"Long before it's in the papers"
January 27, 2015


Dinos out of way, mammals ballooned to record sizes

Nov. 26, 2010
Courtesy of the Uni­vers­ity of New Mex­i­co
and World Science staff

The di­no­saurs’ de­mise 65 mil­lion years ago paved the way for mam­mals to even­tu­ally grow over a thou­sand­fold in size, hit­ting rec­ords for heft some 34 mil­lion years ago, a new study sug­gests.

“Size im­pacts all as­pects of bi­ol­o­gy, from re­pro­duc­tion to ex­tinc­tion,” said Uni­vers­ity of New Mex­i­co bi­ol­o­gist Fe­lisa Smith, who led the re­search. “Un­der­stand­ing the con­straints op­er­at­ing on size is cru­cial to un­der­stand­ing how ecosys­tems work.”

The largest land mam­mals ever, Indri­cotherium (shown in pale blue) and Deino­the­rium, early re­la­tives of ele­phants (dark blue), would have dwarfed to­day's big­gest, the Af­ric­an Ele­phant (gray). (Image: UNM)

Smith and col­leagues col­lect­ed da­ta on the max­i­mum size for each con­ti­nen­t’s ma­jor groups of land mam­mals. These in­clud­ed Pe­ris­so­dac­tyla or so-called odd-toed un­gu­lates, such as hors­es and rhi­nos; Pro­bos­cid­ea, which in­cludes ele­phants, mam­moth and mas­to­don; Xe­nar­thra, the ant­eaters, tree sloths, and ar­madil­los; and oth­er, ex­tinct groups.

“We es­ti­mat­ed body size from fos­sil teeth,” Smith said, “the most com­monly pre­served parts of mam­mals.” It turned out, the sci­en­tists said, that mam­mals grew from a max­i­mum of about 10 kg (22 lbs) when they shared the earth with di­no­saurs to a max­i­mum of 15,400 kg (17 tons) af­ter­wards. The pat­tern was sur­pris­ingly con­sist­ent across lin­eages, re­gions and di­e­tary types, they added.

Max­i­mum sizes started bal­loon­ing as the di­no­saurs fad­ed out, they ex­plained, and peak­ed in the Ol­i­go­cene Ep­och (a­bout 34 mil­lion years ago) in Eur­a­sia. Those glo­ry days wit­nessed the larg­est mam­mal ev­er: Ind­ri­co­th­erium tran­sour­a­li­cum, a horn­less rhino-like plant-eater weigh­ing about 17 tons and about 18 feet (5.5 m) tall at the shoul­der. A sec­ond peak in max­i­mum mam­malian sizes was iden­ti­fied lat­er, in the Mi­o­cene (some 10 mil­lion years ago) in Eur­a­sia and Af­ri­ca.

“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, said mem­bers of Smith’s group: the amount of space avail­a­ble to each an­i­mal and the pre­vail­ing tem­per­a­tures. The colder the cli­mate, the big­ger the mam­mals seem to get, as big­ger an­i­mals con­serve heat bet­ter. It al­so shows, they added, that no one group of mam­mals dom­i­nates the larg­est size class: dif­fer­ent lin­eages pro­duced rec­ord-breakers in dif­fer­ent times and re­gions.

“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.”

Smith said her in­ter­est in the size of mam­mals be­gan years ago as 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.” The new find­ings are pub­lished in the Nov. 25 is­sue of the re­search jour­nal Sci­ence.

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The dinosaurs’ demise 65 million years ago paved the way for mammals to eventually grow over a thousandfold in size, hitting record sizes some 34 million years ago, a new study suggests. “Size impacts all aspects of biology, from reproduction to extinction,” said University of New Mexico biologist Felisa Smith, who led the research. “Understanding the constraints operating on size is crucial to understanding how ecosystems work.” Smith and colleagues collected data on the maximum size for each continent’s major groups of land mammals. These included Perissodactyla or so-called odd-toed ungulates, such as horses and rhinos; Proboscidea, which includes elephants, mammoth and mastodon; Xenarthra, the anteaters, tree sloths, and armadillos; and other, extinct groups. “We estimated body size from fossil teeth,” Smith said, “the most commonly preserved parts of mammals.” It turned out, the scientists said, that mammals grew from a maximum of about 10 kg (22 lbs) when they shared the earth with dinosaurs to a maximum of 15,400 kg (17 tons) afterwards. The pattern was surprisingly consistent across lineages, regions and dietary types, they added. Maximum sizes started ballooning as the dinosaurs faded out, they explained, and peaked in the Oligocene Epoch (about 34 million years ago) in Eurasia. Those glory days witnessed the largest mammal ever: Indricotherium transouralicum, a hornless rhino-like plant-eater weighing about 17 tons and about 18 feet tall at the shoulder. A second peak in maximum mammalian sizes was identified later, in the Miocene (some 10 million years ago) in Eurasia and Africa. “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, said members of Smith’s group: the amount of space available to each animal and the prevailing temperatures. The colder the climate, the bigger the mammals seem to get, as bigger animals conserve heat better. It also shows, they added, that no one group of mammals dominates the largest size class: different lineages produced record-breakers in different times and regions. “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 [dietary] and taxonomic groups.” Smith said her interest in the size of mammals began years ago as 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.” The new findings are published in the Nov. 25 issue of the research journal Science.