Monday, January 31, 2011

Some Dinosaurs Survived the K-T Extinction Event



Dinosaur fossils are relatively rare, although high concentrations of remains do occur at a few localities. Moreover, the endemic nature of dinosaurs, in even closely spaced localities, has hindered the ability of science to confidently determine the biogeographic diversity, evolution, and radiation of these animals. These problems have been exacerbated by the fact that precise age determinations of dinosaur-bearing rocks have generally not been possible, due to a lack of precisely dateable rock layers, such as altered volcanic ash beds, in dinosaur-bone bearing strata. The San Juan Basin (SWB), of northwest New Mexico and southwest Colorado is one of the few places where a series of radiometric ages through Upper Cretaceous strata provides precise age constraints for the abundant and diverse dinosaur fossils found in these rocks. In addition, the ages of dinosaurs from Paleocene strata in the SWB have been tightly bracketed by fossil pollen and paleomagnetic data. James E. Fassett and colleagues present data that, for the first time, directly date SWB Cretaceous and Paleocene dinosaur-bone samples  based on laser-ablation, U-Pb methodology. Zircon U/Pb geochronology using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) is rapidly being adopted in the earth sciences. The use of this new tool to directly date dinosaur fossils may well revolutionize our understanding evolution. University of Alberta Press Release for Fasset et al. follows.


University of Alberta researchers determined that a fossilized dinosaur bone found in New Mexico confounds the long established paradigm that the age of dinosaurs ended between 65.5 and 66 million years ago.The U of A team, led by Larry Heaman from the Department of Earth and Atmospheric Sciences, determined the femur bone of a hadrosaur as being only 64.8 million years old. That means this particular plant eater was alive about 700,000 years after the mass extinction event many paleontologists believe wiped all non-avian dinosaurs off the face of earth, forever.


Heaman and colleagues used a new direct-dating method called U-Pb (uranium-lead) dating. A laser beam unseats minute particles of the fossil, which then undergo isotopic analysis. This new technique 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 low levels of uranium but during fossilization (typically less than 1000 years after death) bone is enriched in elements like uranium. The uranium atoms in bone decay spontaneously to lead over time and once fossilization is complete the uranium-lead clock starts ticking. The isotopic composition of lead determined in the hadrosaur's femur bone is therefore a measure of its absolute age.


Currently, paleontologists date dinosaur fossils using a technique called relative chronology. Where possible, a fossil's age is estimated relative to the known depositional age of a layer of sediment in which it was found or constrained by the known depositional ages of layers above and below the fossil-bearing horizon. However, obtaining accurate depositional ages for sedimentary rocks is very difficult and as a consequence the depositional age of most fossil horizons is poorly constrained. A potential weakness for the relative chronology approach is that over millions of years geologic and environmental forces may cause erosion of a fossil-bearing horizon and therefore a fossil can drift or migrate from its original layer in the strata. The researchers say their direct-dating method precludes the reworking process.


It's widely believed that a mass extinction of the dinosaurs happened between 65.5 and 66 million years ago. It's commonly believed debris from a giant meteorite impact blocked out the Sun, causing extreme climate conditions and killing vegetation worldwide.


Heaman and his research colleagues say there could be several reasons why the New Mexico hadrosaur came from a line of dinosaurs that survived the great mass extinction events of the late Cretaceous period (KT extinction event). Heaman says it's possible that in some areas the vegetation wasn't wiped out and a number of the hadrosaur species survived. The researchers also say the potential survival of dinosaur eggs during extreme climatic conditions needs to be explored.


Heaman and his colleagues believe if their new uranium-lead dating technique bears out on more fossil samples then the KT extinction paradigm and the end of the dinosaurs will have to be revised. The research was published online, January 26, in the journal, Geology.


Citations
Fassett, J.E., L.M. Heaman, and A. Simonetti,  2011. Direct U-Pb dating of Cretaceous and Paleocene dinosaur bones, San Juan Basin, New Mexico. Geology (2011),39(2):159. doi:10.1130/G31466.1


Paton, C., J. D. Woodhead, J. C. Hellstrom, J. M. Hergt, A. Greig, and R. Maas. 2010.Improved laser ablation U-Pb zircon geochronology through robust downhole fractionation correction, Geochem. Geophys. Geosyst., 11, Q0AA06, doi:10.1029/2009GC002618.

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