Size of Mammals Exploded After Dinosaur Extinction

张立涛

Researchers have demon strated that the extinc tion of dinosaurs 65 mil lion years ago paved the way for mam mals to get big ger -- about a thou sand times big ger than they had been. The study titled, "The Evo lu tion of Max i mum Body Size of Ter res trial Mam mals," released in the jour nal Sci ence, is the first to quan ti ta tively explore the pat terns of body size of mam mals after the demise of the dinosaurs.

The research, funded by a National Sci ence Foun da tion Research Coor di na tion Net work grant and led by Uni ver sity of New Mex ico Biol ogy Asso ciate Pro fes sor Felisa Smith, brought together an inter na tional team of pale on tol o gists, evo lu tion ary biol o gists and macro e col o gists from uni ver si ties around the world.

The goal of the research was to revisit key ques tions about size, specif i cally in mam mals. "Size impacts all aspects of biol ogy, from repro duc tion to extinc tion," said Smith. "Under stand ing the con straints oper at ing on size is cru cial to under stand ing how ecosys tems work."
In order to doc u ment what hap pened to mam mals after the extinc tion of dinosaurs, researchers col lected data on the max i mum size for major groups of land mam mals on each con ti nent, includ ing Peris so dactyla, odd-toed ungu lates such as horses and rhi nos; Pro boscidea, which includes ele phants, mam moth and mastodon; Xenarthra, the anteaters, tree sloths, and armadil los; as well as a num ber of other extinct groups. The researchers spent three years assem bling the data.
"The data base is unique," said Smith "because it's com pre hen sive, includ ing mam mals from all con ti nents since the extinc tion of the dinosaurs. We esti mated body size from fos sil teeth, which are the most com monly pre served parts of mammals."
Mam mals grew from a max i mum of about 10 kilo grams when they were shar ing the earth with dinosaurs to a max i mum of 17 tons after wards. The researchers found that the pat tern was sur pris ingly con sis tent, not only glob ally but also across time and across trophic groups and lin eages -- that is, ani mals with dif fer ing diets and descended from dif fer ent ances tors -- as well.
The max i mum size of mam mals began to increase sharply about 65 mil lion years ago, peak ing in the Oligocene Epoch (about 34 mil lion years ago) in Eura sia, and again in the Miocene Epoch (about 10 mil lion years ago) in Eura sia and Africa. The largest mam mal that ever walked the earth -- Indri cotherium tran souralicum, a horn less rhinoceros-like her bi vore that weighed approx i mately 17 tons and stood about 18 feet high at the shoul der -- lived in Eura sia almost 34 mil lion years ago.
"The remark able sim i lar ity in the evo lu tion of max i mum size on the dif fer ent con ti nents sug gests that there were sim i lar eco log i cal roles to be filled by giant mam mals across the globe," said Smith. "This strongly implies that mam mals were respond ing to the same eco log i cal constraints."
The results give clues as to what sets the lim its on max i mum body size on land; the amount of space avail able to each ani mal and the cli mate they live in. The colder the cli mate, the big ger the mam mals seem to get, as big ger ani mals con serve heat bet ter. It also shows that no one group of mam mals dom i nates the largest size class -- the absolute largest mam mal belongs to dif fer ent groups over time and space.
"The results were strik ing. Global tem per a ture and ter res trial land area set con straints on the upper limit of mam mal body size," said Smith, "with larger mam mals evolv ing when the earth was cooler and the ter res trial land area greater. Our analy sis reflected processes oper at ing con sis tently across trophic and tax o nomic groups, and inde pen dent of the phys io graphic his tory of each continent."
The inter est in the size of mam mals for Smith began years ago when she was a grad u ate stu dent at the Uni ver sity of Cal i for nia. "I worked on a num ber of islands off the coast of Baja, Cal i for nia where rodents had evolved into gigan tic body sizes. I've been inter ested in size ever since."
Smith's col leagues in the project include from UNM Dis tin guished Pro fes sor of Biol ogy Jim Brown, Mar cus Hamil ton and Jor dan Okie. Other coau thors are: Ali son Boyer, Daniel Costa, Tamar Dayan, Mor gan Ernest, Alis tair Evans, Mikael Fortelius, John Git tle man, Lar isa Hard ing, Kari Lin tu laakso, Kath leen Lyons, Christy McCain, Juha J. Saari nen, Richard Sibly, Patrick Stephens, Jes sica Theodor and Mark Uhen.

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