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Size of mammals exploded after dinosaur extinction, researchers confirm

November 26, 2010
Uni­ver­sity of New Mex­ico
Researchers have demonstrated that the extinction of dinosaurs 65 million years ago made way for mammals to get bigger -- about a thousand times bigger than they had been, as well as confirming the dramatic growth in mammalian size after the dinosaurs. The study also showed that the ecosystem is able to reset itself relatively quickly.

The largest land mammals that ever lived, Indricotherium and Deinotherium, would have towered over the living African elephant. The tallest on diagram, Indricotherium, an extinct rhino relative, lived during the Eocene to the Oligocene Epoch (37 to 23 million years ago) and reached a mass of 15,000 kg, while Deinotherium (an extinct proboscidean, related to modern elephants) was around from the late-Miocene until the early Pleistocene (8.5 to 2.7 million years ago) and weighed as much as 17,000 kg.
Credit: Alison Boyer/Yale University

Researchers have demon­strated that the extinc­tion of dinosaurs 65 mil­lion years ago paved the way for mam­mals to get big­ger -- about a thou­sand times big­ger than they had been. The study titled, "The Evo­lu­tion of Max­i­mum Body Size of Ter­res­trial Mam­mals," released in the jour­nal Sci­ence, is the first to quan­ti­ta­tively explore the pat­terns of body size of mam­mals after the demise of the dinosaurs.

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.

Story Source:

Materials provided by Uni­ver­sity of New Mex­ico. Original written by Steve Carr. Note: Content may be edited for style and length.

Journal Reference:

  1. 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, 2010; 330 (6008 1216-1219 DOI: 10.1126/science.1194830

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Uni­ver­sity of New Mex­ico. "Size of mammals exploded after dinosaur extinction, researchers confirm." ScienceDaily. ScienceDaily, 26 November 2010. <>.
Uni­ver­sity of New Mex­ico. (2010, November 26). Size of mammals exploded after dinosaur extinction, researchers confirm. ScienceDaily. Retrieved May 8, 2017 from
Uni­ver­sity of New Mex­ico. "Size of mammals exploded after dinosaur extinction, researchers confirm." ScienceDaily. (accessed May 8, 2017).