DECOUPLED DYNAMICS IN BODY MASS DISTRIBUTION BETWEEN LARGE NORTH AMERICAN MAMMALIAN HERBIVORE AND CARNIVORE GUILDS

An organism’s body mass is closely related to its physiology, life history, and evolutionary dynamics. Previous studies indicate that both large herbivore and carnivore body masses show persistent, and at times coincident, increases through the Cenozoic. These parallel increases in body mass suggest an ecological link between the two guilds. Here, we further explore this by characterizing distribution of body masses of species within each guild, then testing for shifts in these distributions over the Cenozoic. We estimated the body masses of 876 herbivore species using the lengths and widths of upper and lower cheek teeth and those of 362 predator species using the length of the lower carnassial (m1).

The herbivore guild shows eight significant changes regularly throughout the Cenozoic. In contrast, the predators show only five significant shifts in body mass distribution at 57, 41, 25, 21, and 11 Ma. The shift at 57 Ma is most likely the result of the appearance and diversification of several new families. The second shift (~41 Ma) corresponds with the loss of many of the smaller bodied (<1 kg) taxa and the continued concentration of taxa within the 1-100 kg range. The shift at 21 Ma corresponds to the diversification of taxa that are greater than 7 kg, and further decline of smallest bodied taxa. Interestingly, this coincides with a similar shift in herbivores toward medium and large sizes, at the expense of smaller species. The shift at 11 Ma is due largely to the diversification of small (<1 kg) mustelids, and clearly not a direct response to large herbivores. In all, most shifts in carnivore body mass distributions seem to be distinct in time and nature from those in large herbivores. Despite the long-term parallel trends of increasing average body mass in both guilds, the distributions of body masses of each guild show more complex and mostly decoupled dynamics.