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Newfound fossils said to clarify dinosaur evolution

Dec. 10, 2009
Courtesy Science
and World Science staff

Paleontologists, helped by am­a­teur vol­un­teers, re­port that they have found a pre­vi­ously un­known meat-eating di­no­saur fos­sil, set­tling a de­ba­te about early di­no­saur ev­o­lu­tion, re­veal­ing a pe­ri­od of ex­plo­sive di­ver­sifica­t­ion and hint­ing at how di­no­saurs spread across the su­per­con­ti­nent Pan­gaea.

Some re­search­ers now pro­pose that di­no­saurs orig­i­nat­ed in what is now South Amer­i­ca and soon af­ter di­verged in­to or­nithis­chi­ans (like Tricer­atops), sauropodomorphs (like Ap­atasaurus) and theropods (like Her­rarasaurus, Tawa and T. rex), be­fore dis­pers­ing across the Tri­as­sic world more than 220 mil­lion years ago. The theropods are thought to have evolved in­to modern-day birds, al­though Tawa split off from the an­ces­tral branch ear­ly on and was not a di­rect bird an­ces­tor. (Im­agey cour­te­sy Zina Deretsky, Na­tion­al Sci­ence Founda­tion)


The new spe­cies, found in north­ern New Mex­i­co and giv­en the name Tawa hal­lae, is de­scribed in the Dec. 10 is­sue of the re­search jour­nal Sci­ence.

Fos­sil bones of sev­er­al in­di­vid­u­als were found, in­clud­ing a nearly com­plete ske­l­e­ton of a ju­ve­nile about the size of a large dog, but with a much long­er tail. It stood about 28 inches (70 cm) tall at the hips and was about 6 feet (2 me­ters) long from snout to tail, sci­en­tists said. It is es­ti­mat­ed to have lived 214 mil­lion years ago. 

Re­mark­ably, the skel­e­tons show lit­tle sign of hav­ing be­ing flat­tened dur­ing fos­sil­iz­a­tion, as usu­ally oc­curs, pa­le­on­tol­o­gists not­ed.

Tawa, named after a Ho­pi In­di­an word for a sun god, is clas­si­fied as part of a group of di­no­saurs called the­ro­pods, which in­cludes Ty­ran­no­saur­us Rex and Ve­loci­rap­tor. Theropods mostly ate meat, walked on two legs and had feath­ers. Though most went ex­tinct by 65 mil­lion years ago, sci­en­tists gen­er­ally be­lieve some lin­eages sur­vived to spawn mod­ern birds.

Ster­ling Nes­bitt, a post­doc­tor­al re­searcher at The Un­ivers­ity of Tex­as at Aus­tin, con­ducted the re­search with col­leagues while a gradua­te stu­dent at Co­lum­bia Un­ivers­ity’s Lamont-Doherty Earth Ob­serv­a­to­ry and the Amer­i­can Mu­se­um of Nat­u­ral His­to­ry in New York.

One of Tawa’s big­gest con­tri­bu­tions to sci­ence has to do with what it said about an­oth­er di­no­saur, Her­rera­saur­us, ac­cord­ing to Nes­bitt and col­leagues. Her­rera­saur­us has been at the cen­ter of a lively de­ba­te since its dis­cov­ery in Ar­gen­ti­na in the 1960s.

Diagrams of Tawa bones. (Courtesy Science/AAAS)


Her­rerasaurus had some si­m­i­lar­i­ties with the­ro­pods—in­clud­ing large claws, car­niv­o­rous teeth and cer­tain pel­vic fea­tures—but lacked oth­er the­ro­pod traits. Some pa­le­on­tol­ogists claimed it was so un­usu­al it was out­side the ev­o­lu­tion­ary tree of the­ro­pods, or even of di­no­saurs. Oth­ers placed it among the ear­li­est the­ro­pods.

“The ques­tion was did those car­niv­o­rous traits arise in Her­rera­saur­us and in the­ro­pods in­de­pend­ently or were they traits from a re­cent com­mon an­ces­tor that got passed down,” said Nes­bitt. “Now that we have Tawa, we think we have an an­swer.”

Tawa had a mix of Her­rera­saur­us-like char­ac­ter­is­tics—such as in the pelvis—and fea­tures found in firmly es­tab­lished the­ro­pods, such as pock­ets for air­sacs in the back­bone, ac­cord­ing to the dis­cov­er­ers. This con­firms, they said, that char­ac­ter­is­tics that Her­rera­saur­us shares un­iquely with the­ro­pods such as Tawa did­n’t arise in­de­pend­ently and that Her­rera­saur­us is in­deed a the­ro­pod.

The firm clas­sifica­t­ion of Her­rera­saur­us as a the­ro­pod points up an in­ter­est­ing fact about di­no­saur ev­o­lu­tion, sci­en­tists said: once di­no­saurs ap­peared, they very rap­idly di­ver­si­fied in­to the three main lin­eages that per­sisted for more than 170 mil­lion years. Her­rera­saur­us was found in a South Amer­i­can rock lay­er along­side the old­est mem­bers of two ma­jor lin­eages—the sauropods and the or­nithis­chi­ans.

Tawa pulls Her­rera­saur­us in­to the the­ro­pod line­age, so that means all three lin­eages are pre­s­ent in South Amer­i­ca pret­ty much as soon as di­no­saurs evolved,” said Nes­bitt.

Tawa skel­e­tons were found be­side two oth­er the­ro­pod di­no­saurs from around the same pe­ri­od. Nes­bitt not­ed that each of the three is more closely rela­ted to a known di­no­saur from South Amer­i­ca than they are to each oth­er. This sug­gests these three spe­cies each de­scended from a separa­te line­age in South Amer­i­ca, rath­er than all evolv­ing from a lo­cal an­ces­tor, Nes­bitt and col­leagues said. They la­ter would have dis­persed to North Amer­i­ca and oth­er parts of Pan­gaea, a su­per­con­ti­nent that in­clud­ed all of to­day’s con­ti­nents gath­ered up as one, be­fore about 200 mil­lion years ago.

The find­ing al­so sug­gests there were mul­ti­ple dis­per­sals out of South Amer­i­ca, ac­cord­ing to the group.

The first Tawa fos­sils were dis­cov­ered in 2004 by vol­un­teers tak­ing a week-long pa­le­on­tol­ogy sem­i­nar with ex­perts at the Ruth Hall Mu­se­um of Pa­le­on­tol­o in Abiquiu, New Mex­i­co. The dig site, known as Hay­den Quar­ry, is in a hill­side on Ghost Ranch made fa­mous by the paint­er Geor­gia O’­Keefe. Al­ex Downs, an in­struc­tor for the course, con­tacted Nes­bitt and a col­league to ask if they’d like to see the fos­sils.

“When we saw them, our jaws dropped,” said Nes­bitt. “A lot of these the­ro­pods have really hol­low bones, so when they get pre­served, they get really crunched. But these were in al­most per­fect con­di­tion.” He was al­so sur­prised by how much ma­terial was pre­served at this one site. The fos­sil bone bed ex­tends for tens of me­ters along the hill­side, prom­is­ing years of painstak­ing work and per­haps ad­di­tion­al sig­nif­i­cant dis­cov­er­ies, he said.


* * *

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Homepage image: the dinosaur-bearing rocks at Ghost Ranch, N.M. (Courtesy Sterling Nesbitt)







 

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Paleontologists, helped by amateur volunteers, report that they have found a previously unknown meat-eating dinosaur fossil, settling a debate about early dinosaur evolution, revealing a period of explosive diversification and hinting at how dinosaurs spread across the supercontinent Pangaea. The new species, found in northern New Mexico named Tawa after the Hopi Indian word for the Puebloan sun god, is described in the Dec. 10 issue of the research journal Science. Fossil bones of several individuals were found, including a nearly complete skeleton of a juvenile about the size of a large dog, but with a much longer tail. It stood about 28 inches (70 cm) tall at the hips and was about 6 feet (2 meters) long from snout to tail, scientists said. It is estimated to have lived 214 million years ago. Remarkably, the skeletons show little sign of having being flattened during fossilization, as usually occurs, paleontologists noted. Tawa is classified as part of a group of dinosaurs called theropods, which includes T. Rex and Velociraptor. Theropods mostly ate meat, walked on two legs and had feathers. Though most went extinct by 65 million years ago, scientists generally believe some lineages survived to spawn modern birds. Sterling Nesbitt, a postdoctoral researcher at The University of Texas at Austin, conducted the research with colleagues while a graduate student at Columbia University’s Lamont-Doherty Earth Observatory and the American Museum of Natural History. One of Tawa’s biggest contributions to science has to do with what it said about another dinosaur, Herrerasaurus, according to Nesbitt and colleagues. Herrerasaurus has been at the center of a lively debate since its discovery in Argentina in the 1960s. Herrerasaurus had some similarities with theropods—including large claws, carnivorous teeth and certain pelvic features—but lacked other theropod traits. Some paleontologists claimed it was so unusual it was outside the evolutionary tree of theropods, or even of dinosaurs. Others placed it among the earliest theropods. “The question was did those carnivorous traits arise in Herrerasaurus and in theropods independently or were they traits from a recent common ancestor that got passed down,” said Nesbitt. “We had so few specimens of early theropods that it was hard to answer that question. But now that we have Tawa, we think we have an answer.” Tawa had a mix of Herrerasaurus-like characteristics—such as in the pelvis—and features found in firmly established theropods, such as pockets for airsacs in the backbone, according to the discoverers. This confirms, they said, that characteristics that Herrerasaurus shares uniquely with theropods such as Tawa didn’t arise independently and that Herrerasaurus is indeed a theropod. The firm classification of Herrerasaurus as a theropods points up an interesting fact about dinosaur evolution, scientists said: once dinosaurs appeared, they very rapidly diversified into the three main lineages that persisted for more than 170 million years. Herrerasaurus was found in a South American rock layer alongside the oldest members of two major lineages—the sauropods and the ornithischians. “Tawa pulls Herrerasaurus into the theropod lineage, so that means all three lineages are present in South America pretty much as soon as dinosaurs evolved,” said Nesbitt. Tawa skeletons were found beside two other theropod dinosaurs from around the same period. Nesbitt noted that each of the three is more closely related to a known dinosaur from South America than they are to each other. This suggests these three species each descended from a separate lineage in South America, rather than all evolving from a local ancestor, Nesbitt and colleagues said. They later would have dispersed to North America and other parts of Pangaea, a supercontinent that included all of today’s continents gathered up together, before about 200 million years ago. The finding also suggests there were multiple dispersals out of South America, according to the group. The first Tawa fossils were discovered in 2004 by volunteers taking a week-long paleontology seminar with experts at the Ruth Hall Museum of Paleontology in Abiquiu, New Mexico. The dig site, known as Hayden Quarry, is in a hillside on Ghost Ranch made famous by the painter Georgia O’Keefe. Alex Downs, an instructor for the course, contacted Nesbitt and a colleague to ask if they’d like to see the fossils. “When we saw them, our jaws dropped,” said Nesbitt. “A lot of these theropods have really hollow bones, so when they get preserved, they get really crunched. But these were in almost perfect condition.” He was also surprised by how much material was preserved at this one site. The fossil bone bed extends for tens of meters along the hillside, promising years of painstaking work and perhaps additional significant discoveries, he said.