Leptocyon sp. A

TEDFORD R. H., WANG X. & TAYLOR B. E., 2009, Phylogenetic Systematics Of The North American Fossil Caninae (Carnivora: Canidae), Bulletin of the American Museum of Natural History 2009 (325), pp. 1-218 : 17-18

Leptocyon sp. A Figure 4

Leptocyon sp. , Wang and Tedford, 1996: 447, fig. 9.

Material: UNSM 25354, fragment of left ramus with p3–p4, and m1 from the Orella Member, Brule Formation, White River Group (Orellan), Sioux County, Nebraska.

Description and Comparison: This specimen is the sole evidence for the presence of Leptocyon and hence the Caninae at the beginning of the Oligocene. The premolars are the most diagnostic elements in this attribution. The p3 is a thin blade (5.3 mm in length, 2.0 mm in width), and its principal cusp is as tall as that of P4. There are no anterior or posterior cusps, but the anterior cingulum is produced into a cuspule-like structure. The posterior cingulum forms a short shelf but is not produced into a cuspule nor does it join the anterior cingulum along the labial face of the crown. The p3 and p4 touch one another in a slightly imbricate fashion. A prominent posterior cusp is present on p4, set slightly labially from the longitudinal midline of the tooth. Anterior and posterior cingula are present and the labial side of the crown bears a low cingulum. The posterior cingulum extends anteriorly on the lingual side of the tooth to a point opposite the posterior cusp. The crown of p4 is slender (6.0 mm in length, 2.4 mm in width) and is 79% of m1 length, with the principal cusp exceeding the height of the paraconid of m1. The m1 is relatively short (7.6 mm in length, 3.7 mm in trigonid width, 3.4 mm in talonid width) and its talonid is 68% of the trigonid length. The length of the paraconid blade is equal to that of the protoconid; it is deflected inward, forming a closed trigonid. The protoconid is a high cusp made more conspicuous by the shortness of the tooth as a whole. The protoconid is also labially inflated to a greater degree than in other members of the genus. The metaconid is large and its tip lies above that of the paraconid. The talonid is bicuspid, with the entoconid about as high as the hypoconid. Both outline a deep talonid basin partly closed posteriorly by a tiny hypoconulid that arises from the postentocristid. There is a well-developed cingulum that can be traced completely along the labial side of the tooth. The ramal fragment is shallow (about the height of the m1 protoconid), tapering only slightly forward. There is a small posterior mental foramen beneath the short diastema between p2 and p3.

This specimen is slightly smaller than the holotype and referred specimens of L. douglassi . It differs from the latter and other species of the genus in the lack of a posterior cusp on p3, the relatively short m1 with a short and inwardly deflected paraconid, a high and labially inflated protoconid, and a conspicuous labial cingulum. Except for the well-developed entoconid, the m1 resembles those of contemporary early Oligocene Hesperocyon gregarius rather than later Oligo- cene Leptocyon douglassi with its elongate m1, open trigonid, and subdued labial cingulum.

Discussion: Wang and Tedford (1996) brought this specimen to notice as the earliest defensible member of the Caninae . Its early Oligocene (Orellan) occurrence corroborates the predicted synchronous appearance of members of the sister taxa, the Borophaginae and Caninae . Nevertheless, there are some decidedly primitive features of the m1 that recall Prohesperocyon wilsoni , especially the short, closed trigonid and the labial inflation of the high protoconid that togeth- er resemble the former taxon more than Hesperocyon gregarius and recall the condition of the trigonid in species of Miacis ( Wang and Tedford, 1994) . Despite these primitive features of the trigonid, the broad, strongly bicuspidate talonid and the elongate, slender premolars lacking accessory cusps on p3 and having only a posterior cusp on p4 are characters that ally Leptocyon sp. A first with the borophagines and secondly with the canines. This mosaic of features gives some idea of the sequence of character acquisition in an early member of the Caninae as already pointed out by us ( Wang et al., 1999).