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June 27, 2011

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Big dinos were about as warm as people, study finds

June 26, 2011
Courtesy of the U.S. National Science Foundation
and World Science staff

Were di­no­saurs slow and lum­ber­ing, or quick and ag­ile?

It de­pends largely on wheth­er they were cold- or warm-blood­ed.

When the gi­ant rep­tiles were first dis­cov­ered in the mid-1800s, pa­le­on­tol­ogists as­sumed they were plod­ding beasts that re­lied on warm sur­round­ings to keep warm, like mod­ern rep­tiles. But re­search in the last few dec­ades sug­gests they were faster crea­tures, nim­ble like the ve­loci­rap­tors or T. rex de­picted in the mov­ie Ju­ras­sic Park, re­quir­ing warm­er, reg­u­lat­ed body tem­per­a­tures.

Now, re­search­ers say they have de­vel­oped a way to meas­ure di­no­saurs’ body tem­per­a­tures, some­thing pre­vi­ously con­sid­ered “im­pos­si­ble.”

Skull re­con­struc­tion of Ca­ma­ra­saur­us. (Cred­it: Sau­ri­er­mu­seum Aa­thal, Switz­er­land )

Their find­ing: that “the body tem­per­a­ture of di­no­saurs was close to that of mam­mals,” in­clud­ing hu­mans, “and that the di­no­saurs’ phys­i­ol­o­gy al­lowed them to reg­u­late that tem­per­a­ture. The re­sult has im­plica­t­ions for our un­der­stand­ing of di­no­saurs’ ecol­o­gy and demise,” said Li­sa Boush, pro­gram di­rec­tor in the U.S. Na­t­ional Sci­ence Founda­t­ion’s Di­vi­sion of Earth Sci­ences, which funded the re­search.

The sci­en­tists chem­ic­ally an­a­lyzed teeth of sau­ro­pods—long-tailed, long-necked di­no­saurs that were the big­gest land an­i­mals ev­er to have lived. It was “like be­ing able to stick a ther­mom­e­ter in an an­i­mal that has been ex­tinct for 150 mil­lion years,” said Rob­ert Ea­gle of the Cal­i­for­nia In­sti­tute of Tech­nol­o­gy, a ge­o­chem­ist and lead au­thor of a pa­per pub­lished in the cur­rent on­line edi­tion of the re­search jour­nal Sci­ence.

“The con­sen­sus was that no one would ev­er meas­ure di­no­saur body tem­per­a­tures, that it’s im­pos­si­ble,” said John Eiler, a co-au­thor and ge­o­chem­ist at Cal­tech. But us­ing a tech­nique de­vel­oped in Eil­er’s lab, the team did just that. The re­search­ers an­a­lyzed 11 teeth, un­earthed up in Tan­za­nia, Wy­o­ming and Ok­la­ho­ma, from the di­no­saurs Bra­chi­o­saur­us and Ca­ma­ra­saur­us.

They meas­ured the con­centra­t­ions of iso­topes, or forms of chem­ical el­e­ments, called carbon-13 and oxygen-18 in bioa­p­atite, a min­er­al found in teeth and bone. How of­ten these iso­topes bond with each oth­er, or “clump,” de­pends on tem­per­a­ture. The in­vest­i­ga­tion con­clud­ed that Bra­chi­o­saur­us had a tem­per­a­ture of about 38.2 de­grees Cel­si­us (100.8 de­grees Fahr­en­heit) and Ca­ma­ra­saur­us, about 35.7 de­grees Cel­si­us (96.3 de­grees Fahr­en­heit), warm­er than crocodiles and al­li­ga­tors, but cool­er than birds.

Close-up of a Ca­ma­rasaurus skull. (Cred­it: Sauri­er­mu­seum Aathal, Switz­er­land )

The meas­urements are ac­cu­rate to with­in one or two de­grees Cel­si­us, the re­search­ers claim. “Nobody has used this ap­proach to look at di­no­saur body tem­per­a­tures be­fore, so our study pro­vides a com­pletely dif­fer­ent an­gle,” Ea­gle said.

The fact that the tem­per­a­tures were si­m­i­lar to those of most mod­ern mam­mals might seem to imply that di­no­saurs had a warm-blood­ed me­tab­o­lism. But, the re­search­ers say, the is­sue is more com­plex. Be­cause sau­ro­pod di­no­saurs were so huge, they could re­tain their body heat much more ef­fi­ciently than smaller mam­mals like hu­mans.

The meas­ured tem­per­a­tures are low­er than what’s pre­dicted by some mod­els of di­no­saur body tem­per­a­tures, sug­gest­ing there is some­thing mis­sing in sci­en­tists’ un­der­stand­ing of di­no­saur phys­i­ol­o­gy. These mod­els imply that di­no­saurs were so-called gi­gan­to­therms, that they main­tained warm tem­per­a­tures by their sheer size.

To ex­plain the low­er tem­per­a­tures, the re­search­ers sug­gest that di­no­saurs could have had phys­i­o­logic­al or be­hav­ior­al adapta­t­ions that al­lowed them to avoid get­ting too hot. The di­no­saurs could have had low­er met­a­bol­ic rates to re­duce the amount of in­ter­nal heat. They could al­so have had some­thing like an air-sac sys­tem to dis­si­pate heat. Or, they could have dis­pelled heat through their long necks and tails.

Pre­vi­ously, re­search­ers could only gauge di­no­saur me­tab­o­lism or body tem­per­a­tures in­di­rect­ly. For ex­am­ple, they in­ferred be­hav­ior and phys­i­ol­o­gy by fig­ur­ing out how fast di­no­saurs ran based on the spac­ing of di­no­saur tracks, stu­dying the ra­tio of preda­tors to prey in the fos­sil rec­ord, or meas­ur­ing the growth rates of bone. But these lines of ev­i­dence of­ten con­flicted.

“We’re get­ting at body tem­per­a­ture through a line of rea­son­ing that I think is rel­a­tively bullet-proof, pro­vided you can find well-pre­served sam­ples,” Eiler said.

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Were dinosaurs slow and lumbering, or quick and agile? It depends largely on whether they were cold- or warm-blooded. When the giant reptiles were first discovered in the mid-1800s, paleontologists assumed they were plodding beasts that relied on warm surroundings to keep warm, like modern reptiles. But research in the last few decades suggests they were faster creatures, nimble like the velociraptors or T. rex depicted in the movie Jurassic Park, requiring warmer, regulated body temperatures. Now, researchers say they have developed a way to measure dinosaurs’ body temperatures, something previously considered “impossible.” Their finding: that “the body temperature of dinosaurs was close to that of mammals,” including humans, “and that the dinosaurs’ physiology allowed them to regulate that temperature. The result has implications for our understanding of dinosaurs’ ecology and demise,” said Lisa Boush, program director in the U.S. National Science Foundation’s Division of Earth Sciences, which funded the research. The scientists chemically analyzed teeth of sauropods—long-tailed, long-necked dinosaurs that were the biggest land animals ever to have lived. It was “like being able to stick a thermometer in an animal that has been extinct for 150 million years,” said Robert Eagle of the California Institute of Technology, a geochemist and lead author of a paper published in the current online edition of the research journal Science. “The consensus was that no one would ever measure dinosaur body temperatures, that it’s impossible,” said John Eiler, a co-author and geochemist at Caltech. But using a technique developed in Eiler’s lab, the team did just that. The researchers analyzed 11 teeth, unearthed up in Tanzania, Wyoming and Oklahoma, that belonged to the dinosaurs Brachiosaurus and Camarasaurus. They measured the concentrations of isotopes, or forms of chemical elements, called carbon-13 and oxygen-18 in bioapatite, a mineral found in teeth and bone. How often these isotopes bond with each other, or “clump,” depends on temperature. They concluded that Brachiosaurus had a temperature of about 38.2 degrees Celsius (100.8 degrees Fahrenheit) and Camarasaurus had one of about 35.7 degrees Celsius (96.3 degrees Fahrenheit), warmer than modern and extinct crocodiles and alligators, but cooler than birds. The measurements are accurate to within one or two degrees Celsius, the researchers claim. “Nobody has used this approach to look at dinosaur body temperatures before, so our study provides a completely different angle,” Eagle said. The fact that the temperatures were similar to those of most modern mammals might seem to imply that dinosaurs had a warm-blooded metabolism. But, the researchers say, the issue is more complex. Because sauropod dinosaurs were so huge, they could retain their body heat much more efficiently than smaller mammals like humans. “The body temperatures we’ve estimated provide key information that any model of dinosaur physiology has to be able to explain,” said Aradhna Tripati, a co-author who’s a geochemist at University of California, Los Angeles and visiting geochemist at Caltech. “As a result, the data can help scientists test physiological models to explain how these organisms lived.” The measured temperatures are lower than what’s predicted by some models of dinosaur body temperatures, suggesting there is something missing in scientists’ understanding of dinosaur physiology. These models imply that dinosaurs were so-called gigantotherms, that they maintained warm temperatures by their sheer size. To explain the lower temperatures, the researchers suggest that dinosaurs could have had physiological or behavioral adaptations that allowed them to avoid getting too hot. The dinosaurs could have had lower metabolic rates to reduce the amount of internal heat. They could also have had something like an air-sac system to dissipate heat. Or, they could have dispelled heat through their long necks and tails. Previously, researchers could only gauged dinosaur metabolism or body temperatures indirectly. For example, they inferred behavior and physiology by figuring out how fast dinosaurs ran based on the spacing of dinosaur tracks, studying the ratio of predators to prey in the fossil record, or measuring the growth rates of bone. But these lines of evidence often conflicted. “We’re getting at body temperature through a line of reasoning that I think is relatively bullet-proof, provided you can find well-preserved samples,” Eiler said.