THE EVOLUTION OF CARNIVORAN HUNTING STYLES AT JOHN DAY FOSSIL BEDS NATIONAL MONUMENT

The shift from closed to open environments from the late Oligocene through the Miocene spurred adaptions for distance running in mammals across North America. While this general pattern is well-documented, the impact of environmental change on locomotor evolution may have varied between different clades of mammals within different ecological niches, and evolutionary constraints or competition may have been more influential on certain groups. We used the near-continuous record of ecosystem change from 31 to 7 Ma preserved by in the John Day Basin of eastern Oregon to test for differing evolutionary responses to the same regional environmental change, focusing on change in carnivore hunting style within these formations. Among extant predators ambush hunters, which hide then charge their prey, are more frequently found in closed, forested environments, while pounce-pursuit and pursuit hunters, adapted for longer chases, inhabit more open environments. An increase in pounce-pursuit and pursuit hunters relative to ambush hunters would be expected with the spread of open habitat in the Miocene if the environment was the major driver in carnivore locomotor evolution. We used geometric morphometrics of the distal humeral trochlea in conjunction with discriminant function analysis to classify each specimen as ambush, pounce-pursuit, or pursuit hunters. We applied these specimen specific classifications to the entire species duration at John Day Fossil Beds using the geology as time bins. We found that most predators from the Turtle Cove Member of the John Day Formation (~30 Ma) were ambush hunters, with three of 18 confidently classified as pounce-pursuit. Among canids we found an increase in pounce-pursuit hunters through time, supporting changing environment as a major driver for canid locomotor evolution. All nimravids strongly classified as ambush predators and did not occur after the Kimberly Member of the John Day Formation (~26 Ma). Among amphyicyonids, one from the Turtle Cove Member classified as pounce-pursuit, in the Kimberly Member, one species classified as a pursuit hunter, but the last, from the Mascall Formation (~16 Ma), classified as an ambush hunter. These patterns of locomotor evolution among amphyicyonids and nimravids suggest factors other than environment at play.