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“King” of dinos called more hyena than lion

Feb. 22, 2011
Courtesy of the University of California - Berkeley
and World Science staff

The fe­ro­cious Ty­ran­no­saur­us rex is of­ten de­picted as the bloody top dog of the Cre­ta­ceous pe­ri­od, ruth­lessly stalk­ing herds of duck-billed di­no­saurs.

If this is so, it would seem some­one is now in­sult­ing the king.

Scientists say a cen­sus of di­no­saur skele­tons from all over a swath of east­ern Mon­tana shows that the beast—whose La­tin name means tyrant liz­ard king—was too abun­dant to have lived only on live di­no­saurs it took down with its scythe-like teeth.

This cast of a T. rex is on dis­play in UC Berke­ley's Val­ley Life Sci­ences Build­ing. The orig­i­nal fos­sil ske­l­e­ton from Mon­tana's Hell Creek For­ma­tion is in the Mu­se­um of the Rock­ies in Boze­man, Mont. (Cred­it: Randy Ir­mis)


So rath­er than the “li­on” of its realm, the pa­le­on­tol­o­gists say, T. rex was probably more of an “op­por­tunis­tic” pred­a­tor. That is, it was like the hy­e­na in Af­ri­ca, which both hunts live prey and scav­enges dead meat, and ei­ther way is­n’t picky about which an­i­mals are its fod­der.

“In our cen­sus, T. rex came out very high, equiv­a­lent in num­bers to Ed­mon­to­saur­us, which many peo­ple had thought was its pri­ma­ry prey,” said Mon­tana State Uni­vers­ity’s John “Jack” Horner, who is also cu­ra­tor of pa­le­on­tol­ogy at the Mu­se­um of the Rock­ies in Boze­man, Mont. “This said that T. rex is not a chee­tah, it’s not a li­on. It’s more like a hy­e­na.”

“This pu­ta­tive ‘apex pred­a­tor’ is as abun­dant in the up­per lay­ers of the Hell Creek Forma­t­ion as the her­bi­vores, its re­put­ed pri­ma­ry food source,” added co-re­searcher Mark B. Good­win of the Uni­vers­ity of Cal­i­for­nia, Berke­ley. “And it’s even more plen­ti­ful in the oth­er two-thirds of the forma­t­ion.”

Nor­mal­ly, Good­win said, top pred­a­tors are one-third or one-fourth as abun­dant as their prey, be­cause they need more en­er­gy. But op­por­tunis­tic hunters like the hy­e­na can be twice as abun­dant as the top pred­a­tors. “If you count the li­ons and the leop­ards and the chee­tahs in the Serengeti [de­sert of Af­rica], the num­ber still does not equal the num­ber of hy­e­nas, be­cause hy­e­nas have a much wid­er food source,” Horner said. “Chee­tahs, for ex­am­ple, only go af­ter things that are really fast. They don’t eat tur­tles. But a hy­e­na will eat a tur­tle, or an­y­thing else that it can catch or is dead.”

Sim­i­lar­ly, T. rex was eat­ing an­y­thing it could, he said. “There’s no ev­i­dence that T. rex could run very fast, so it was­n’t out there be­ing a chee­tah. If it could get a sick an­i­mal, it would.”

Horner sug­gests that ju­ve­nile and young adult T. rex may have been pri­marily flesh eaters, while the old­er adults, which de­vel­oped pro­por­tion­ally larg­er, bone-crushing teeth as they aged, al­so con­sumed the bones and mar­row of their prey.

The di­no­saur cen­sus in the Hell Creek Forma­t­ion of Mon­tana, cov­er­ing fossils that date from 65 milli­on to 95 milli­on years ago, was be­gun in 1999. Re­sults were pub­lished Feb. 9 in the re­search jour­nal P­LoS One.

Horner and Good­win, to­geth­er and sep­a­rate­ly, have been di­no­saur-digging in East­ern Mon­tana for dec­ades. The fos­sils date from a time when the ar­ea bor­dered an in­land sea, which pe­ri­odic­ally ad­vanced and with­drew over coast­al plains, de­posit­ing sed­i­ment that was lat­er ex­posed and heavily erod­ed. When Hor­ner started his cen­sus of di­no­saurs in the Hell Creek Forma­t­ion around Fort Peck Lake in 1999, he teamed up with Good­win to re-ex­am­ine some of the di­no­saurs disco­vered in the ar­ea.

Since then, through lab anal­y­sis and an­nu­al sum­mer digs, they have con­clud­ed that one named spe­cies, Toro­saur­us, was just a big, aged Tri­cer­a­tops; two dome-headed di­no­saurs, Dra­corex and Sty­gi­moloch, were merely young­er mem­bers of the ge­nus Pachy­ce­pha­lo­sau­rus; and the so-called Nan­o­tyran­nus was just a ju­ve­nile T. rex.

Once these fos­sils had been prop­erly iden­ti­fied, Hom­er and Good­win said, they cat­a­logued the spe­cies and rel­a­tive ages of known di­no­saurs in the forma­t­ion, which is about 100 me­ters (yards) thick at ex­posed ar­e­as co­vering some 1,000 square kilome­ters (400 square miles).

“Small ju­ve­niles and old­er adults were rel­a­tively rare com­pared to large ju­ve­niles and sub-adults for all the di­no­saurs,” Good­win said. This could make sense if ju­ve­niles lived in oth­er loca­t­ions, which is not un­com­mon in some spe­cies, he added; “this adds to an emerg­ing pic­ture of what the di­no­saur fau­na looked like dur­ing the late Cre­ta­ceous.”

Horner not­ed a great­er va­ri­e­ty of di­no­saurs in the old­er sed­i­ments, the Low­er Hell Creek Forma­t­ion, com­pared to the young­er “Up­per” forma­t­ion. “Defi­nitely there was a change in popula­t­ion lead­ing up to the Cre­ta­ceous-Tertiary bound­ary,” the time when the di­no­saurs went ex­tinct due to a pre­sumed as­ter­oid im­pact, Horner said. “So some­thing was hap­pen­ing to the fau­nas pri­or to the im­pact. dur­ing the 10 milli­on years af­ter di­no­saur di­vers­ity peak­ed 75 milli­on years ago, the di­no­saurs dwin­dled pret­ty fast, and there weren’t many left at the end.”


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The ferocious Tyrannosaurus rex is often depicted as the bloody top dog of the Cretaceous period, ruthlessly stalking herds of duck-billed dinosaurs. But a new census of all dinosaur skeletons unearthed over much of eastern Montana, scientists say, shows that the “tyrant lizard king” was too abundant to have lived only on dinosaurs it tracked and killed with its scythe-like teeth. So rather than the “lion” of its realm, the paleontologists say, T. rex was probably more of an “opportunistic” predator. That is, it was like the hyena in Africa, which both hunts live prey and scavenges dead meat, and either way isn’t picky about which animals are its sustenance. “In our census, T. rex came out very high, equivalent in numbers to Edmontosaurus, which many people had thought was its primary prey,” said John “Jack” Horner, curator of paleontology at the Museum of the Rockies in Bozeman, Mont., and a professor at Montana State University. “This said that T. rex is not a cheetah, it’s not a lion. It’s more like a hyena.” “This putative ‘apex predator’ is as abundant in the upper layers of the Hell Creek Formation as the herbivores, its reputed primary food source,” added co-researcher Mark B. Goodwin of the University of California, Berkeley. “And it’s even more plentiful in the other two-thirds of the formation.” Normally, Goodwin said, top predators are one-third or one-fourth as abundant as their prey, because of carnivores’ larger energy needs. Opportunistic hunters like the hyena, however, can be twice as abundant as the top predators. “If you count the lions and the leopards and the cheetahs in the Serengeti, the number still does not equal the number of hyenas, because hyenas have a much wider food source,” Horner said. “Cheetahs, for example, only go after things that are really fast. They don’t eat turtles. But a hyena will eat a turtle, or anything else that it can catch or is dead.” Similarly, T. rex was eating anything it could, he said. “There’s no evidence that T. rex could run very fast, so it wasn’t out there being a cheetah. If it could get a sick animal, it would.” Horner suggests that juvenile and young adult T. rex may have been primarily flesh eaters, while the older adults, which developed proportionally larger, bone-crushing teeth as they aged, also consumed the bones and marrow of their prey. The dinosaur census in the Hell Creek Formation of Montana, which dates from 65 to 95 million years ago, was begun in 1999 by Horner and Goodwin with the financial and occasional field support of Nathan Myhrvold, former chief technology officer for Microsoft Corp. and co-founder of Intellectual Ventures of Bellevue, Wash. The results, authored by Horner, Goodwin and Myhrvold, were published Feb. 9 in the open-access research journal PLoS One. Horner and Goodwin, together and separately, have been dinosaur-digging in Eastern Montana for decades. The fossils date from a time when the area bordered an inland sea, which periodically advanced and withdrew over coastal plains, depositing sediment that was later exposed and heavily eroded. When Horner started his census of dinosaurs in the Hell Creek Formation around Fort Peck Lake in 1999, he teamed up with Goodwin to re-examine some of the dinosaurs discovered in the area. Since then, through lab analysis and annual summer digs, they have concluded that one named species, Torosaurus, was just a big, aged Triceratops; two dome-headed dinosaurs, Dracorex and Stygimoloch, were merely younger members of the genus Pachycephalosaurus; and the so-called Nanotyrannus was just a juvenile T. rex. Once these fossils had been properly identified, Homer and Goodwin said, they catalogued the species and relative ages of known dinosaurs in the formation, which is about 100 meters thick at exposed areas covering some 1,000 square kilometers. The census included only skeletal remains, not teeth, because the paleontologists wanted a record of the maturity of each specimen, and teeth tell little about the age of a dinosaur at death, Goodwin said. “Small juveniles and older adults were relatively rare compared to large juveniles and subadults for all the dinosaurs,” Goodwin said. This could be explained if juveniles lived in other locations, which is not uncommon in some species. The largest adults may simply have been relatively rare. “This adds to an emerging picture of what the dinosaur fauna looked like during the late Cretaceous,” he said. Horner noted the greater variety of dinosaurs in the older sediments, the Lower Hell Creek Formation, compared to the younger “Upper” formation. “Definitely there was a change in population leading up to the Cretaceous-Tertiary boundary,” the time when the dinosaurs went extinct due to a presumed asteroid impact, Horner said. “So something was happening to the faunas prior to the impact.. during the 10 million years after dinosaur diversity peaked 75 million years ago, the dinosaurs dwindled pretty fast, and there weren’t many left at the end.”