BODY-SIZE EVOLUTION AND ECOLOGICAL REPLACEMENT IN TERRESTRIAL VERTEBRATES

The paleontological literature contains many examples of putative ecological replacements of one clade by another over evolutionary timescales. For example, many studies have documented an expansion of the body size niche by mammals after the K/Pg extinction, which is widely thought to have resulted from ecological release after the extinction of the non-avian dinosaurs. Here, we examine the evolution of body size in 4 clades of terrestrial vertebrates (dinosaurs, mammals, pterosaurs, and birds) by compiling data for 3684 species (63 pterosaurs, 45 birds, 3035 mammals, and 541 non-avian dinosaurs) spanning the last 250 million years (Triassic-Holocene). Although dinosaurs and mammals originated at roughly the same time (~210-220 Ma), dinosaurs quickly expanded their body size niche and filled most ecological roles in terrestrial communities, whereas contemporaneous (Mesozoic) mammals remained restricted to small-bodied species (<10-15 kg) and inhabited a limited set of ecological roles. Unlike the evolution of body size in mammals, which is consistent with a driven trend, the evolution of body size in dinosaurs is consistent with a random walk. Flying organisms show a different pattern. Pterosaurs evolved during the Mesozoic and remained fairly small (~1 kg) for the first 100 Ma of their evolution. However, after birds evolved, pterosaurs underwent a driven shift to larger size with birds occupying the smaller body size niche. After the extinction of the pterosaurs and the non-avian dinosaurs, mammals rapidly expanded their body size niche and birds underwent a driven shift to larger size. In both cases, an ecological release is a viable causative mechanism. Body size data from these four clades supports the role of ecological interactions in driving the body size evolution of terrestrial vertebrates over geological timescales.

Additional co-authors: Paula A. Spaeth Anich, Steve C. Wang and Meng Chen