"Long before it's in the papers"
January 30, 2011

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Some dinos may have survived dieoff—for a while

Jan. 30, 2011
Courtesy of the University of Alberta
and World Science staff

The dinosaurs are supposed to have died out between 65.5 and 66 mil­lion years ago. But at least one duck-billed dino­saur doesn’t seem to have got­ten the me­mo as it plod­ded over what is today New Mexico about 64.8 million years ago, some scient­ists say.

The finding, they claim, con­founds the ac­cept­ed chro­nol­o­gy for the demise of di­no­saurs, and suggests that others also lived on some­what past their sup­posed ex­pir­ation date. The re­search­ers led by Lar­ry Hea­man at the Uni­vers­ity of Al­ber­ta in Can­a­da say the plant eat­er was alive about 700,000 years af­ter the mass ex­tinc­tion that many pa­le­on­tol­ogists be­lieve wiped out all “non-avian” di­no­saurs.

An artist's re­con­struc­tion of had­ro­saurs in pre­s­ent-day New Mex­i­co around 70 to 80 mil­lion years ago, rel­a­tives of a fos­sil­ized rep­tile dat­ed to about 64.8 mil­lion years ago in a new stu­dy. (Im­age by Rich­ard Pen­ney, NM Mu­se­um of Nat­u­ral His­to­ry and Sci­ence)
Hea­man and col­leagues used a new dat­ing meth­od called ura­ni­um-lead dat­ing. A la­ser beam dis­lodges min­ute par­t­i­cles of the fos­sil, which are then an­a­lyzed for their iso­topes, or lev­els of dif­fer­ent vari­ants of chem­i­cal el­e­ments. The new tech­nique, sci­en­tists say, not only al­lows the age of fos­sil bone to be de­ter­mined but po­ten­tially can dis­tin­guish the type of food a di­no­saur eats. 

Liv­ing bone con­tains very lit­tle ura­ni­um, but dur­ing fos­sil­iz­a­tion, typ­ic­ally with­in 1,000 years af­ter death, the bone gains in ura­ni­um con­tent. Af­ter fos­sil­iz­a­tion, these ura­ni­um atoms de­ter­i­o­rate at a known rate to be­come lead. Thus the pre­s­ent lev­els of these sub­stances give a meas­ure of the amount of time since fos­sil­iz­a­tion.

Cur­rent­ly, pa­le­on­tol­ogists date di­no­saur fos­sils us­ing a tech­nique called rel­a­tive chro­nol­o­gy, the Uni­vers­ity of Al­ber­ta re­search­ers ex­plained. Where pos­si­ble, a fos­sil’s age is es­ti­mat­ed rel­a­tive to the known age of a lay­er of earth in which it was found. Fail­ing that, the lay­ers above and be­low the fos­sil are used. But ob­tain­ing ac­cu­rate es­ti­mates by this meth­od is hard, all the more so be­cause a fos­sil can drift out of its orig­i­nal lay­er in the rocks. The ura­ni­um-lead meth­od is de­signed to side­step these prob­lems. 

Sci­en­tists widely be­lieve that the di­no­saurs were killed off af­ter de­bris from a gi­ant me­te­or­ite im­pact blocked out the Sun, caus­ing ex­treme cli­mate con­di­tions and kill­ing vegeta­t­ion world­wide. 

Hea­man and his col­leagues say there could be sev­er­al rea­sons why the New Mex­i­co had­ro­saur, or duck-billed dino­saur, came from a line­age that sur­vived the great mass ex­tinc­tion of the pe­ri­od, called the late Cre­ta­ceous. Hea­man said it could be that in some ar­eas the vegeta­t­ion was­n’t wiped out and some had­ro­saur spe­cies sur­vived. It’s even pos­si­ble, the re­search­ers said, that di­no­saur eggs might have been able to sur­vive ex­treme cli­mate con­di­tions, though this idea needs more re­search.

If the new dat­ing tech­nique bears out on more fos­sil sam­ples then con­ven­tion­al views on the end of the di­no­saurs may need re­vi­sion, ac­cord­ing to Hea­man and col­leagues, whose find­ings ap­peared online Jan. 26 in the jour­nal Ge­ol­o­gy.

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A fossilized dinosaur bone found in New Mexico confounds the accepted chronology that the age of dinosaurs ended between 65.5 and 66 million years ago, researchers say. Researchers led by Larry Heaman at the University of Alberta in Canada dated the thigh bone of a hadrosaur, or duck-billed dinosaur, as being only 64.8 million years old. That would mean the plant eater was alive about 700,000 years after the mass extinction many paleontologists believe permanently wiped out all “non-avian” dinosaurs. Heaman and colleagues used a new direct-dating method called uranium-lead dating. A laser beam dislodges minute particles of the fossil, which are then analyzed for their isotopes, or levels of different variants of chemical elements. The new technique, scientists say, not only allows the age of fossil bone to be determined but potentially can distinguish the type of food a dinosaur eats. Living bone contains very little uranium, but during fossilization, typically within 1,000 years after death, the bone gains in uranium content. After fossilization, these uranium atoms deteriorate at a known rate to become lead. Thus the present levels of these substances give a measure of the amount of time since fossilization. Currently, paleontologists date dinosaur fossils using a technique called relative chronology, the University of Alberta researchers explained. Where possible, a fossil’s age is estimated relative to the known age of a layer of earth in which it was found, or failing that, the layers above and below the fossil. But obtaining accurate estimates by this method is hard, all the more so because a fossil can drift out of its original layer in the rocks. The uranium-lead method is designed to sidestep these problems. Scientists widely believe that the dinosaurs were killed off after debris from a giant meteorite impact blocked out the Sun, causing extreme climate conditions and killing vegetation worldwide. Heaman and his colleagues say there could be several reasons why the New Mexico hadrosaur came from a line of dinosaurs that survived the great mass extinction of the period, called the late Cretaceous. Heaman said it could be that in some areas the vegetation wasn’t wiped out and some hadrosaur species survived. It’s even possible, the researchers said, that dinosaur eggs might have been able to survive extreme climate conditions, though this idea needs more research. If the new dating technique bears out on more fossil samples then conventional views on the end of the dinosaurs may need revision, according to Heaman and colleagues, whose findings appear in the Jan. 26 issue of the journal Geology.