How big is a giant? The importance of method in estimating body size of extinct mammals

Abstract

Estimating body size of extinct mammals presents problems when size can be estimated only by extrapolation. I examined the influence of phylogenetic, biomechanical, and statistical assumptions on body size estimates for 2 species of fossil castorids, the Pleistocene "giant" beaver Castoroides and the fossorial Miocene beaver Palaeocastor. Prior descriptions of Castoroides as "black-bear sized" were greatly exaggerated; new analyses estimated body masses at 60-100 kg. Estimates for Palaeocastor were similar to previously published estimates (0.8-1.2 kg). Biologically realistic size estimates were based on femur length and on interspecific data covering a wide range of body mass; skull length measurements or extrapolations from an ontogenetic single-species data set resulted in excessively large body mass estimates. Body mass must be estimated with due attention to the choice of both morphological trait and reference taxa.