Fossils discovered in the oft-painted arroyos of northern New Mexico show for the first time that dinosaurs and their non-dinosaur ancestors lived side by side for tens of millions of years, disproving the notion that dinosaurs rapidly replaced their supposedly outmoded predecessors.
The fossils were excavated from the Hayden Quarry at Ghost Ranch, an area made famous through the paintings of Georgia O'Keefe, by a team of paleontologists from the University of California, Berkeley, the American Museum of Natural History and The Field Museum. The finds, including fossil bones of a new dinosaur predecessor the researchers have named Dromomeron romeri, are described in a cover story in the July 20 issue of Science.
"Up to now, paleontologists have thought that dinosaur precursors disappeared long before the dinosaurs appeared, that their ancestors probably were out-competed and replaced by dinosaurs and didn't survive," said co-author Kevin Padian, professor of integrative biology at UC Berkeley and a curator in the campus's Museum of Paleontology. "Now, the evidence shows that they may have coexisted for 15 or 20 million years or more."
According to primary authors Randall Irmis and Sterling Nesbitt, graduate students, respectively, at UC Berkeley and at New York's American Museum, the new bones provide anatomical information that tells paleontologists about the evolution of dinosaur precursors, their transition into true dinosaurs and how dinosaurs diversified.
"Finding dinosaur precursors, or basal dinosauromorphs, together with dinosaurs tells us something about the pace of changeover," Irmis said. "If there was any competition between the precursors and dinosaurs, then it was a very prolonged competition."
An alternative hypothesis held that the sudden extinction of many animals in the Late Triassic period allowed dinosaurs to diversify and eventually populate the globe. Based on the new findings, however, "quite a few of the groups proposed to go extinct survived well into the Late Triassic," Irmis said.
Sampling Size! Sampling Size! It all depends on the Sampling Size!
I suspect that over the next century, the story of evolution and mass extinctions is going to be shaken up quite a bit. A lot of the generalizations we make about what happens during these events are based on a handful of sites. This is often really true of the terrestrial environments.
The only good transitions for the KT Boundary for the terrestrial environment are, iirc, in North America. The more famous of these is Hell's Creek. A lot of theories have been built on what has happened based on the fossils of this one, single or sometimes two locales. This is exceedingly disingenuous. While it is understandable that there is a huge temptation to try to build big theories on what data you have, people ought to take a step back and think about the fact they are working with such small, small samples and in very similar environs. For all they know, the dinosaurs went extinct in the rest of the world sometime before this. OTOH, they may have declined MORE here at Hell's Creek than other places. true, the nature of geology makes it very unlikely there will be lots of spots that are fossiliferous, terrestrial, and have contiguous sediment through the whole period that is to be studied.
Likewise, there are two locales with good spots for the terrestrial PT Boundaries. Fortunately, these are in Russia and South Africa, making them almost polar opposites of the same megacontinent. However, what was happening in the terrestrial realm in, say, the tropical realm? We have an idea in the marine environs, but not, alas, in the terrestrial. We can make extrapolations, to some extent, with the help of the marine environment: if the biota hasn't radically changed there, it's not that likely to have changed on land. We hope.
Sampling issues, like the above can and do lead us into iniquity, erm, I mean, error. There's a big debate about the Big Five Mass Extinctions. There are those - like Hallam, frex - that have been arguing that based on the sampling issues we have with the Cambrian and Late Triassic that there may not be the mass extinction that we think is there. There have been fossils lately out of Britain that show critters that were supposed to have died off earlier in the Triassic during the extinction event did not. Additionally, there seems to be evidence of a dicynodont in Australia during the early Cretaceous. Now we have the cousins or precursors of the dinosaurs - as noted above - surviving past the Late Triassic Event. Because these critters had the gall to not die where we could find their remains as fossils, little - or not so little - ingrates! we often get the wrong impression as to when they died out or that they were even there the first place (cue New Zealand Miocene mammals). Something not all that different is happening with the Cambrian fossils. More and more of the relatives of the ones found in the Burgess Shale are being found in the Ordovician, lessening the number of critters or their close relatives that seemed to have gone extinct at the end of Cambrian.
In the end, I think that the mass extinctions are going to undergo some revisions as we find more sites that have good samples of the time frames we want to study wrt the extinction events. There will be surprises. There will be some 'oh duh' moments. there will be 'how could they have thought that' moments. I even better there will one or two events that might even fade from the top five or grow into the 6th (barring our current one).
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