http://www.sciencemag.org/content/330/6008/1216.full

The Evolution of Maximum Body Size of Terrestrial Mammals

Felisa A. Smith, Alison G. Boyer, James H. Brown, Daniel P. Costa, Tamar
Dayan, S. K. Morgan Ernest, Alistair R. Evans, Mikael Fortelius, John L.
Gittleman, Marcus J. Hamilton, Larisa E. Harding, Kari Lintulaakso, S.
Kathleen Lyons, Christy McCain, Jordan G. Okie, Juha J. Saarinen, Richard M.
Sibly, Patrick R. Stephens, Jessica Theodor, and Mark D. Uhen:

The Evolution of Maximum Body Size of Terrestrial Mammals

–Science: Vol. 330, no. 6008, pp. 1216-1219
26 November 2010, DOI: 10.1126/science.1194830

Abstract

The extinction of dinosaurs at the Cretaceous/Paleogene (K/Pg) boundary was
the seminal event that opened the door for the subsequent diversification of
terrestrial mammals. Our compilation of maximum body size at the ordinal
level by sub-epoch shows a near-exponential increase after the K/Pg. On each
continent, the maximum size of mammals leveled off after 40 million years
ago and thereafter remained approximately constant. There was remarkable
congruence in the rate, trajectory, and upper limit across continents,
orders, and trophic guilds, despite differences in geological and climatic
history, turnover of lineages, and ecological variation. Our analysis
suggests that although the primary driver for the evolution of giant mammals
was diversification to fill ecological niches, environmental temperature and
land area may have ultimately constrained the maximum size achieved.

-Mikko Haaramo