ANTILOCAPRIDAE Gray, 1866

Brian, Larry, L. Beatty, Martin, D., Ny & Ks, 2009, The earliest North American record of the Antilocapridae (Artiodactyla, Mammalia), PaleoBios 29 (1), pp. 29-35 : 30-32

FAMILY ANTILOCAPRIDAE Gray, 1866 GENUS: indeterminate

Referred material — KUVP 48020, a partial left mandible.

Diagnosis —Moderately hypsodont selenodont lower molars with small ectostylids, but lacking other accessory features (such as metastylids found in later Paracosoryx ). Length of m2 = 6.8 mm, width = 5.73 mm (N=1). The hypoconulid of the m3 has a sulcus on the posterolabial aspect, extending dorsoventrally for the entire exposed length.

Comments —The only antilocaprid yet known from the Arikareean. Taxonomic assignment is provisional pending revision of the Merycodontinae .

DESCRIPTION Mandible and dentition KUVP 48020 ( Figs. 2–4) is a partial mandible with all

three molars intact and fully erupted. Except for the broken distal root of the p4, the mandible anterior to the first molar is missing. The body of the mandible immediately below the teeth is complete and the inferior beginning of the ramus is preserved. Nothing remains of the angle of the mandible, the coronoid process or the mandibular condyle posteriorly, though the mandibular foramen is preserved. The depth of the mandible below the m2 = 11 mm. It steadily increases to 19mm at the level of the m3 hypoconulid, most likely to accommodate a more hypsodont m3. Other merycodontines, such as Paracosoryx , maintain a roughly uniform mandibular depth along these teeth, suggesting that this Arikareean merycodontine had slightly more hypsodont m3s or an enlarged site of attachment for the masseter and/or medial pterygoid muscles, or both.

The dentition is in an advanced, but not senescent stage of wear, with all dentine lakes connected, including that of the hypoconulid. The infundibulum of the m1 is nearly worn away, and if there was an infundibulum of the m3 hypoconulid, it is now missing. All that remains of the p4 is part of the distal root in the broken end of the mandibular body.

Unlike Merycodus, KUVP 48020 possesses ectostylids on the lower m1 and m2 (this region in the m3 is covered in calculus). Unlike Paracosoryx , it does not have any metastylids. An apomorphic feature of KUVP 48020 is the hypoconulid of the m3. A shallow vertical groove on its postero-labial edge makes the hypoconulid C-shaped in cross section (visible in occlusal view). Due to the advanced stage of wear, it is uncertain whether this hypoconulid was one solid cusp or a double posterior lobe. Because this shallow vertical groove is on the labial aspect, it differs from both conditions (open and closed) of the posterior lobe sensu Janis and Scott (1987). This posterolabial groove of the m3 hypoconulid is not reported in Paracosoryx or Merycodus ( Janis and Manning 1998) , and is not present in any specimens identified as such in the AMNH collections.

Comparisons

Differentiating this mandible from other Late Arikareean artiodactyls of similar size is difficult, particularly because many taxa within this size range lack both cranial appendages and well-diagnosed dental characteristics. Future studies of intraspecific variation from large samples of these relatively smaller artiodactyls may better clarify their range of variation. The combination of dental characteristics and mandibular depth preserved in KUVP 48020 is not found in other contemporaneous selenodont artiodactyl groups, including gelocids, leptomerycids, hypertragulids, moschids and camelids.

The Family Gelocidae is typically characterized by an anterior cingulum on the lower molars ( Janis and Scott 1987) and brachydont lower molars ( Métais and Vislobokova 2007). Like KUVP 48020, gelocids lack metastylids, although this is where the similarities end. Gelocids have only a remnant of a paraconid present, crowded metaconids and entoconids and a trace of the Dorcatherium fold ( Métais and Vislobokova 2007). Pseudoceras , the only known North American gelocid ( Frick 1937, Webb and Perrigo 1984), differs from KUVP 48020 in having a posteriorly directed metaconid and a hypoconid enclosing a narrow fossettid ( Métais and Vislobokova 2007).

Leptomerycids are perhaps the best candidate outside of merycodontines for the placement of KUVP 48020 based primarily on size and the lack of distinctive lower molar characters, although their mandibular ramus depth is much less. Pronodens is larger than KUVP 48020 with broader lower molars (molar width holotype m2 = 7.2 mm) ( Koerner 1940, Métais and Vislobokova 2007).

The hypertragulids, including Hypertragulus and Nanotragulus , share one feature with KUVP 48020: an enlarged masseteric fossa and mandibular angle ( Frick 1937). Though the mandibular angle is not preserved in KUVP 48020, the posterior portion of the inferior rim of the mandibular ramus preserves the ventral projection of the most anterior part of an enlarged masseteric fossa. This ventral projection can be seen in Hypertragulus as well, though it should be noted that hypertragulids (including the holotype of Hypertragulus, AMNH 6815) otherwise differ from KUVP 48020 in having brachydont lower molars with a shallow mandibular ramus depth. Also, Hypertragulus has prominent anterior cingula as well as accessory cuspids, unlike KUVP 48020 ( Vislobokova 1998, Webb 1998, Métais and Vislobokova 2007).

The only members of the Moschidae that immigrated to North America were the Blastomerycinae, which consist of six genera ( Prothero 2007). Unlike gelocids and KUVP 48020, moschid lower molars have metastylids like Paracosoryx . Unlike moschids, antilocaprid metaconid crests (including that of Paracosoryx and KUVP 48020) are not anteriorly directed to meet the paraconid crest ( Webb 1998).

KUVP 48020 can be differentiated from camelids in general because it lacks the labial ribs and entostylids of the postcanine dentition that are considered characteristic of the Camelidae . Of the stenomylines, Stenomylus is the only known Arikareean taxon, and although its molars are also hypsodont, the m3s of stenomyline camels are characteristically extremely elongated anteroposteriorly ( Honey et al. 1998), which KUVP 48020 is not. The protolabine camelid Michenia is known from this time period, but is differentiated from KUVP 48020 by its more slender mandibular ramus and larger teeth (length of m2 = 16–24.8 mm) ( Frick and Taylor 1971, Honey et al. 1998).

Lastly, KUVP 48020 is differentiated from the contemporary ruminant, Delahomeryx , on the basis of molar morphology. The dentition of Delahomeryx is larger than KUVP 48020 ( Delahomeryx m2 length 14.4 mm). The entoconid overlaps the hypoconulid and the protocone has unusual intercolumnar tubercles ( Stevens et al. 1969), unlike KUVP 48020.