Paleomammology: Body mass evolution

Fig.1. "Indricotherium" as imagined by Z. Burian in 1972 (found in SPINAR 1976).

The teeth of mammals are distinctive features; in most cases it is possible from a recovered single tooth to identify the family and often even the species. The size and kind of the tooth also helps to estimate the body weight and the life habits of its former owner.
A new database compiled with these estimated values, especially body masses, helped to reconstruct mammalian evolution since the end of the Cretaceous. The mass extinction that marks the Cretaceous-Palaeogene limit was the starting point for the increase in body mass of the mammals.
In a geological short time of 25 to 30 million years mammals increased their body weight, ranging between 3 grams to 15 kilograms during the Cretaceous, by a factor of thousand on all continents and in almost all mammalian lineages.
After the demise of large plant eating dinosaurs the newly available ecological niche was soon filled by the mammals and larger bodies were an evolutionary advantage for this replacement. Large animals lose energy in form of body-heat dispersion more slowly, have more room for large digestive organs, to handle the plants, and have fewer predators to fear. Predators on their own had to growth to catch such mighty preys.
However after this period with rapid growth stagnation took place, the largest land-mammal actually known are the herbivorous genus Paraceratherium (syn. Indricotherium), reaching 15-17 tons and living 37 million years ago in Asia, and Deinotherium, reaching 17 tons, and living 8-2 million years in Africa, marking both the acme of body size and weight of land dwelling mammals.

Fig.1. Examples of the largest and heaviest land mammals that ever lived, Paraceratherium (ca. 37-23 Ma ago), Deinotherium (8,5-2,7Ma) and the living African elephant (Loxodonta africana). The tallest on diagram, Paraceratherium, an extinct rhino relative, reached a mass of 15 tons, while Deinotherium, an extinct proboscidean, weighed as much as 17 tons. In comparison a modern elephant reaches 2 to 5 tons (figure credit Alison Boyer, Yale University).

After these giants the body mass of land mammals slightly decreased in the following periods. Comparing the fossil record to the geology and climate of the corresponding time, the research suggests also the factors that limited the evolutionary growth of mammals: the relatively warm climate delimited the dispersion of superfluous body heat, and the landmass delimited the available food resources.

Fig.2. The correlation between land size (resources) and body mass is well known, the maximum body size is determined by the number of territories that can fit into a given land area. As example the body masses of top endothermic and ectothermic carnivores and herbivores, as a function of Holocene area of landmass inhabited. Larger landmasses support larger top species, and that, for a given area of landmass, body masses decrease in the sequence (BURNESS et al. 2001).

Bibliography:

BURNESS, G.P.; DIAMOND, J. & FLANNERY, T. (2001): Dinosaurs, dragons, and dwarfs: The evolution of maximal body size. PNAS Vol. 98(25): 14518-14523
SMITH, F.A.; BOYER, A.G.; BROWN, J.H.; COSTA, D.P.; DAYAN, T. et al. (2010): The Evolution of Maximum Body Size of Terrestrial Mammals. Science 26 Vol.330 (6008): 1216-1219 DOI: 10.1126/science.1194830
SPINAR, Z.V. (1976): Quando l´uomo non c´era. Fratelli Fabbri Editori, Milano: 228

Online Resources:

SMITH, F.A. (2010): The evolution of giant mammals: Trajectory and constraints on body size over the Cenozoic. Paper No. 77-14 2010 GSA Denver Annual Meeting (31 October -3 November 2010)