Size of mammals exploded after dinosaur extinction, researchers confirm

The research, funded by a National Sci­ence Foun­da­tion Research Coor­di­na­tion Net­work grant and led by Uni­ver­sity of New Mex­ico Biol­ogy Asso­ciate Pro­fes­sor Felisa Smith, brought together an inter­na­tional team of pale­on­tol­o­gists, evo­lu­tion­ary biol­o­gists and macro­e­col­o­gists from uni­ver­si­ties around the world.

The goal of the research was to revisit key ques­tions about size, specif­i­cally in mam­mals. "Size impacts all aspects of biol­ogy, from repro­duc­tion to extinc­tion," said Smith. "Under­stand­ing the con­straints oper­at­ing on size is cru­cial to under­stand­ing how ecosys­tems work."

In order to doc­u­ment what hap­pened to mam­mals after the extinc­tion of dinosaurs, researchers col­lected data on the max­i­mum size for major groups of land mam­mals on each con­ti­nent, includ­ing Peris­so­dactyla, odd-toed ungu­lates such as horses and rhi­nos; Pro­boscidea, which includes ele­phants, mam­moth and mastodon; Xenarthra, the anteaters, tree sloths, and armadil­los; as well as a num­ber of other extinct groups. The researchers spent three years assem­bling the data.

"The data­base is unique," said Smith "because it's com­pre­hen­sive, includ­ing mam­mals from all con­ti­nents since the extinc­tion of the dinosaurs. We esti­mated body size from fos­sil teeth, which are the most com­monly pre­served parts of mammals."

Mam­mals grew from a max­i­mum of about 10 kilo­grams when they were shar­ing the earth with dinosaurs to a max­i­mum of 17 tons after­wards. The researchers found that the pat­tern was sur­pris­ingly con­sis­tent, not only glob­ally but also across time and across trophic groups and lin­eages -- that is, ani­mals with dif­fer­ing diets and descended from dif­fer­ent ances­tors -- as well.

The max­i­mum size of mam­mals began to increase sharply about 65 mil­lion years ago, peak­ing in the Oligocene Epoch (about 34 mil­lion years ago) in Eura­sia, and again in the Miocene Epoch (about 10 mil­lion years ago) in Eura­sia and Africa. The largest mam­mal that ever walked the earth -- Indri­cotherium tran­souralicum, a horn­less rhinoceros-like her­bi­vore that weighed approx­i­mately 17 tons and stood about 18 feet high at the shoul­der -- lived in Eura­sia almost 34 mil­lion years ago.

"The remark­able sim­i­lar­ity in the evo­lu­tion of max­i­mum size on the dif­fer­ent con­ti­nents sug­gests that there were sim­i­lar eco­log­i­cal roles to be filled by giant mam­mals across the globe," said Smith. "This strongly implies that mam­mals were respond­ing to the same eco­log­i­cal constraints."

The results give clues as to what sets the lim­its on max­i­mum body size on land; the amount of space avail­able to each ani­mal and the cli­mate they live in. The colder the cli­mate, the big­ger the mam­mals seem to get, as big­ger ani­mals con­serve heat bet­ter. It also shows that no one group of mam­mals dom­i­nates the largest size class -- the absolute largest mam­mal belongs to dif­fer­ent groups over time and space.

"The results were strik­ing. Global tem­per­a­ture and ter­res­trial land area set con­straints on the upper limit of mam­mal body size," said Smith, "with larger mam­mals evolv­ing when the earth was cooler and the ter­res­trial land area greater. Our analy­sis reflected processes oper­at­ing con­sis­tently across trophic and tax­o­nomic groups, and inde­pen­dent of the phys­io­graphic his­tory of each continent."

The inter­est in the size of mam­mals for Smith began years ago when she was a grad­u­ate stu­dent at the Uni­ver­sity of Cal­i­for­nia. "I worked on a num­ber of islands off the coast of Baja, Cal­i­for­nia where rodents had evolved into gigan­tic body sizes. I've been inter­ested in size ever since."

Smith's col­leagues in the project include from UNM Dis­tin­guished Pro­fes­sor of Biol­ogy Jim Brown, Mar­cus Hamil­ton and Jor­dan Okie. Other coau­thors are: Ali­son Boyer, Daniel Costa, Tamar Dayan, Mor­gan Ernest, Alis­tair Evans, Mikael Fortelius, John Git­tle­man, Lar­isa Hard­ing, Kari Lin­tu­laakso, Kath­leen Lyons, Christy McCain, Juha J. Saari­nen, Richard Sibly, Patrick Stephens, Jes­sica Theodor and Mark Uhen.