Series: Bulletin of the American Museum of Natural History (AMNH Bulletins) Volume: 221
207 pages, b/w photos, illustrations
The subfamily Hesperocyoninae includes the oldest and most primitive members of the Canidae. It has a geological range from late Eocene (Duchesnean) to middle Miocene (early Barstovian), lasting more than 20 million 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 place. It achieved a maximum diversity of 14 species in the early Arikareean.
The present study describes the extraordinary fossil canids from the Frick Collection. It contains many new taxa that provide new insight into the phylogeny of the hesperocyonines, and also more complete materials of known forms that allow better assessments of morphological variations. Besides the basal canid Prohesperocyon, a total of 10 genera and 26 species of hesperocyonines are recognized; among these, 4 genera and 8 species are new. A phylogenetic analysis is performed using cladistic methodology, with Miacis as an outgroup.
Four major lineages can be defined on the basis of shared derived characters: the Mesocyon-Enhydrocyon clade, the Osbornodon clade, the Paraenhydrocyon clade, and the Ectopocynus clade. The latter three clades are newly recognized, although some of their included species had previously been referred to various other taxa. In addition, a monotypic Caedocyon probably forms a distinct clade of its own. Hesperocyon occupies the central position of possibly being ancestral to many of the lineages. Its lack of shared derived characters, however, precludes it from being closely linked to any particular derived clade, although there is evidence that the Paraenhydrocyon clade may be directly descendant from H. gregarius. A new, large-sized Hesperocyon ('H.' coloradensis) provides an important link between H. gregarius and the Mesocyon-Enhydrocyon clade. The Mesocyon-Enhydrocyon clade is the most diverse lineage of hesperocyonines and includes 10 species.
Mesocyon and Cynodesmus, two medium-sized genera with numerous species previously referred to, are now restricted to include two species each. Sunkahetanka and Philotrox represent two transitional taxa leading to the hypercarnivorous Enhydrocyon. Trends within this lineage include elongation of the posterior portion of the skull so that the rostrum appears to be short, enlargement of carnassials, and increasingly robust premolars. Materials of a large species from mostly the Frick Collection lead to the recognition of a new genus and species Osbornodon fricki. Four additional species are referred to the genus, three of them having been previously described (O. sesnoni, O. iamonensis, and O. brachypus) and one new species (O. renjiei) being the most primitive of the genus. Osbornodon is the only genus of the Hesperocyoninae that developed a basined talonid in the lower molars and long upper molars, features paralleling those of the Borophaginae and Caninae. In addition, Osbornodon has an elongated rostrum and an extensive frontal sinus (in derived species), a combination of characteristics also seen in the Caninae.
A sister-group relationship between the Osbornodon clade and the Mesocyon-Enhydrocyon clade is suggested by their common possession of a ventrally directed paroccipital process. The three species presently referred to Paraenhydrocyon (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-tipped premolars, clearly contrasts with the robust premolars of the Mesocyon-Enhydrocyon clade. Paraenhydrocyon does, however, parallel the Mesocyon-Enhydrocyon clade in having a reduced metaconid of the lower molars. Represented by a single partial cranium, Caedocyon is a highly apomorphic form, with enlarged, caniniform upper third incisors, shortened upper premolars, and reduced upper molars. Its phylogenetic position is not clear, although its simple, high premolars and the precision of its occlusion suggest affinity with Paraenhydrocyon. Ectopocynus (new genus with three species, all from the Frick Collection) is mainly characterized by its simple, robust, and blunt premolars, and approaches many of the characteristics of Enhydrocyon, such as massive premolars and reduced or lost metaconid on lower molars. Materials of Ectopocynus are limited to mandibles and teeth only.
Its unknown cranial anatomy does not permit a firm answer about its phylogenetic affinity, although a primitive member of this lineage (Ec. antiquus) suggests affinity with the Hesperocyon-like forms of the Whitneyan. Hesperocyoninae, as presently defined, is likely a paraphyletic taxon. The subfamily may be characterized by possession of a trenchant talonid of lower first molars, in contrast to basined talonids in the Borophaginae and Caninae. The trenchant talonid, however, is clearly primitive for the Hesperocyoninae (and also for the family Canidae), and no synapomorphy is found to support a monophyletic Hesperocyoninae. The paraphyly of the Hesperocyoninae is also sugested by its primitive genus Hesperocyon, which is probably ancestral to both borophagines and canines. Evolutionary trends in hesperocyonines include repeated tendencies toward hypercarnivory, involving large body size, strong cranial construction, robust premolars, and increasing length of carnassials. Such tendencies can be observed in nearly all of the clades, although each has a different 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.
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