Monday, April 28, 2008

When Did Dinosaurs Go Extinct? Cretaceous-Tertiary Boundary Dating Refined

ScienceDaily (Apr. 28, 2008) — Scientists at the University of California, Berkeley, and the Berkeley Geochronology Center have pinpointed the date of the dinosaurs' extinction more precisely than ever thanks to refinements to a common technique for dating rocks and fossils.
The argon-argon dating method has been widely used to determine the age of rocks, whether they're thousands or billions of years old. Nevertheless, the technique had systematic errors that produced dates with uncertainties of about 2.5 percent, according to Paul Renne, director of the Berkeley Geochronology Center and an adjunct professor of earth and planetary science at UC Berkeley.
Renne and his colleagues in Berkeley and in the Netherlands now have lowered this uncertainty to 0.25 percent and brought it into agreement with other isotopic methods of dating rocks, such as uranium/lead dating. As a result, argon-argon dating today can provide more precise absolute dates for many geologic events, ranging from volcanic eruptions and earthquakes to the extinction of the dinosaurs and many other creatures at the end of the Cretaceous period and the beginning of the Tertiary period. That boundary had previously been dated at 65.5 million years ago, give or take 300,000 years.
According to a paper by Renne's team in the April 25 issue of Science, the best date for the Cretaceous-Tertiary, or K/T, boundary is now 65.95 million years, give or take 40,000 years.
"The importance of the argon-argon technique is that it is the only technique that has the dynamic range to cover nearly all of Earth's history," Renne said. "What this refinement means is that you can use different chronometers now and get the same answer, whereas, that wasn't true before."
Renne noted that the greater precision matters little for recent events, such as the emergence of human ancestors in Africa 6 million years ago, because the uncertainty is only a few tens of thousands of years.
"Where it really adds up is in dating events in the early solar system," Renne said. "A 1 percent difference at 4.5 billion years is almost 50 million years."
One major implication of the revision involves the formation of meteorites, planetessimals and planets in the early solar system, he said. Argon-argon dating was giving a lower date than other methods for the formation of meteorites, suggesting that they cooled slowly during the solar system's infancy.
"The new result implies that many of these meteorites cooled very, very quickly, which is consistent with what is known or suggested from other studies using other isotopic systems," he said. "The evolution of the early solar system - the accretion of planetessimals, the differentiation of bodies by gravity while still hot - happened very fast. Argon-argon dating is now no longer at odds with that evidence, but is very consistent with it."
Renne has warned geologists for a decade of uncertainty in the argon-argon method and has been correcting his own data since 2000, but it took a collaboration that he initiated in 1998 with Jan R. Wijbrans of the Free University in the Netherlands to obtain convincing evidence. Wijbrans and his Dutch colleagues were studying a unique series of sediments from the Messinian Melilla-Nador Basin on the coast of Morocco that contain records of cycles in Earth's climate that reflect changes in Earth's orbit that can be precisely calculated.
Wijbrans' colleague Frits Hilgen at the University of Utrecht, a coauthor of the study, has been one of the world's leaders in translating the record of orbital cycles into a time scale for geologists, according to Renne. Renne's group had proposed using the astronomical tuning approach to calibrate the argon-argon method as early as 1994, but lacked ideal sedimentary sequences to realize the full power of this approach. The collaboration brought together all the appropriate expertise to bring this approach to fruition, he said.
"The problem with astronomical dating of much older sediments, even when they contain clear records of astronomical cycles, is that you're talking about a pattern that is not anchored anywhere," Renne said. "You see a bunch of repetitions of features in sediments, but you don't know where to start counting."
Argon-argon dating of volcanic ash, or tephra, in these sediments provided that anchor, he said, synchronizing the methods and making each one more precise. The argon-argon analyses were conducted both in Berkeley and Amsterdam to eliminate interlaboratory bias.
Argon-argon dating, developed at UC Berkeley in the 1960s, is based on the fact that the naturally-occurring isotope potassium-40 decays to argon-40 with a 1.25-billion-year half-life. Single-grain rock samples are irradiated with neutrons to convert potassium-40 to argon-39, which is normally not present in nature. The ratio of argon-39 to argon-39 then provides a measurement of the age of the sample.
"This should be the last big revision of argon-argon dating," Renne said. "We've finally narrowed it down to where we are talking about fractions-of-a-percent revisions in the future, at most."
Klaudia Kuiper, the lead author of the Science paper, was a Ph.D. student in Amsterdam working with study coauthors Wijbrans, Hilgen and Wout Krijgsman when the study was initiated. She also conducted lab work with Renne and Alan Deino, a geochronologist with Renne at the Berkeley Geochronology Center who was also one of the study's coauthors.
The work was funded by the U.S. and Dutch National Science Foundations and the Ann and Gordon Getty Foundation.
Adapted from materials provided by University of California - Berkeley.
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Friday, April 25, 2008

Molecular Analysis Confirms Tyrannosaurus Rex's Evolutionary Link To Birds

ScienceDaily (Apr. 25, 2008) — Putting more meat on the theory that dinosaurs' closest living relatives are modern-day birds, molecular analysis of a shred of 68-million-year-old Tyrannosaurus rex protein -- along with that of 21 modern species -- confirms that dinosaurs share common ancestry with chickens, ostriches, and to a lesser extent, alligators.
The work, published in the journal Science, represents the first use of molecular data to place a non-avian dinosaur in a phylogenetic tree that traces the evolution of species. The scientists also report that similar analysis of 160,000- to 600,000-year-old collagen protein sequences derived from mastodon bone establishes a close phylogenetic relationship between that extinct species and modern elephants.
"These results match predictions made from skeletal anatomy, providing the first molecular evidence for the evolutionary relationships of a non-avian dinosaur," says co-author Chris Organ, a postdoctoral researcher in organismic and evolutionary biology at Harvard University. "Even though we only had six peptides -- just 89 amino acids -- from T. rex, we were able to establish these relationships with a relatively high degree of support. With more data, we'd likely see the T. rex branch on the phylogenetic tree between alligators and chickens and ostriches, though we can't resolve this position with currently available data."
The current paper builds on work reported in Science last year. In that paper, a team headed by John M. Asara and Lewis C. Cantley, both of Beth Israel Deaconess Medi-cal Center (BIDMC) and Harvard Medical School (HMS), first captured and sequenced tiny pieces of collagen protein from T. rex. For the current work, Organ and Asara and their colleagues used sophisticated algorithms to compare collagen protein from several dozen species. The goal: placing T. rex on the animal kingdom's family tree using molecu-lar evidence.
"Most of the collagen sequence was obtained from protein and genome databases but we also needed to sequence some critical organisms, including modern alligator and modern ostrich, by mass spectrometry," says Asara, director of the mass spectrometry core facility at BIDMC and instructor in pathology at HMS. "We determined that T. rex, in fact, grouped with birds -- ostrich and chicken -- better than any other organism that we studied. We also show that it groups better with birds than modern reptiles, such as alliga-tors and green anole lizards."
While scientists have long suspected that birds, and not more basal reptiles, are di-nosaurs' closest living relatives, for years that hypothesis rested largely on morphological similarities in bird and dinosaur skeletons.
The scraps of dinosaur protein were wrested from a fossil femur discovered in 2003 by John Horner of the Museum of the Rockies in a barren fossil-rich stretch of land that spans Wyoming and Montana. Mary H. Schweitzer of North Carolina State Univer-sity (NCSU) and the North Carolina Museum of Natural Sciences discovered soft-tissue preservation in the T. rex bone in 2005; Asara became involved in analysis of the colla-gen protein because of his expertise in mass spectrometry techniques capable of se-quencing minute amounts of protein from human tumors. While it appears impossible to salvage DNA from the bone, Asara was able to extract precious slivers of protein.
The current work by Organ and Asara suggests that the extracted protein from the fossilized dinosaur tissue is authentic, rather than contamination from a living spe-cies.
"These results support the endogenous origin of the preserved collagen mole-cules," the researchers write.
Organ, Asara, Schweitzer, and Cantley's co-authors on the Science paper are Wenxia Zheng of NCSU and Lisa M. Freimark of BIDMC. Their research was funded by the National Institutes of Health, the National Science Foundation, the Paul F. Glenn Foundation, and the David and Lucile Packard Foundation.
Adapted from materials provided by Harvard University.

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Sunday, April 20, 2008

Early Elephant 'Was Amphibious'


ScienceDaily (Apr. 20, 2008) — The scientists were investigating the lifestyle of two early elephants (proboscideans) Moeritherium and Barytherium that lived in the Eocene period, over 37 million years ago. By analysing isotopes in tooth enamel from Moeritherium they were able to deduce that it was very likely a semi-aquatic mammal, spending its days in water eating freshwater plants.‘
We know from molecular data that modern elephants share a common ancestry with the sirenians - aquatic sea cows and dugongs,’ said Alexander Liu of Oxford’s Department of Earth Sciences, lead author of a report of the research published online in PNAS. ‘It suggests that elephants may have an ancestor which was amphibious in its mode of life and we wanted to know if Moeritherium or Barytherium was this semi-aquatic ancient relative. Unfortunately only fragments of the skeletons of these early elephants survive, so instead of looking at their bones we looked at the chemical composition of their teeth to determine what they ate and how they lived.’
Alex Liu, with colleagues Erik Seiffert from Stony Brook University (USA) and Elwyn Simons from the Duke Lemur Center (USA), analysed the oxygen and carbon isotope ratios contained within tooth enamel from both extinct proboscideans.
While carbon isotopes can give clues as to an animal’s diet, oxygen isotopes found in teeth come from local water sources - and variations in the ratios of these isotopes can indicate the type of environment the animal lived in. They compared the ratios of these isotopes to definitely terrestrial animals from the same period and these results – when combined with results from studies of embryology, molecular data, and sedimentology – lead them to believe that Moeritherium was semi-aquatic.
Alex Liu commented: ‘We now have substantial evidence to suggest that modern elephants do have ancient relatives which lived primarily in water. The next steps are to conduct similar analyses on other elephant ancestors to determine when the switch from water to land occurred, and to determine exactly when the now fully-aquatic sirenians split from their semi-aquatic proboscidean relatives.’
Adapted from materials provided by University Of Oxford.

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