamel base. I 2–3 in UNSM 47800 show no differences from those incisors in ACM 4804.

The upper canine measures 37.9 mm in height from its posterolabial enamel base to its unworn tip. Length and width at the base of the enamel are 19.3 and 13.4 mm. An enamel ridge runs from the tip to the enamel base on the posterior surface; a second ridge runs from tip to base on the anterolingual surface.

P1 is a small, conical tooth 9.4 mm in length, 6.9 mm in greatest width. There is a single principal cusp and no accessory cusp. The principal cusp is placed forward of center, and from this cusp a fine enamel ridge extends down both the anterior and posterior faces of the tooth.

P2 is much larger than P1, measuring 15.8 mm in length, 8.7 mm in posterior width (in UNSM 47800, P2 measures 15.9 mm and 8.0 mm). P2 is tall, triangular in lateral view, with a vertical, nearly rectilinear anterior face and a more gradually sloping posterior face. Of importance is the small anterior root relative to the larger posterior root, a diagnostic feature of Delotrochanter , indicating that the shortening of p 2 in the lower jaw is reciprocated in the upper dentition. Alveolar length of the P2 anterior root is 5.6 mm; posterior root, 10.0 mm; length of the roots themselves are, respectively, 5.2 and 8.8 mm. The posterior heel of P2 is widened and a weak accessory cusp is labially placed low on the broad heel. Thin enamel ridges are present on the anterior and posterior faces of P2, running to the respective anterolingual and posterolabial corners.

P3 is lost from the skull of ACM 4804 but is present in UNSM 47800 (length, 18.2 mm; width, 9.8 mm) where it is about the same height as P2 but posteriorly wider with a small accessory cusp slightly labially situated on the posterior slope.

P4 measures 21.4 mm in labial length, 18.8 mm in greatest transverse width across the protocone. This massive tooth has a short metastylar blade 9 mm in length that forms a moderately developed shear surface with the large paracone. Small vertical wear facets are developed on the lingual faces of metastylar blade and paracone but the predominant crushing mode of occlusion is evidenced in this young individual by bluntcusp wear on the paracone. As in the larger temnocyonines, the rounded blunt protocone is enlarged for crushing but is not as massive as in species of Mammacyon . The protocone in Delotrochanter does not extend as far toward the midline of the palate as it does in Mammacyon , and does not reach the lingual margin of M1. P4 length is 22.8 mm and width is 17.9 mm in UNSM 47800, too young an individual to develop shear facets on P4.

M1 measures 17.7 mm in length of the labial margin, 25.4 mm in greatest transverse width, and 14.7 mm in anteroposterior width of the protocone region. In UNSM 47800, M1 length is 17.5 mm; greatest width is 23.9 mm; and width across the protocone is 14.1 mm, thus a smaller molar relative to the M 1 in ACM 4804. An M1 that is constricted at the level of the protocone basin so that the tooth has a ‘‘waist’’ is typical of Delotrochanter and Mammacyon . The labial half of M1 is formed by a prominent, yet rather blunt, paracone and somewhat smaller metacone bordered by a labial cingulum thickened in the parastylar region. The lingual faces of the paracone and metacone form a vertical shear surface but these cusps are lower and shear is not as developed as in large species of Temnocyon . The low protocone is isolated within an enamel platform, creating a flat crushing surface surrounded by a muchthickened lingual cingulum. A preprotocrista runs from the protocone to the anterior cingulum at the base of the paracone but there is no postprotocrista. On the M1s of both ACM 4804 and UNSM 47800 the lingual cingulum thins abruptly at the level of the protocone basin. The lingual cingulum of UNSM 47800 is not as developed as in ACM 4804.

M2 is a rectangular tooth, much smaller than M1, measuring 9.3 mm in labial length, 17.4 mm in greatest width (M2 length, 8.9 mm; width, 15.3 mm in UNSM 47800). The paracone is much larger than the strongly reduced metacone, both bordered by a labial cingulum thickened in the parastylar region. The lingual faces of para- and metacone form a low shear surface that continues the paracone- metacone shear plane of M1 (this plane less evident in UNSM 47800 in which M2 is more reduced). The M2 protocone is a low, blunt cusp isolated on an enamel platform surrounded by a slightly thickened lingual cingulum. There is no evidence of M3 although that part of the maxilla where M3 would occur is not preserved. However, UNSM 47800 has an intact maxilla in which M3 is certainly absent.

The skull (UNSM 47800) found in 1981 within a burrow at Beardog Hill, Agate Fossil Beds National Monument preserves the left P1–M2 (figs. 37–38). This is the only known skull of Delotrochanter . The left side of the skull and upper dentition are largely intact, whereas the right side was destroyed by plant roots that invaded the burrow. The skull is remarkably similar in form to the skull of the hyena, Crocuta crocuta . Both skulls are broad and rather short with laterally extended zygomatic arches and large temporal fossae indicating well-developed temporal jaw musculature. Both have blunt, short snouts of similar length: the preorbital proportion of skull length in UNSM 47800 is 40 % and in the hyena 35 %. UNSM 47800 differs from Crocuta in possessing a more inflated frontal region and more massive snout with larger canines. The infraorbital foramen of UNSM 47800 is much larger than this foramen in the hyena, suggesting a more developed neurovascular supply to the nasal region. The braincase of UNSM 47800, despite the rather short skull, is the largest of any known temnocyonine (table 7).

The upper teeth are extremely similar in form to those of ACM 4804 but are somewhat smaller and more gracile. P4 is longer and its protocone not as developed, and the M1–2 are smaller, not as hypertrophied as in ACM 4804. The crushing function of P4–M2 is better developed in ACM 4804. ACM 4804 is a male (a baculum was found with the skeleton) and, based on the gracile teeth, UNSM 47800 could be a female. Sexual dimorphism is common among amphicyonids and it would not be surprising to find it present in temnocyonines. Dimorphism in temnocyonine species is difficult to demonstrate because of the lack of multiple individuals from a single locality.

DISCUSSION: Delotrochanter is defined on the basis of the holotype dentition and skeleton of D. oryktes (ACM 4804) supplemented by information derived from the individual (UNSM 47800) found in the carnivore dens at Agate National Monument. These two individuals are clearly closely related and are assigned here to a single species, diagnosed by short, posteriorly wide premolars (p2–4, P2–3) with centrally (not labially) placed posterior accessory cusp on p4. These premolars differ from the longer premolars and labially placed p4 posterior accessory cusp found in Temnocyon and Mammacyon . Although the p4 of Mammacyon ferocior is broader than that of M. obtusidens , the posterior accessory cusp in both species remains labial in position. Also, Delotrochanter oryktes does not have as prominent a protocone on P4 as in Mammacyon nor is its m2 as elongate; and it lacks the m1 metaconid found in Temnocyon . These differences, however, are most readily observed in relatively complete dentitions: isolated teeth will in many instances be difficult to assign.

The mandible of ACM 4804 is only, 2 cm longer than that of a mature female Alaskan wolf (ZM 17459: mandibular length, 18.5 cm; basilar length, 22 cm); yet mandibular depth and the height of the coronoid process are much greater and the canine, premolars, and m2 are conspicuously larger and more robust. The m1 is about the same length, but is a wider, blunt-cusped tooth, its protoconid-paraconid lacking the sharp edges seen on the m1 shearing blade of the wolf. The feeding strategy of D. oryktes relied on powerful jaw musculature applied to a deep, rigid mandibular ‘‘beam’’ carrying the heavy, blunt premolar and molar crushing dentition with enormous canines.

The four individuals of D. oryktes from Agate National Monument occur at three superposed stratigraphic levels: (1) ACM 4804, the holotype, was found, 12–15 m above the two principal camel-producing horizons at Stenomylus Quarry in the Harrison Formation (a second individual, ACM 4804A, a juvenile, was found with ACM 4804 but is known only from postcranials); (2) The calcaneum (YPM-PU 24872) indicates the presence of D. oryktes in the Agate waterhole bonebed (basal Anderson Ranch Fm.), which occurs stratigraphically between the levels of ACM 4804 and UNSM 48700; (3) The adult (UNSM 47800) discovered in the carnivore dens at Beardog Hill ( Hunt et al., 1983: fig. 1, den 2, burrow C) also occurs in the basal Anderson Ranch beds (the dens are excavated into the waterhole bonebed). The amount of time intervening between these stratigraphic levels remains uncertain. The age of ACM 4804 is approximated by the Agate Ash, recently redated at,22.9 Ma ( Izett and Obradovich, 2001).

Despite its younger stratigraphic age, UNSM 47800 is the smaller, more gracile animal relative to ACM 4804. Given the baculum found with ACM 4804, the size difference between UNSM 47800 and ACM 4804 is more likely due to sexual dimorphism and not relative stratigraphic position.

The postcranial skeleton of ACM 4804 is discussed under Postcranial Osetology.

Figures

Fig. 34. Holotype mandible of Delotrochanter oryktes (ACM 4804) with left canine, p4–m2, alveoli of p1–3, from Stenomylus Quarry, Harrison Formation, Agate Fossil Beds National Monument, Sioux Co., Nebraska. A, labial and B, lingual views. Male individual found with baculum.

Fig. 35. Holotype mandible of Delotrochanter oryktes (ACM 4804) with left canine, p2–m2 (p2–3 inserted in mandibular alveoli to demonstrate small diameter of anterior roots).

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. 37. A, Delotrochanter oryktes (UNSM 47800), cranium with P1–M2 of presumed female from carnivore den, Beardog Hill, Agate Fossil Beds National Monument, Sioux Co., Nebraska. The short, deep hyenalike skull is unique among temnocyonines. B, Plan map of dens showing location of burrow (den 2) that contained the skull and foot bones of D. oryktes (UNSM 47800). The beardog Daphoenodon superbus was the principal occupant of dens 1, 3, and 4; mustelids were found in dens 5 and 6.

Fig. 38. Left maxilla of the Delotrochanter oryktes cranium (UNSM 47800) with P1–M2, presumed female, Beardog Hill, Agate Fossil Beds National Monument, Sioux Co., Nebraska. The crushing P4–M1 is more gracile than these teeth in the male (ACM 4804, fig. 39).

Fig. 39. Right maxilla with P4–M2 of Delotrochanter oryktes (ACM 4804, male), Stenomylus Quarry, Harrison Formation, Agate Fossil Beds National Monument, Sioux Co., Nebraska.