Leptocyon delicatus (Loomis), 1932

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 : 25-26

Leptocyon delicatus (Loomis), 1932 Figure 12A–B; appendix 3

Pachycynodon delicatus Loomis, 1932: 325 , fig. 8. Neocynodesmus delicatus: Macdonald, 1963: 212 .

Type: ACM 31102, left partial ramus with incisor alveoli, c broken, p1 alveolus, and p2– m2 from the ‘‘Lower Rosebud beds,’’ Porcupine Creek, South Dakota. Macdonald (1963: 212) concluded that it is impossible to determine if this locality is equivalent to the Monroe Creek or Harrison formations.

Distribution: Known only from the locality of the type, early medial Arikareean of South Dakota.

Revised Diagnosis: Differs from all other species of Leptocyon in being approximately 40% smaller; premolar cusplets and cingular cusps generally weaker and especially weaker than those of contemporaneous taxa; m1 metaconid proportionally larger and taller crowned relative to size and height of protoconid; and m2 lacks a paraconid.

Description and Comparison: To our knowledge, the type and only known specimen of L. delicatus ( Loomis, 1932) is the smallest known canid. Although the canine is broken in the type ramus, its size relative to that of the ramus corresponds to that of Leptocyon and the canine is separated from the p1 by a moderately long diastema. The first lower premolar is absent, but the alveolus indicates a small single-rooted tooth that is separated from p2 by a short diastema. The premolars are especially like those of Leptocyon : p2–p4 are widely spaced, slender, elongate, and tall-crowned, with the crown height of the p4 exceeding the height of the m1 paraconid. The second premolar lacks an anterior cingular cusp that is present in some species of Leptocyon , but it has a minute posterior cingular cusp. Both p3 and p4 have anterior cingular cusps as in contemporary larger species of Leptocyon . Additionally, p3 has a minute posterior cusp and p4 a weak posterior cusp. The posterior cusps and cingular cups are relatively weaker than those of other species of Leptocyon .

Morphologically, the molars are similar to those of the much larger jaws referred to L. gregorii (F: AM 49063) from rocks equated with the Harrison Formation in Wyoming. The m1 is slender with a widely open trigonid and a tall-crowned protoconid as in the latter specimen. The metaconid, however, differs from L. gregorii in being proportionally larger and taller crowned relative to the size and height of the protoconid. The features of the metaconid are plesiomorphous and held in common with Hesperocyon . The m1 talonid is narrow and short relative to the length of the trigonid, with a strong hypoconid and a small conate entoconid that are often present in Leptocyon and always found in Vulpes . The m2 is small relative to the size of the m1 and morphologically resembles that of the above specimen referred to L. gregorii (F: AM 49063). An extremely weak anterolabial cingulum is present on m2, as in early occurring species of Leptocyon . A distinct paraconid is absent, but a shelf is formed by the joining of weak crests extending anteriorly from the protoconid and metaconid. The protoconid and metaconid are situated anteriorly and subequal in size. A narrow talonid has a welldeveloped hypoconid and a low crestlike entoconid. X-ray examination of the type shows that the m3 was not present in this individual.

Discussion: Loomis (1932: 325) described Pachycynodon delicatus from the Lower Rosebud of South Dakota, and stated, ‘‘this tiny lower jaw is almost identical in size with P. tenius of the Phosphorites of Quercy, differing only in that the teeth are slightly longer.’’ Macdonald (1963: 212) erected a new genus, Neocynodesmus , and designated P. delicatus Loomis (1932) its genotypic species. In his study of the Miocene faunas of the Wounded Knee Creek area, South Dakota, Macdonald limited his comparison of the type of Neocynodesmus delicatus to that of a new species, Cynodesmus cooki , which he described in the same report. Macdonald commented that the ‘‘close resemblance of the carnassial to that of Cynodesmus cooki suggests that this form may have been derived from that slightly older species.’’ Additional F:AM specimens of ‘‘ C. ’’ cooki (now placed in the borophagine genus Otarocyon Wang et al., 1999 ) show that the premolars of L. delicatus are more slender with weaker anterior and posterior cingular cusps and less prominent posterior cusp on p3 and p4. Furthermore, the lower carnassial of L. delicatus differs markedly from that of ‘‘ C. ’’ cooki in the manner typical of the canines in that its m1 is more elongate and slenderer, the trigonid lower, the paraconid less oblique, and the talonid narrower with a much smaller entoconid relative to the size of the hypoconid.

Macdonald failed to compare L. delicatus with specimens of Leptocyon that also occur in the ‘‘Lower Rosebud beds.’’ Except for the lack of an m3, the mandibular and dental features of this tiny ramus are foxlike. Because all canids possess a third lower molar, an X-ray was taken of the type jaw but no unerupted m3 is present. The absence of the m3 prompted a comparison with the mustelids and procyonids. The outstanding general features of the type jaw of L. delicatus that separate it from members of those arctoid families are: (1) mandibular ramus shallow, elongate, and tapering anteriorly; (2) premolars elongate, slender, and widely spaced; (3) slender m1 with trigonid tall and open with small, but distinct, entoconid that is generally absent in the mustelids; and (4) m2 large, relative to m1, with strong and subequal metaconid and protoconid, and trigonid about equivalent in length to talonid.

Most of the above features that separate L. delicatus from the mustelids and procyonids are shared with Leptocyon , including: m1 entoconid distinct; mandibular ramus shallow and thin; premolars narrow and elongate; p4 with weak posterior cusplet; anterior premolars separated by short diastema; m1 without complete labial cingulum; and m2 with weak anterolabial cingulum. The absence of the m3 is considered an anomalous feature in this otherwise canidlike tiny jaw. Although the type is about 40% smaller than the next smallest jaw ( CM 1300) referred to Leptocyon , it is assigned to this genus because of the listed synapomorphies and the lack of features that would ally it to more derived canine genera.