Mammacyon ferocior, 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 : 64-71

Mammacyon ferocior , new species Figures 28–31

TYPE: Right partial mandible with i3 root, c–m3, and associated limb elements and vertebrae: atlas, caudal vertebra, sacrum, glenoid of scapula, left humerus, distal right humerus, both ulnae, rib fragments, left metacarpal 2, proximal left and right metacarpal 3, innominate fragments, partial left femur, left tibia, proximal right tibia, both calcanea, left astragalus, proximal right astragalus, right navicular, both cuboids, left ectocuneiform, right metatarsal 2, left distal metatarsal 3, proximal right metatarsal 5 (reduced), from?Harrison Formation, north of Keeline, Niobrara County, Wyoming, collected by Charles Falkenbach, 1931 ( F: AM 27562).

DISTRIBUTION: Mid- or early late Arikareean, Arikaree Group, Niobrara County, Wyoming.

ETYMOLOGY: From the Latin, ferocior , ‘‘particularly fierce,’’ in allusion to the large size and presumed disposition of this predator.

DIAGNOSIS: Largest species of the Mammacyon lineage, thus sharing the same dental characters of form and proportion as M. obtusidens but distinguished by larger size. Dental ratio A/B, 1.23; ratio C/D, 1.13 (table 6), lower than all Temnocyon species. Ratio C/D (1.13), lowest of all temnocyonines. Distinguished from other large temnocyonines such as R. amplidens by an elongate m2 (ratio E/F: 1.6 in M. ferocior ,, 1.9 in R. amplidens ); from T. ferox , T. percussor , T. fingeruti , and T. macrogenys by absence of the m1 metaconid; from T. macrogenys by much smaller size; and from Delotrochanter oryktes and D. major by a p4 with labially placed posterior accessory cusp (centrally placed in Delotrochanter ). M. ferocior p4 posteriorly broader than in M. obtusidens . Cranium with greatly inflated frontal sinuses relative to small braincase volume (table 7).

REFERRED SPECIMEN: F:AM 54134, a skull with poorly preserved basicranium yet in other features complete and uncrushed. This is the largest known North American temnocyonine skull (basilar length, 28 cm). Upper dentition with alveoli for six incisors, two canines, left P1–3, right P2–3; complete right P1, P4–M2 (P4 damaged), and left P4–M2; M3 not present in the species. From the?Harrison Formation, north of Keeline, Niobrara County, collected by C. Falkenbach in 1944, from a stratigraphic level ‘‘high in the formation’’ in this area.

DESCRIPTION: FAM 27562—The depth of the mandible (estimated at, 44 mm) is uncertain because the ventral border of the horizontal ramus is damaged. Length of toothrow from the posterior border of the canine alveolus to the posterior limit of m3 is 126.2 mm. The mandibular symphysis extends from the anterior canine border almost to the posterior border of p2, having a greatest length of 48.8 mm and depth of 36.3 mm.

Lower canine height is 47.5 mm above the labial alveolar border. The tooth is grooved on its posterolabial face by the upper canine, identical to the similarly positioned groove in LACM 9194 ( M. obtusidens ). At the enamel base the canine measures 14 mm in width, 20.3 mm in anteroposterior length, and is somewhat posteriorly recurved. In addition to the groove worn by the upper canine, there is a deep elliptical wear facet on the anterolingual face produced by the I3 which is much larger than I1–2 based on alveolar dimensions.

The p1 is a small, peglike tooth that measures 9 mm in length, 5.9 mm in width. The main cusp is anteriorly placed as in all temnocyonines, with a long posterior slope and no accessory or cingular cusps.

The p2 is much larger than p1, measuring 17.9 mm in length, 8.1 mm in greatest width posterior to the main cusp. There are no accessory or cingular cusps. The posterior face is longer and more gently inclined relative to the more vertical anterior face. Fine enamel ridges are present on both anterior and posterior faces and, as in M. obtusidens , the anterior ridge descends to the anterolingual cingulum whereas the posterior ridge runs to the posterolabial cingulum. The tooth itself is angled outward; that is, the anterior p2 root is more labial in position than the posterior root. A somewhat swollen cingulum is more defined along the lingual bases of p2, p3, and p4 relative to its less pronounced labial expression.

The p3 is simply a taller, larger example of p2; it measures 19.3 mm in length, 9 mm in width. There are no accessory or cingular cusps despite the broadened heel of p3 relative to p2. Thin enamel ridges run down the anterior and posterior faces to the barely discernible cingulum, the anterior ridge contacting the anterolingual corner, the posterior touching the posterolabial corner where the cingulum is better developed along the posterior margin.

The p4 is more massive and robust than p3. It measures 21.9 mm in length, 10.7 mm in width. A large posterior accessory cusp occurs about halfway between the tip of the main cusp and the crown base. It lies labial not only to the main cusp but also to the basal cusp occupying the center of the broad posterior shelf of p4. The main cusp of p4 is nearly as tall and robust as the protoconid of m1. Both p3 and p4 are particularly tall premolars; their principal cusps are each, 4 mm taller than the m1 paraconid.

Flat apical wear facets are present on the principal cusps of p1–4 and on the m1 protoconid.

The m1 measures 27.7 mm in length,, 13 mm in width (the labial talonid is lost). The carnassial is large and robust yet appears rather low and almost dwarfed relative to the tall, massive premolars. The metaconid is absent. Both protoconid and paraconid are massive, broad cusps, the protoconid showing strong apical wear. However, carnassial shear is evidenced by a near-vertical facet on the labial face of the paraconid-protoconid. The paraconid, placed anterolingual to the protoconid, has not rotated into a position directly anterior to the protoconid as in carnivorans with highly developed carnassial shear. The hypoconid is large, blunt, centrally placed on the talonid, and the entoconid is absent. The cingulum is not well defined and appears only as a vague swelling around the base of the tooth. However, a prominent swelling of the enamel occurs at the posterolingual corner of m1 and represents a small cingular shelf; anterior to this swelling on the lingual margin of the talonid, the base of m1 is indented or ‘‘notched.’’ Both the cingular swelling and indentation also occur in M. obtusidens .

The m2 measures 17.3 mm in length, 10.3 mm in width. This is the longest North American temnocyonine m2, yet the m1 of this same individual is exceeded in length by the m1 of three other species ( T. macrogenys , D. oryktes , R. amplidens ). A low, blunt protoconid occupies the center of the trigonid; a hypoconid of nearly equal height lies directly behind the protoconid in the center of the talonid. There is neither metaconid nor entoconid. A vestigial paraconid appears as a low platform at the anterolingual corner of m2.

The m3 is a small, oval tooth with minimal surface relief. It measures 8.6 mm in length, 7.2 mm in width. Because the enamel is damaged, cusp pattern is uncertain; however, a low protoconid was the principal cusp and a small, reduced talonid is present.

FAM 54134—The skull is largely uncrushed and the largest known for the subfamily. It displays ‘‘bearlike’’ proportions reflected in the swollen muzzle, inflated frontal region, heavy zygomatic arches, and pronounced sagittal and lambdoid crests. The upper teeth represent the most highly specialized durophagous dentition developed by a temnocyonine, a crushing dentition unlike that of bears ( Ursinae ). The locus of crushing in the upper teeth of F:AM 54134 involves P3–M1/p4–m2 whereas in living ursine bears the M1–M2/m1–m3 serve this function.

Basilar length of skull is 280 mm comparable in size to adult male Ursus americanus . The braincase is proportionately small relative to overall skull size; in fact the expanded frontal sinuses appear to have had a volume greater than the cranial cavity (table 7). The secondary palate is long (14 cm) and narrow for a skull of this length: palatal width measured across the M1s is 8.7 cm whereas only 3 cm of this is occupied by the palatal bone between the teeth, with the remainder taken up by the enlarged molars. The tall, narrow infraorbital foramen (height, 18 mm; width, 9 mm), opens on the maxilla above the upper carnassial. The orbitotemporal region is long: postorbital length is, 19 cm, preorbital, 12.5 cm. The left orbital region preserves the elongate depression in alisphenoid and orbitosphenoid bones for the optic foramen, sphenorbital fissure, and anterior aperture of the alisphenoid canal. The canal is 20 mm in length; its posterior opening shares a common fossa with the foramen ovale. The foramen rotundum opens internally into the canal as in other temnocyonine crania.

The basicranium is foreshortened: the length from the common fossa for the posterior opening of the alisphenoid canal and the foramen ovale to the ventral notch of the foramen magnum (an estimate of basicranial length) is only 5.5 cm, approximately one-fifth the basilar skull length. Although the basicranium is damaged there is an evident similarity to the M. obtusidens basicranium (ACM 34-41). Enough of the basicranial axis is preserved to show that, on the right side, the margin of the basioccipital was deeply excavated for an enlarged inferior petrosal venous sinus. This is the most pronounced development of the sinus in any temnocyonine skull and is related to the large size of M. ferocior . The sinus includes a deep central pocket also seen in Temnocyon altigenis (UCMP 9999), penetrating 13 mm into the basioccipital.

The morphology of the upper dentition is comparable to ACM 34-41, however the Keeline skull has larger, more robust teeth, a massive P3–M2 crushing dental battery, close-spaced P2–4 (no diastemata), huge canines, and a full complement of incisors with large I3.

Incisor alveoli show that the upper incisors became larger from I1 to I3: I1 alveolus measures 9.1 mm in length, 4 mm in width; I2 alveolus, 11.2 mm X 6.1 mm; I3 alveolus, 11.5 mm X 10 mm. There is a diastema of, 9 mm between I3 and the canine alveolus. The canine alveolus measures 24 mm X 15 mm; the maxilla surrounding the canine roots is swollen to accommodate the large canines, indicating a male individual.

The upper premolars were somewhat crowded judging from the placement of P1– 3 alveoli. This is not the crowding seen in many young amphicyonids since the wear on cheek teeth shows F:AM 54134 to be a mature adult. As is the case in the mandible, P2 is angled outward and shows the effect of crowding more than any other tooth, its posterior alveolus more lingual than the anterior. This degree of crowding also occurs in the upper teeth of M. obtusidens (ACM 34- 41) and must be characteristic of large species of Mammacyon .

P1 is a small rounded peg (7.6 mm in length, 6.4 mm in width) preserved only on the right side close behind the large canine. The alveoli for P2–3 measure 18.6 mm by 9.4 mm, and 21.1 mm by 13.4 mm, respectively. The larger posterior alveolus common to each of these premolars shows that both P2 and P3 were posteriorly broad, especially P3.

The enormous P4 with its short metastylar blade and massive paracone and protocone represents the culmination of the trend toward a crushing dentition within the genus. Its 26 mm length is almost equalled by its 23.1 mm width. Length of the metastylar blade is 10 mm; length of paracone including the parastylar region is 17.4 mm. The embrasure between the protocones of P4 and M1 for the m1 trigonid is reduced to 8.5 mm in anteroposterior length, notably less than the anteroposterior lengths of the protocones themselves (P4 protocone, 12 mm; M1 protocone, 17.4 mm) so that very little of the lower carnassial could have been inserted into this space. Occlusion of the M. ferocior mandible (F:AM 27562) with the upper teeth of the skull demonstrates that the m1 protoconid is arrested at the level of the lingual cingulum of M1 by the narrow embrasure; the m1 paraconid and hypoconid also act as enamel stops against P4 and the M1 protocone basin. Thus, although some shear occurred between upper and lower carnassials, particularly in unworn teeth, a specialized crushing action was the dominant occlusal mode. Following an initial shearing stroke as the mandible brought the lower carnassial into contact with the upper teeth, the final phase of occlusion between p4–m2 and P3–M2 must have been mortar and pestle crushing employing the lingually expanded P4–M1 protocones. Ratios A/B and C/D (table 6) are the lowest in the genus, and are related to the development of a crushing occlusion in M. ferocior .

M1 measures 21.4 mm in length, 29.8 mm in width. Despite its enormous size, it retains the characteristic temnocyonine configuration in which an expanded protocone region is separated from the enlarged labial half of the tooth by a prominent constriction at the level of the protocone basin. The paracone is slightly larger than the metacone. These two cusps are labially bordered by a cingulum, which is more pronounced labial to the paracone and is continuous with a small parastyle at the anterolabial corner of M1. Although the cusps are worn, in all respects they appear as in M. obtusidens . The large protocone is situated in the center of a broad enamel platform more developed than in any other temnocyonine species. The enamel platform is surrounded by an expanded lingual cingulum. A preprotocrista extends from the protocone toward the anterior cingulum, and a weak vestige of a postprotocrista also appears on the surface of the enamel platform trending toward the posterior cingulum. There are no para- or metaconules. Lateral to the protocone itself is a deep protocone basin that receives the m1 hypoconid much as pestle fits against mortar.

M2 is rectangular in occlusal outline with its long axis oriented transversely; it measures 10.8 mm in length, 17.4 mm in width, and is much smaller than M1. The paracone is much larger than the metacone. A strong labial cingulum borders only the paracone; the metacone cingulum is weak. The paracone-metacone region forms an elevated labial rim that overlooks the flat protocone region. The centrally situated protocone on this enamel flat is but a reduced version of the same cusp on M1. Wear has reduced the protocone to a flat surface nearly coplanar with the enamel flat on which it rests.

M3 was no longer present in this species. The maxilla terminates abruptly behind M 2 in smooth bone indicating that in life there was no tooth posterior to M2.

Length of the upper toothrow from the posterior border of the canine alveolus to the posterior border of M2 is 104.2 mm. Length of the left P1–3 based on an alveolar measurement is, 51 mm. Greatest length of P4–M2 is 55.7 mm.

DISCUSSION: Although the mandible (F:AM 27562) and skull (F:AM 54134) here assigned to Mammacyon ferocior were not associated, the teeth in the mandible not only correspond in size and form to the upper teeth but also occlude perfectly. The collector, Charles Falkenbach, did not provide exact locality data for these fossils, but we know that they were derived from the same general locality north of Keeline, Wyoming, from gray volcaniclastic fine-grained sandstone of the Arikaree Group. Recent geologic study of the area north of Keeline and examination of the fossils previously collected from the area by the Frick Laboratory suggest that the fauna including M. ferocior can be considered as mid- to early late Arikareean in age. The fauna lacks a number of mammalian species that typify the late Arikareean (Ar3) Harrison Formation fauna of Sioux County to the east (see Age and Correlation). Although a radioisotopic age is not available for the M. ferocior hypodigm, faunal relationships suggest that the species is older than,23 Ma. Mammacyon has not been found in any known latest Arikareean (Ar4) fauna.

Associated limb elements and vertebrae were collected with the M. ferocior mandible (F:AM 27562) in 1931; they are alike in form but larger in size than the postcranials associated with the genoholotype of Mammacyon (ACM 34-41). Additional limb and foot bones (F:AM 107758) collected by Falkenbach in 1950 ‘‘from brownish sandstone 15 feet below highest exposure, west end, north of Keeline, Wyo.’’ belong to a large temnocyonine and also possibly represent limb elements of M. ferocior . Included are a humerus (length, 234 mm; width of distal end, 50.3 mm), partial scapula (only the glenoid and adjacent blade), distal?tibia, distal?radius, and two elongate metapodials. M. ferocior retained a postcranial skeleton much like (if not identical to) M. obtusidens , indicative of a digitigrade cursorial gait in which the forelimb was characterized by a narrow distal humerus, a lengthening of radius and ulna with limited ability for pronation/supination, and an elongate tarsus and carpus (see Postcranial Osteology). These are the first large Cenozoic carnivorans to achieve a limb skeleton modified for a digitigrade stance, restricted fore-aft limb excursion, and a striding cursorial gait. This recommends a behavioral mode in which a cursorial habit was coupled with durophagous feeding, an ecology to some extent paralleling spotted hyaenids ( Crocuta ) in the Old World.

None of the other very large North American temnocyonine species ( Temnocyon macrogenys , Delotrochanter major , Rudiocyon amplidens ) are known from skulls, although Delotrochanter oryktes is represented by a substantial partial cranium (UNSM 47800) from the carnivore dens at Beardog Hill, Agate National Monument. The skull of M. ferocior (F:AM 54134) is the most complete cranium of a large temnocyonine and demonstrates that the terminal species of the Mammacyon lineage possessed a rather ursidlike profile, strong inflation of the frontal sinuses relative to volume of the cranial cavity, a broad rostrum swollen around the large canine alveoli, and a robust dentition with cheek teeth adapted for crushing bone, fibrous sinew, and muscle.

Figures

Fig. 28. Holotype mandible of Mammacyon ferocior (F:AM 27562) with right canine, p1–m3, from the Arikaree Group, north of Keeline, Niobrara Co., Wyoming. The large crushing p4 nearly equals the carnassial in size. A, labial and B, lingual views.

Fig. 29. Occlusal stereopair of the mandibular dentition (p4–m3) of Mammacyon ferocior (F:AM 27562). Note absence of m1 and m2 metaconids, apical wear on p4–m2 cusps, a labial shear facet on m1 protoconid-paraconid, and the broad p4 (compare fig. 23).

Fig. 30. Cranium of Mammacyon ferocior (F:AM 54134) in (A) right lateral; (B) dorsal; and (C; opposite page) left lateral (restored) views, from the Arikaree Group, north of Keeline, Niobrara Co., Wyoming, showing (B) width of the frontal paranasal sinuses relative to braincase volume.

Fig. 31. Mammacyon ferocior (F:AM 54134) rostrum (A) in palatal view with right and left P4–M2, right P1, and alveoli for I1–3, C, P2–3, from the Arikaree Group, north of Keeline, Niobrara Co., Wyoming. Stereophotos (B) of crushing P4–M2. This is the largest species of Mammacyon, and represents the climax of durophagy in the genus.