Abstracts and program for the February 14-15, 2015, WAVP Annual Meeting, California State University, Stanislaus, Turlock, CA, USA.

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. SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP 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.

Oncorhynchus rastrosus was a very large, spike-toothed, Pacific Salmon from the mid-Miocene to early Pliocene of the Pacific Northwest (California to Washington). It had two large premaxillary (breeding-fighting) teeth that stuck out laterally from the snout like spikes. It migrated from the Pacific Ocean to inland rivers to spawn, as extant Pacific salmon do today. It was planktivorous, based on numerous, long, over-lapping gill-rakers, and few, small teeth. There are gaps in our knowledge about this interesting salmon. First, one of the localities where many of the paratype specimens were collected (Turlock Lake, California), was not described geologically in the original paper beyond, ‘from the Mehrten Formation’. Here we describe these deposits as cross-bedded sands, gravels and large, rounded cobbles indicative of a fast-flowing, river, which we coin here as the “proto-Tuolumne River,” that periodically overflowed its banks. Paleocurrent directions indicate the river flowed to the southwest, where it joined the San Joaquin River in the Central Valley and flowed southward at this time (~5 Ma), emptying into a marine embayment near Bakersfield. Thus, the Turlock Lake O. rastrosus specimens would have migrated up from this embayment. Second, we investigated whether O. rastrosus developmentally changed before migration upriver to spawn, as extant Pacific salmon do today, by comparing premaxillary teeth from freshwater and coastal marine deposits in California. We found that the largest teeth (with the largest osseous tooth bases) were from freshwater deposits, and that these specimens had the most worn and blunt tooth tips. We propose that this was due to use in territorial defense during spawning and redd (nesting site) construction.

Borophagine canids have long been hypothesized to be North American ecological 'avatars' of living hyenas in Africa and Asia, but direct fossil evidence of hyena-like bone consumption is hitherto unknown. We report rare coprolites (fossilized feces) of Borophagus parvus from the late Miocene of California and, for the first time, describe unambiguous evidence that these predatory canids ingested large amounts of bone. Surface morphology, micro-CT analyses, and contextual information reveal (1) droppings in concentrations signifying scent-marking behavior, similar to latrines used by living social carnivorans; (2) routine consumption of skeletons; (3) undissolved bones inside coprolites indicating gastrointestinal similarity to modern striped and brown hyenas; (4) B. parvus body weight of ~24 kg, reaching sizes of obligatory large-prey hunters; and (5) prey size ranging ~35-100 kg. This combination of traits suggests that bone-crushing Borophagus potentially hunted in collaborative social groups and occupied a niche no longer present in North American ecosystems.