PHYLOGENETIC SYSTEMATICS OF THE HESPEROCYONINAE (CARNIVORA, CANIDAE)

The subfamily Hesperocyoninae includes the oldest and most primitive m embers of the Canidae. It has a geological range from Late Eocene (Duc hesnean) to Middle Miocene (Early Barstovian), lasting more than 20 mi llion years. Representatives of the Hesperocyoninae are known only in North America. Initial diversification of the Hesperocyoninae began in the Orellan, and by the Whitneyan, all of its main lineages were in p lace. It achieved a maximum diversity of 14 species in the early Arika reean. The present study describes the extraordinary fossil canids fro m the Frick Collection. It contains many new taxa that provide new ins ight into the phylogeny of the hesperocyonines, and also more complete materials of known forms that allow better assessments of morphologic al variations. Besides the basal canid Prohesperocyon, a total of 10 g enera and 26 species of hesperocyonines are recognized; among these, 4 genera and 8 species are new. A phylogenetic analysis is performed us ing cladistic methodology, with Miacis as an outgroup. Four major line ages can be defined on the basis of shared derived characters: the Mes ocyon-Enhydrocyon clade, the Osbornodon clade, the Paraenhydrocyon cla de, and the Ectopocynus clade. The latter three clades are newly recog nized, although some of their included species had previously been ref erred to various other taxa. In addition, a monotypic Caedecyon probab ly forms a distinct clade of its own. Hesperocyon occupies the central position of possibly being ancestral to many of the lineages. Its lac k of shared derived characters, however, precludes it from being close ly linked to any particular derived clade, although there is evidence that the Paraenhydrocyon clade may be directly descendant from H. greg arius. A new, large-sized Hesperocyon (''H.'' coloradensis) provides a n important link between H. gregarius and the Mesocyon-Enhydrocyon cla de. The Mesocyon-Enhydrocyon clade is the most diverse lineage of hesp erocyonines and includes 10 species. Mesocyon and Cynodesmus, two medi um-sized genera with numerous species previously referred to, are now restricted to include two species each. Sunkahetanka and Philotrox rep resent two transitional taxa leading to the hypercarnivorous Enhydrocy on. Trends within this lineage include elongation of the posterior por tion of the skull so that the rostrum appears to be short, enlargement of carnassials, and increasingly robust premolars. Materials of a lar ge species from mostly the Frick Collection lead to the recognition of a new genus and species Osbornodon fricki. Four additional species ar e referred to the genus, three of them having been previously describe d (O. sesnoni, O. iamonensis, and O. brachypus) and one new species (O . renjiei) being the most primitive of the genus. Osbornodon is the on ly genus of the Hesperocyoninae that developed a basined talonid in th e lower molars and long upper molars, features paralleling those of th e Borophaginae and Caninae. In addition, Osbornodon has an elongated r ostrum and an extensive frontal sinus (in derived species), a combinat ion of characteristics also seen in the Caninae. A sister-group relati onship between the Osbornodon clade and the Mesocyon-Enhydrocyon clade is suggested by their common possession of a ventrally directed paroc cipital process. The three species presently referred to Paraenhydrocy on (a new genus) were previously included in Mesocyon, and now form a clade of their own. In addition to its retention of several primitive cranial characters, Paraenhydrocyon, with its unique slender, sharp-ti pped premolars, clearly contrasts with the robust premolars of the Mes ocyon-Enhydrocyon clade. Paraenhydrocyon does, however, parallel the M esocyon-Enhydrocyon clade in having a reduced metaconid of the lower m olars. Represented by a single partial cranium, Caedocyon is a highly apomorphic form, with enlarged, caniniform upper third incisors, short ened upper premolars, and reduced upper molars. Its phylogenetic posit ion is not clear, although its simple, high premolars and the precisio n of its occlusion suggest affinity with Paraenhydrocyon. Ectopocynus (new genus with three species, all from the Frick Collection) is mainl y characterized by its simple, robust, and blunt premolars, and approa ches many of the characteristics of Enhydrocyon, such as massive premo lars and reduced or lost metaconid on lower molars. Materials of Ectop ocynus are limited to mandibles and teeth only. Its unknown cranial an atomy does not permit a firm answer about its phylogenetic affinity, a lthough a primitive member of this lineage (Ec. antiquus) suggests aff inity with the Hesperocyon-like forms of the Whitneyan. Hesperocyonina e, as presently defined, is likely a paraphyletic taxon. The subfamily may be characterized by possession of a trenchant talonid of lower fi rst molars, in contrast to basined talonids in the Borophaginae and Ca ninae. The trenchant talonid, however, is clearly primitive for the He sperocyoninae (and also for the family Canidae), and no synapomorphy i s found to support a monophyletic Hesperocyoninae. The paraphyly of th e Hesperocyoninae is also suggested by its primitive genus Hesperocyon , which is probably ancestral to both borophagines and canines. Evolut ionary trends in hesperocyonines include repeated tendencies toward hy percarnivory, involving large body size, strong cranial construction, robust premolars, and increasing length of carnassials. Such tendencie s can be observed in nearly all of the clades, although each has a dif ferent combination of these tendencies and achieves different degrees of hypercarnivory. Truly hypocarnivorous taxa, on the other hand, did not develop as in Borophaginae and Caninae.