Posted on 12/16/2009 9:05:33 AM PST by null and void
Pristine Skeletons Shed Light on Early Dinosaur Evolution
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| A reconstruction of the newly discovered Triassic, carnivorous dinosaur, Tawa hallae. Courtesy of Jorge Gonzalez |
When Darwin's finches diverged from their common ancestor, the isolation of their island home allowed many species to arise from one. When their dinosaur ancestors emerged in the Triassic, the island home was the unified landmass Pangea, and the evolution was far more complicated.
In the December 11, 2009, issue of Science, a team of paleontologists introduced the Triassic dinosaur Tawa hallae, an animal that may answer longstanding questions about the earliest years of dinosaur evolution. The Tawa fossils, collected along with other specimens during recent field excursions to Ghost Ranch in New Mexico, are some of the most complete and best preserved dinosaur skeletons from the Late-Triassic time period.
Tawa possesses features that appear in its contemporaries and features that do not, a finding that helps unite all Triassic carnivorous dinosaurs into one group, the theropods, the same group that included Tyrannosaurus rex, and now includes birds. The recent finds also support the hypothesis that dinosaurs first originated in what is now South America and soon after diverged into theropods, sauropodomorphs (the line that includes the ground-shaking giants like Apatosaurus) and ornithischians (a line that includes a range of body types, including Stegosaurus and Triceratops). Only after this divergence did dinosaurs disperse across the Triassic world more than 220 million years ago.
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| Based on an analysis of how Tawa relates to other early dinosaurs, researchers hypothesize that dinosaurs originated in what is now South America, and soon after diverged into ornithischians (like Triceratops), sauropodomorphs (like Apatosaurus) and theropods (like Tyrannosaurus rex), before dispersing across the Triassic world more than 220 million years ago. The theropods evolved into modern-day birds, although Tawa split off from the ancestral branch early on and was not a direct bird ancestor. Courtesy of Zina Deretsky, National Science Foundation |
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"Tawa gives us an unprecedented window into early dinosaur evolution, solidifying the relationships of early dinosaurs, revealing how they spread across the globe, and providing new insights into the evolution of their characteristics," says Sterling Nesbitt of the University of Texas at Austin, the lead author on the paper.
"If you have continents splitting apart, you get isolation," says Nesbitt. "So when barriers develop, you would expect that multiple carnivorous dinosaurs in a region should represent a closely related, endemic radiation," similar to what occurred with Darwin's finches. "But that is what we don't see in early dinosaur evolution," adds Nesbitt.
Instead, Nesbitt and his colleagues observed three distantly related carnivores in the fossil-rich, Late Triassic beds, implying that each carnivore descended from a separate lineage before arriving in North America, instead of all evolving from a local ancestor. In addition to Tawa, the researchers found fossils from a carnivorous dinosaur related to Coelophysis, common to that region, and fossils from a carnivore closely related to Herrerasaurus, which lived in South America.
The two- to four-meter-long skeletons of Tawa display characteristics that exist in both of its contemporaries, and features found in neither, implying a separate lineage. Unlike many theropods, Tawa's lineage does not lead directly to birds.
According to Nesbitt, the old view held that Herrerasaurus split off of the family tree after the ornithischians, but before the sauropods and theropods diverged. "Tawa now appears to show that the three groups split from each other as soon as dinosaurs evolved," he adds, though paleontologists have not yet found a concrete example of a dinosaur that existed before the divergence.
"Tawa is a very good example of a fossil that fills in what we call a morphological gap," says Nesbitt, referring to a gap in knowledge about how morphology, or body structures, changed over time, a result of the incomplete nature of the fossil record. While theropods were changing quickly in the Triassic, paleontologists have found few animals that preserve the "steps" that define the sequence of changes.
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| A reconstruction of the Tawa hallae skeleton. Courtesy of Sterling Nesbitt |
One of the most significant morphological gaps for early dinosaurs lies between Herrerasaurus and animals that are clearly more closely related to birds, such as Coelophysis. According to Nesbitt, Tawa fits perfectly in between. "It is not a missing link," he adds, "It evolved on its own lineage, but it retains characteristics that existed in Herrerasaurus that we thought were more primitive while also possessing features seen in unmistakable theropods, including birds, such as the presence of air sacs surrounding the braincase and neck."
"Usually, early dinosaur specimens are not as complete or well preserved, so they spur a lot more questions than answers," says Nesbitt. "Tawa is so well preserved that every bone we have, we can examine it in three dimensions. And we can analyze five of the skeletons this way, with examples of both mature and immature animals. This is just the tip of the iceberg. All dinosaurs share a common feature, an open hip socket, and you can dissect your Thanksgiving turkey and still see that original feature. But the earliest lineages that lie in between are far from understood."
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| A reconstruction of the head of Tawa hallae. Courtesy of Jorge Gonzalez |
Nesbitt's co-authors included Nathan Smith of the University of Chicago and the Field Museum of Natural History; Randall Irmis of the Utah Museum of Natural History, University of Utah; Alan Turner of Stony Brook University; Alex Downs of the Ruth Hall Museum of Paleontology in Abiquiu, N.M.; and Mark Norell of the American Museum of Natural History, where Nesbitt was a researcher at the time of the discovery.
The research was supported by the National Science Foundation (NSF) through grants to Mark Norell (0228607), Alan Turner (0608003), Nathan Smith (0808250) and NSF Graduate Research Fellowships to Sterling Nesbitt and Randall Irmis. The research was also featured in the NSF-funded IMAX 3D movie "Dinosaurs Alive!" (itself supported by NSF grant 0337269).
The research was also sponsored by the National Geographic Society with other participating institutions including the University of Chicago, the Field Museum of Natural History, the Utah Museum of Natural History, the University of Utah, Stony Brook University and the Ruth Hall Museum of Paleontology.
Easily? Maybe so, but I doubt a finch would cross 20 miles of open ocean if it didn't have to.
Darwin, apparently responding to critics, threw them a sop by writing of “life, with its several powers, having been originally breathed by the Creator into a few forms or into one . . ..”
I'm not following you here. Are you complaining that he acknowledged a Creator? I can see how that it wouldn't fit an anti-Darwin/anti-science biased world view, and how that would be disturbing to a fanatic. I don't think you are one though.
Continuing, we read “It was in fact because of their very proximity, as Darwin humbly admitted, that he didn’t keep very straight what specimens came from which island.”
Yes. I suppose the case would be strengthened if he'd lied about it, or if hundreds of other scientist didn't go back to the Galapagos and personally check his observations.
I rest my case.
Uneasily...
No, as was quite plain, I neither complained nor rejoiced that Darwin grudgingly allowed that a creator might have breathed life into one or more forms of life.
It is clear that no matter what I write, or read, nor whom or what I quote, your opinion of my supposed beliefs and convictions remains fixed.
Farewell.
'have to' is the key.
Follow the mating habits of the penguins, and explain why they trek 50-100 miles to the most uninhabitable part of the planet to reproduce.
You say that as if you are the only one it happens to.
: )
What a critter needs has everything to do with evolution.
Follow the mating habits of the penguins, and explain why they trek 50-100 miles to the most uninhabitable part of the planet to reproduce.
Touché!
;^P
Just sayin'
If you are saying that because a critter "needs" a longer neck to reach the tasty leaves, it will grow one, no. That's the Lamarckian theory of evolution. Stalin loved it, as it meant that if you beat people into the proper mold, you could create the "New Soviet Man".
This time you get the missing words award: "to survive".
What a critter needs to survive has everything to do with evolution.
In times of hardship the members of the population with longer necks eat just a little bit better than their short-necked cousins. Sometimes that makes the razor thin difference between starving and barely surviving.
The short-necked cousins really needed a longer neck to survive. They didn't have one. They didn't survive. Or leave descendants.
The ones that live long enough to reproduce have an input to what the next generation looks like. The ones who don't simply don't. Unfortunately, neither you, nor I, nor any living creature "needs" to survive. We all die in the end anyway.
(Although I will allow the possibility that members of 'the last generation' need never die)...
What a critter needs to survive has everything to do with evolution.
Unfortunately, neither you, nor I, nor any living creature "needs" to survive.
This last quote sounds more like the thoughts of Stalin.
If you are saying that because a critter "needs" a longer neck to reach the tasty leaves, it will grow one, no.
Not odd. Predictable, yes.
The problem stems from the conviction that one knows the truth, and that the opposition are idiots with false beliefs.
The truth is that we really just don't know exactly how it all works (life, evolution), nor how in the heck it all even started.
Aye, but giraffes didn’t get longer necks because they wanted them.
No single giraffe ever wished its neck longer, but ones who already had longer necks could gather more food and had a better chance of passing on their long neck genes.
Really no different than making a wiener dog breed out of generic dog stock.
Just in one case humans determine who wins the reproduction lottery, and in the other nature “selects” the survivors.
Arguably the most accurate, rational and sane comment ever made on a crevo thread.
(It doesn't change the fact that I'm right, you're wrong, and that settles it!)
};^P>
Well, at least you are honest.
: )
Due to the environmental conditions (competition with other 'leaf eaters'), they certainly needed them.... to survive.
We are becoming a bigger factor in the selection process. Not only through selective reproduction, but through alteration of environment.
Yeah. In spades!
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Note: this topic is from 12/16/2009."If you have continents splitting apart, you get isolation," says Nesbitt. "So when barriers develop, you would expect that multiple carnivorous dinosaurs in a region should represent a closely related, endemic radiation... But that is what we don't see in early dinosaur evolution," adds Nesbitt... three distantly related carnivores in the fossil-rich, Late Triassic beds, implying that each carnivore descended from a separate lineage before arriving in North America, instead of all evolving from a local ancestor. In addition to Tawa, the researchers found fossils from a carnivorous dinosaur related to Coelophysis, common to that region, and fossils from a carnivore closely related to Herrerasaurus, which lived in South America. The two- to four-meter-long skeletons of Tawa display characteristics that exist in both of its contemporaries, and features found in neither, implying a separate lineage. Unlike many theropods, Tawa's lineage does not lead directly to birds. According to Nesbitt, the old view held that Herrerasaurus split off of the family tree after the ornithischians, but before the sauropods and theropods diverged. "Tawa now appears to show that the three groups split from each other as soon as dinosaurs evolved," he adds, though paleontologists have not yet found a concrete example of a dinosaur that existed before the divergence.Thanks null and void. |
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Thanks null and void. |
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