Temnocyon subferox, 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 : 32-36

Temnocyon subferox , new species Figures 9 , 10 , 66

Temnocyon altigenis: Thorpe, 1922: 167–168 . Temnocyon altigenis (in part): Hough, 1948: 100–

101.

TYPE: YPM 10065, nearly complete skull with basicranium but lacking the rostrum anterior to P3 ; from the John Day Formation, John Day River , Oregon (a catalog entry at the Peabody Museum, Yale University, reads ‘‘Middle John Day’’) ; teeth present include right P3–M2, left P4–M2. Collector or date of collection unknown, but cataloged in 1914.

DISTRIBUTION: Earlier Arikareean, John Day Formation, Oregon.

ETYMOLOGY: From the Latin, sub, for ‘‘less than, under’’ and ferox , ‘‘fierce,’’ in the belief that the species precedes Temnocyon ferox .

DIAGNOSIS: Cranium and teeth intermediate in size (basilar length,, 21 cm) between T. altigenis (UCMP 9999, 1549, basilar lengths, 16–18 cm) and T. ferox (YPM-PU 10787, basilar length,, 26 cm) and without strong inflation of the frontal region (table 7). Differs from other species of Temnocyon (except T. altigenis ) by (a) smaller size and more plesiomorphic P4–M 2 in which a marked expansion of M1 protocone region has not occurred (ratio A/B, 1.44, table 6); (b) P4 sectorial with protocone placed in advance of paracone (more dentally derived species of Temnocyon have protocone directly lingual to paracone). No preprotocrista on M1 (present in T. altigenis ). Retains most plesiomorphic auditory bulla of any known temnocyonine (see discussion of Basicranial Anatomy).

REFERRED SPECIMENS: None.

DESCRIPTION: The skull was first mentioned and briefly described by Thorpe (1922) in a discussion of doglike canids and amphicyonids in the Marsh collection at Yale. Later, Hough (1948: 101 —‘‘a specimen in the Peabody Museum, Yale University’’) mentioned the auditory region and recognized the presence of a remnant of the auditory bulla (she does not cite a catalog number but her description can apply only to YPM 10065). Neither Hough nor Thorpe illustrated the skull. The basicranium shows the deeply embayed basioccipital bone typical of amphicyonids, thought to contain an enlarged inferior petrosal venous sinus. The auditory region is reviewed and illustrated in the section on Basicranial Anatomy.

Although the skull is uncrushed and its form intact, the bone itself is fractured and altered by diagenesis after burial so that anatomical detail is lacking. The dolichocephalic skull has a gracile appearance with slender zygomatic arches and a long postorbital distance (orbital margin to occpital condyle is 14.3 cm). The frontal region is not inflated, thus similar to T. ferox , contrasting with the expanded frontal region of T. fingeruti . As in T. altigenis , the maxilla is slightly depressed above the prominent infraorbital foramen. Foramina of the orbital region are as described for T. altigenis : an optic foramen, sphenorbital fissure, and anterior foramen for the alisphenoid canal open into the elongate depression that begins 18 mm anterior to the foramen ovale. The posterior opening of the canal and the foramen ovale share a common fossa. As in UCMP 1549 the foramen rotundum must open internally into the enclosed alisphenoid canal. Despite the loss of the rostrum the palate is preserved from P3 to M2; the cheek teeth seem quite small given the palatal area and skull size (greatest palatal width at P4– M1 is 6.5 cm; P4–M2 occupy 58 % of this width). The mandibles must have been long and slender because of the modest glenoid fossa for a rather small narrow condyle, the small cheek teeth, and the considerable distance (, 9 cm) from the glenoid fossa to the carnassial.

The left P4–M2 and right P3 survive whereas P1–2, incisors, and canines did not.

P3 is laterally compressed, slightly expanded posteriorly, and is taller than in T. altigenis and T. ferox . A fine enamel ridge runs from the principal cusp down the posterior slope to the basal cusp, the latter more prominent than a tiny posterior accessory cusp 2 mm anterior to the basal cusp. There are weak labial and lingual cingula that meet posteriorly to form the prominent basal cusp. P3 length, 12.9 mm; greatest posterior width, 6.6 mm.

P4 has a well-developed protocone, a sectorial paracone/metastylar blade, and is clearly a shearing carnassial. The metastylar blade is placed at a 45 ° angle to the anteroposteriorly aligned paracone. Length of the blade, 8 mm; paracone length, 10.3 mm. The protocone, lingual and slightly anterior to the paracone, is enlarged as is the protocone of T. altigenis . In T. subferox there is a broad embrasure between P4 and M1 into which the shearing m1 trigonid fits during carnassial occlusion. Dimensions of this embrasure in T. subferox are similar to those in T. altigenis (UCMP 9999) and represent the plesiomorphic state. Closure of this shearing embrasure between P4 and M1 occurs in the large species of Mammacyon and Delotrochanter where crushing takes precedence over shear.

P4 is encircled by a cingulum that is pronounced at the labial base of the metastylar blade. The cingulum is swollen at the anterolabial corner to form a parastyle, situated 2 mm anterolabial of the base of the enamel ridge descending the anterior face of the paracone. The protocone is a blunt knoblike cusp separated by a valley from the paracone and would have occluded with the heel of p4.

M1 is similar to the T. altigenis M1 except that the protocone region is more derived. A well-developed parastyle contacts the P4 metastylar blade. The labial cingulum is thickest at the parastylar region and thins posteriorly yet is still well defined along the entire labial margin of M1. The paracone is somewhat larger than the metacone. The internal faces of paracone-metacone form a vertical shearing surface that descends steeply to an enamel flat forming the lingual half of the tooth. A knoblike protocone is isolated on this enamel flat: no preprotocrista runs from the protocone across the enamel flat to the anterior cingulum as in the more plesiomorphic T. altigenis . This differs from T. ferox (YPM-PU 10787) that retains not only a pre- but also a weak postprotocrista.

The knoblike M1 protocone and its enamel flat are surrounded by a prominent cingulum, strongly thickened on the lingual margin, slightly less developed on the posterior margin, and thin but distinct anteriorly. Development of a swollen anterolingual M1 cingulum in temnocyonines accompanies loss of the m1 metaconid during closure of the embrasure between P4 and M1. There is no metaconule; a paraconule may be represented by a small cingular cusp on the anterior edge of M1 midway between protocone and paracone.

M2 is much smaller than M1 but remains a fully functional quadrate tooth with a weak parastyle. The paracone and metacone are elevated as in M1 but here the metacone is much smaller than the paracone. The lingual half of the tooth is a low enamel platform with a low blunt protocone. The tooth is surrounded by a prominent cingulum, lingually thickened as in M1.

M3 was not present in life: the maxillary border is smooth and unbroken posterior to M2, and there is no evidence of alveoli or roots for M3.

DISCUSSION: The skull, which is of considerable importance because of the basicranium, has been long ignored except for Hough’s (1948) brief mention of its auditory region. It was first described by Thorpe (1922), who placed it in T. altigenis . However, the skull is much larger than the known crania of T. altigenis (UCMP 9999, 1549) and exceeds any reasonable upper limit of skull size predicted for the T. altigenis sample from the John Day region. Unfortunately, Yale University collection records for the skull do not establish an exact location, a collector, or a date of collection, only that it was cataloged in 1914 as YPM 10065, and that it may have come from the ‘‘Middle John Day.’’ Whether the skull was found by collectors employed by O.C. Marsh in the late 19th century is unknown but is a possibility. It is surprising that such a wellpreserved and unusual carnivore did not merit a more detailed description of its site of collection, which suggests it was found during one of the early Yale expeditions when exact locality data were rarely obtained. The stage of evolution of the skull relative to other North American temnocyonines indicates that a late Arikareean assignment is unlikely. An earlier Arikareean age is most probable, postdating the T. altigenis hypodigm in the John Day Formation.

The plesiomorphic dentition and weakly inflated frontal region of the skull make it a possible predecessor to T. ferox (YPM-PU 10787) from the ‘‘upper John Day beds.’’ The lack of pre- and postprotocristae on M 1 in YPM 10065, and their presence in T. ferox , seem to conflict with this view. However, with acquisition of larger samples, protocristae may prove to be variable features on the molars of these carnivorans. The morphological ‘‘distance’’ between T. subferox and T. ferox is greater than between skulls of T. subferox and T. altigenis , suggesting that substantial time intervened between YPM 10065 and T. ferox .

The skull of YPM 10065 is important to the determination of the broader relationships of temnocyonines since the basicranium and auditory bulla are preserved and largely undistorted. The bulla of T. subferox represents the most plesiomorphic condition found among temnocyonines. The auditory bulla in North American temnocyonines is known only in T. subferox (YPM 10065), in T. fingeruti (NM 280/61), and in Mammacyon obtusidens (ACM 34–41). These bullae are more fully discussed in the section on Basicranial Anatomy.

No mandible was associated with the skull of T. subferox (YPM 10065), and no isolated jaws from the John Day beds occlude satisfactorily, but there are two mandibles of North American temnocyonines that correspond in size: (a) LACM 15908, a partial mandible from the Sharps Formation of South Dakota; (b) LACM 470, a crushed mandible from Kew Quarry, Sespe Formation, Las Posas Hills, California.

The Sharps mandible (LACM 15908) is unlikely to belong to the same species as the holotype skull despite the size correspondence: the m2 protoconid and hypoconid are placed in the center of the tooth, not on the labial margin as in Temnocyon . The m1 metaconid of the Sharps jaw is reduced, more than expected for an m1 that would occlude with the shearing P4 of the holotype skull. This jaw likely represents an early member of the Mammacyon lineage. On the other hand, the Kew Quarry jaw includes a tall p3–p4 that corresponds to the tall P 3 in YPM 10065. Here the Kew Quarry mandible is referred to T. cf. T. subferox pending discovery of associated upper and lower dentitions of the John Day species.

Figures

Fig. 9. Temnocyon subferox (YPM 10065), John Day Formation, Oregon. Holotype cranium in (A) lateral and (B) high lateral oblique views. Rostrum restored to the left of dashed line.

Fig. 10. Holotype of Temnocyon subferox (YPM 10065), John Day Formation, Oregon. A, maxilla with P4–M2; B, basicranium with remnant of ectotympanic bulla (stereopairs with negatives reversed). See figs. 66, 69. For abbreviations, see p. 5.

Fig. 66. Holotype basicranium of Temnocyon subferox (YPM 10065), John Day Formation, Oregon (stereopair). The bulla fragment in the auditory region is a remnant of an ectotympanic crescent (T) similar to that seen in Daphoenus. YPM 10065 represents the plesiomorphic state of the temnocyonine auditory bulla. For abbreviations, see page 5.