MORPHOLOGY AS A MEANS FOR DETERMINING THE PALEOECOLOGY OF A NORTH AMERICAN OLIGOCENE CANID (MAMMALIA:CARNIVORA)

Over the last 40 million years a variety of canid taxa have evolved, radiated, and gone extinct in North America. This ancient and modern canid diversity is distributed between three well recognized clades: Hesperocyoninae, Borophaginae, and Caninae, of which only the latter is extant. During the late Oligocene (early Arikareean NALMA) stem taxa of each clade coexisted represented by Hesperocyon, Archaeocyon, and Leptocyon, respectively. Canid fossil remains are here reported from the gritstone and sandstone-mudstone members of the Otay Formation (30-28 Ma) of SW San Diego County, California. These fossils, San Diego Natural History Museum (SDNHM) 111027, SDNHM 73132, and ACC 12 preserve articulated limb bones. In addition, SDNHM 111027 and ACC12 include partial cranial and mandibular dentitions that permit identification of these specimens as the diminutive borophagine, Archaeocyon sp. Morphometric analysis of the postcranial remains of the Otay Formation canid were conducted to determine body size (weight) and reconstruct locomotion and hunting behaviors.

Morphometric data suggest that Archaeocyon was a generalist, displaying evolutionary adaptations primarily for scansorial and terrestrial locomotion behavior, with additional evidence suggesting semi-fossorial adaptations as well. The change in climate from the Eocene tropical forests to the spreading of the late Oligocene grasslands may have allowed for these non-arboreal adaptations. Archaeocyon’s small body size, similar to that of a Chihuahua, would have been preyed upon by larger predators, forcing Archaeocyon to seek out shelter. These locomotion adaptations may have allowed Archaeocyon to survive and seek shelter by burrowing, instead of climbing trees, and by increasing terrestrial speed in the open grasslands. Additionally, predatory behavior from the morphology suggests these canids were ambush predators adapting towards a pounce-pursuit hunting behavior, another possible effect from the changing climate, implying a lack of social pack-hunting behavior.