Leptocyon Matthew, 1918

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 : 14-17

Leptocyon Matthew, 1918

Vulpes: Trouessart, 1897: 310 (catalog).

Vulpes: Stirton and McGrew, 1935: 129 (faunal list).

Vulpes: Gregory, 1942: 348 .

Vulpes: Macdonald, 1948: 55 , fig. 1a–b.

Neocynodesmus Macdonald, 1963: 212 .

Type Species: Leptocyon vafer (Leidy), 1858 .

Included Species: L. douglassi , n. sp.; L. mollis (Merriam), 1906 ; L. delicatus (Loomis), 1932 ; L. vulpinus (Matthew), 1907 ; L. gregorii (Matthew), 1907 ; L. vafer (Leidy), 1858 ; L. leidyi , n. sp.; L. tejonensis , n. sp.; L. matthewi , n. sp.; and Leptocyon sp. A and B.

Distribution: Orellan to late Clarendonian; Nebraska, Colorado, Montana, South Dakota, Wyoming, Kansas, Nevada, New Mexico, Arizona, California, and Oregon.

Revised Diagnosis: Leptocyon differs from Vulpes in possession of the following primitive characters: greater postorbital constric- tion; I1–I3 with medial cusplets; M1 and M2 with stronger parastyle; m1 hypoconulid absent, its entoconid relatively smaller and enlarged only in most derived species; m2 with weaker anterolabial cingulum and metaconid smaller than or equal in size to protoconid in less derived species, postparietal foramen still present in early species; humerus with entepicondylar foramen; ulna and fibula short and more robust with larger distal articular facets for radius and tibia; radius with distal exostosis in early species ( Wang and Rothschild, 1992); tibia with longer anteroproximal crest, less anteriorly and distally extended proximolateral and proximomedial articular surfaces for femur; relatively larger entocuneiform and welldeveloped functional metatarsal I with two phalanges.

Discussion: When Leidy (1858: 21) described Canis vafer , he recognized its foxlike features and commented that it did not differ in form from those of the red fox. Trouessart (1897: 310) listed this taxon in his catalog of mammals as Vulpes vafer . It was not until 1918 that Matthew delineated the morphological features that separated this species from those of Canis and Vulpes .

Matthew’s (1918: 189) original description of the genus Leptocyon was largely based on two lower jaws ( AMNH 17201 and 17202; the latter was sent on exchange to the University of Uppsala in 1912) from the early Barstovian Lower Snake Creek Fauna (Olcott Formation, Skinner et al., 1977: 300) in Sioux County, Nebraska. Leptocyon species are now recognized in deposits as old as the Orellan ( Wang and Tedford, 1996). Several species that are considered to belong to Leptocyon (including L. mollis , L. vulpinus , and L. gregorii ) were originally described as Nothocyon , but its genotypic species is now recognized as an arctoid ( Wang and Tedford, 1992). Matthew pointed out the foxlike characters of Leptocyon in his diagnosis, including the slenderness of jaw, long premolar region with diastemata between the teeth, premolars with accessory cusps, m1 talonid with low marginal entoconid crest obscurely divided into two cusps, and m2 with vestigial and shelflike paraconid. A combination of several of these features is useful in distinguishing some jaws of Lepto- cyon from Vulpes , but of all of the characters cited by Matthew, only the primitively low, conical, and sometimes crestlike entoconid serves to separate most jaws of Leptocyon from those of Vulpes .

Some later authors ( Stirton and McGrew, 1935: 129; Gregory, 1942: 348; and Macdonald 1948: 55) placed Leptocyon in Vulpes , but Webb (1969: 40) discussed morphological features that can be used to separate Leptocyon vafer from Vulpes species. Additional evidence permits us to further revise this diagnosis and clearly show how species of the two genera differ. In addition to the low conical or crestlike entoconid on the m1, the talonid in most Leptocyon species is Ushaped in transverse section and the hypoconulid is absent, whereas in Vulpes the entoconid and hypoconid are larger, coalescing at their bases to modify the talonid basin into a V-shaped transverse section, and the hypoconulid is often present. The m2 of Leptocyon differs from that of Vulpes in having a weaker anterolabial cingulum.

Neocynodesmus Macdonald (1963) is placed in synonymy with Leptocyon because the type of the genus and only known species, N. delicatus ( Loomis, 1932) , shares a number of features with Leptocyon . The dental row in the type jaw (ACM 31102) is approximately 40% smaller (appendix 3) than in the next smallest ramus referred to Leptocyon and is thus the smallest known canid.

In comparing the upper dentition of L. vafer with that of V. vulpes, Webb (1969: 40) observed that the protocones on the carnassial and the upper molars are consistently larger and the metaconules weaker than in modern forms. After comparing recent species of Vulpes available in the AMNH(M) collection from Europe, Asia, Africa, and North America, we found that the dental features of Leptocyon , particularly the earlier species, recall those of Hesperocyon . These dental characters are: m1 talonid basin U-shaped in cross-section and hypoconulid absent; m2 with anterolabial cingulum weak to absent, and paraconid present; and M1 and M2 with a relatively strong parastyle and low-crowned paracone and metacone.

Synapomorphous characters that unite Leptocyon and Vulpes are primarily the foxlike characters initially recognized by Matthew, including the slenderness of the jaw and the long premolar region with diastemata separating the premolars. Of special importance, although misinterpreted by Matthew, are the weak premolar cusplets, which are greatly reduced or absent compared to those in hesperocyonines and borophagines.

As indicated by log-ratio diagram (fig. 3), the skull proportions of Leptocyon species show comparable trends. Comparison of Arikareean L. mollis and L. gregorii with Clarendonian L. vafer shows the following differences. In L. mollis and L. gregorii the braincase is relatively narrower than that of L. vafer . The height of the maxilla between the alveolar border and the ventral rim of the orbit and the depth of the jugal are both proportionally less than in L. vafer .

The height of the maxilla between the alveolar border and the ventral rim of the orbit and the depth of the jugal are both proportionally less than in L. vafer . The larger zygomatic process of the jugal in L. vafer is correlated with a deeper and more dorsoventrally expanded depression for the masseter muscle. Furthermore, the skulls of L. mollis and L. gregorii show more cerebellar exposure on the posterior side of the braincase, and the lip of the external auditory meatus is smaller and projects less laterally than in L. vafer . A comparison of the skull of L. vafer with that of V. stenognathus shows somewhat better agreement than it does with more primitive Leptocyon , but L. vafer still differs from Vulpes in having a relatively deeper jugal and greater height of the maxillary from the toothrow to orbit.

A close resemblance exists between the limbs of Leptocyon and Vulpes , but the present study shows that the limbs of Leptocyon are more primitive, with several features in common with Hesperocyon (fig. 9). The entepicondylar foramen is absent in Vulpes , but it is well developed on the humerus of Leptocyon . Although we lack evidence from associated limbs in most cases, we conclude from the unassociated limbs referred to various Leptocyon species that the limb proportions are similar to those of Vulpes . The radius appears to be short relative to both the length of the humerus and the tibia. This is a primitive condition held in common with Hesperocyon and all living foxes. The distomedial radial exostosis in Leptocyon vulpinus is relatively larger than that of Vulpes , but later Leptocyon species show smaller exostoses. This osteochondroma is prevalent in Hesperocyon and seems to have a hereditary basis ( Wang and Rothschild, 1992). Additionally, the ulnae of Leptocyon are more robust than those of Vulpes . Furthermore, the distal articular facets on both the ulna and radius for their mutual articulation are relatively larger than those of Vulpes . The tibia of Leptocyon vafer is shorter and the fibula (F:AM 62780) is more robust and less compressed than that of Vulpes . Moreover, the tibiae of the Leptocyon species differ from those of Vulpes in that the anteroproximal crest is relatively longer, and the proximolateral and medial articular condyles for the femur are less anteriorly and distally extended. These features would restrict flexion of the hindlimb as compared to living foxes.

Although our knowledge of the front foot of most Leptocyon species is limited to a few unassociated elements, a complete rear foot of L. vafer (F:AM 62780) is known from late Barstovian deposits in New Mexico. The tarsals and metatarsals II–V are similar to those of species of Vulpes and are well within the range of variation found in recent foxes. It is the first digit of the pes (fig. 9H) that distinguishes Leptocyon from Vulpes and reveals a surprisingly primitive feature. In Leptocyon vafer the entocuneiform is relatively larger than in Vulpes species , and metatarsal I is well developed and functional. Metatarsal I is 19.0 mm long and has two small phalanges. The length of metatarsal I is approximately 40% that of metatarsal II. Metatarsal I is unknown in the fossil V. stenognathus but in recent foxes it consists of only a short proximal rudiment, although Lonnberg (1916) reported an individual V. vulpes that had both phalanges for this digit. The metatarsal itself had unfortunately been discarded when the animal was skinned.

Primitive limb features of Leptocyon held in common with Hesperocyon are mainly a proportionally short radius relative to the length of the humerus and tibia, a strong radial exostosis, a robust ulna, a strong fibula, and a functional metatarsal I. Thus, the limbs of Leptocyon are less elongate, more robust, and less adapted for running than are those of Vulpes .