Fossil canids from the Mehrten Formation, Late Cenozoic of Northern California
The Hemphillian-age Mehrten Formation has yielded several fossil dogs (family Canidae). Initial study had identified all larger canids as Borophagus secundus ( = Osteoborus cyonoides), a bone-crushing borophagine (a diverse subfamily of extinct canids), and all smaller canids as Canis cf. C. davisi, a canine (a subfamily including all living canids). Later analysis split B. secundus into two taxa and reassigned Canis cf. C. davisi to Eucyon davisi, yet omitted fragmentary remains of possible vulpines. Despite this wealth of Mehrten canid materials, a detailed systematic treatment is lacking. More recently, additional specimens have become available, further necessitating a systematic examination of the canids at this site. The present study finds four canid species: two borophagines (Borophagus parvus and B. secundus) and two canines (E. davisi and Vulpes stenognathus, newly identified). We support the division of ‘B. secundus’ into the smaller B. parvus and the larger, more hypercarnivorous B. secundus and find two morphotypes within B. parvus. A specimen from the Coyote Hill fauna, earliest in the Mehrten sequence, is reassigned to B. secundus, extending the temporal range of the species in California into the early Hemphillian. Borophagus parvus co-occurs with B. secundus only in the lower Modesto Reservoir Member; higher in the sequence, B. parvus co-occurs only with canines. Despite the apparent turnover in its co-inhabitants, B. parvus exhibits no size change. Given that the characteristics of B. parvus and B. secundus are intermediate among other Borophagus, the Mehrten canids may represent a rapid transition early in the evolution of the Borophagus lineage.
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Citation for this article: Balisi, M., X. Wang, J. Sankey, J. Biewer, and D. Garber. 2018. Fossil canids from the Mehrten Formation, Late Cenozoic of Northern California. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2017.1405009.
