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Tiny “T. rex” found

Sept. 17, 2009
Courtesy Science
and World Science staff

When you think of Ty­ran­no­saur­us rex, a few strik­ing phys­i­cal traits come to mind: an over­sized head with pow­er­ful jaws, ti­ny fore­arms, and mus­cu­lar run­ner’s legs.

But re­search­ers have just iden­ti­fied a much smaller di­no­saur from Chi­na, no more than three me­ters (10 feet) long, with the same fea­tures – and it pre­dates the T. rex by tens of mil­lions of years.

The 125-mil­lion year old Rap­tor­ex in front of its big­ger des­cen­d­ant, T. rex. (draw­ing by Todd Mar­shall)


This find­ing, pub­lished on­line by the jour­nal Sci­ence on Sept. 17, means that such spe­cial­ized traits char­ac­ter­ized all sizes of “tyran­nosauroids” dur­ing their reign in the Cre­ta­ceous Pe­ri­od, re­search­ers say.

The ty­ran­no­saur body plan thus evolved at “ba­sic­ally our bod­yweight,” said Un­ivers­ity of Chi­ca­go re­search­er and Na­tional Ge­o­graph­ic explorer-in-residence Paul Sereno. “That’s pret­ty stag­ger­ing, be­cause there’s no oth­er ex­am­ple that I can think of where an an­i­mal has been so fi­nely de­signed” at a size about one nine­ti­eth, by weight, of that which its de­scen­dants would be­come.

Sereno and col­leagues stud­ied the new fos­sil, nam­ing it Rap­torex krieg­steini, and es­ti­mat­ed that it was a young adult when it died. The sci­en­tists ex­am­ined the skull, teeth, nose, spine, shoul­ders, fore­arms, pel­vis, and hind legs. They com­pared the fea­tures to larg­er ty­ran­no­saur­oids, the ev­o­lu­tion­ary group of T. rex-like di­no­saurs.

“First, we used the best me­chan­i­cal prepara­t­ion of the spec­i­men pos­si­ble, which en­tails the fi­nest nee­dles and air abra­sives un­der a mi­cro­scope,” Se­re­no said. “Then we made molds and casts of the cra­ni­al bones, as­sem­bled a cast skull, and sent that skull through a CT scan­ner at the Un­ivers­ity of Chi­ca­go hos­pi­tal to get the snout cross-section.”

A mi­cro­scop­ic ex­amina­t­ion indi­cated the beast “had lived to be five or six,” he added.

The re­search­ers con­clude that the “preda­tory skele­tal de­sign” of R. krieg­steini was simply scaled up with lit­tle change in its car­niv­o­rous de­scen­dants. Se­re­no and his col­leagues al­so use this new fos­sil to pro­pose and de­scribe three ma­jor stages in the ev­o­lu­tion of body form in ty­ran­no­sauroids.

Rap­torex is es­ti­mat­ed to have lived at least 125 mil­lion years ago. The much big­ger de­scen­dants, start­ing from about 90 mil­lion years ago, dom­i­nat­ed Asian and North Amer­i­can land­scapes un­til the great di­no­saur ex­tinc­tion 65 mil­lion years ago.

Hen­ry Krieg­stein, a pri­vate fos­sil col­lec­tor, brought the nearly com­plete Rap­torex ske­l­e­ton to Sereno’s at­ten­tion af­ter buy­ing it from a ven­dor. Af­ter Sereno and col­leagues fin­ish a more de­tailed study of Rap­torex, re­search­ers plan to re­turn it to a mu­se­um in In­ner Mon­go­lia, where they say it was il­lic­itly ex­ca­vat­ed.


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When you think of Tyrannosaurus rex, a few striking physical traits come to mind: an immense skull with powerful jaws, tiny forearms, and muscular runner’s legs. But researchers have just identified a much smaller dinosaur from China, no more than three meters (10 feet) long, with the same features – and it predates the T. rex by tens of millions of years. This finding, published online by the journal Science on Sept. 17, means that such specialized traits characterized all sizes of “tyrannosauroids” during their reign in the Cretaceous Period, researchers say. The tyrannosaur body plan thus evolved at “basically our bodyweight,” said University of Chicago researcher and National Geographic explorer-in-residence Paul Sereno. “That’s pretty staggering, because there’s no other example that I can think of where an animal has been so finely designed” at a size about one ninetieth, by weight, of that which its descendants would become. Sereno and colleagues studied the new fossil, naming it Raptorex kriegsteini, and estimated that it was a young adult when it died. The scientists examined the skull, teeth, nose, spine, shoulders, forearms, pelvis, and hind legs. They comparing the features to larger tyrannosauroids, the evolutionary group of T. rex-like dinosaurs. “First, we used the best mechanical preparation of the specimen possible, which entails the finest needles and air abrasives under a microscope,” Sereno said in an email interview. “Then we made molds and casts of the cranial bones, assembled a cast skull, and sent that skull through a CT scanner at the University of Chicago hospital to get the snout cross-section.” A microscopic examination determined that the animal “had lived to be five or six,” he added. The researchers conclude that the “predatory skeletal design” of R. kriegsteini was simply scaled up with little modification in its carnivorous descendants. Sereno and his colleagues also use this new fossil to propose and describe three major stages in the evolution of body form in tyrannosauroid dinosaurs. Raptorex is estimated to have lived at least 125 million years ago. The much bigger descendants, starting from about 90 million years ago, dominated Asian and North American landscapes until the great dinosaur extinction 65 million years ago. Henry Kriegstein, a private fossil collector, brought the nearly complete Raptorex skeleton to Sereno’s attention after buying it from a vendor. After Sereno and colleagues finish amore detailed study of Raptorex, researchers plan to return it to a museum in Inner Mongolia, where the fossil was illicitly excavated.