March 4, 2009
An Early Theropod Leaves an Impression on Scientists
A restoration of Dilophosaurus forming the body impression found in Utah. By H. Ky Luterman.
If there is one top complaint paleontologists have about restorations of dinosaurs in movies, it is that the filmmakers never get the hands right. Theropods, be they Tyrannosaurus or Velociraptor, are always shown with their “palms” facing downwards—even though this would have been anatomically impossible. Paleontologists have long known that they held their hands so that their palms faced each other, almost as if they were holding a basketball.
A new paper published in PLoS One by a team of scientists from Utah and Colorado confirms what paleontologists have long known from the structure of the bones. About 198 million years ago, a large theropod dinosaur trudged along a muddy stretch of land, leaving well-defined tracks behind it. It also stopped every now and then to crouch down on a small berm near an ancient lake. When it did so, impressions were made of the positions of other parts of its body like its tail, hips, and hands.
The hand impressions showed that this dinosaur held its hands facing each other. This is important because it means that this way of holding the hands, which is also seen in modern birds, appeared relatively early among theropod dinosaurs. It is yet another trait considered “bird-like” that appeared in dinosaurs first.
As is typically the case, the precise identity of the trackmaker cannot be determined. Footprints, like fossils of organisms, are traditionally given their own genus name so that scientists can identify the same types of prints from different locations. The Utah tracks were attributed to a well-known track genus called Eubrontes, although the theropod Dilophosaurus (which is known from the same area, even if it is geologically a little younger than the tracks) seems like a good candidate for the animal that made the marks.
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Are these the same prints that some interpreted as evidence of feathers for Dilophosaurus back before all the Yixian discoveries or are they new?
I mean, new in a very limited sense of the word…
Just to clarify, this also means that they did not have any sort of “wrist” as well, correct?
Hi All — As one of the paper’s co-authors, I thought I’d respond to these comments:
These are new…well, they were discovered in 2004, but other than a quick mention in a couple of places, they had not yet been published. The ones you’re thinking of that had been attributed to a Dilophosaurus-like animal, and that had been interpreted as possibly exhibiting “protofeather” impressions, are about the same age as these new tracks, but were found way out east — they’re from the Portland Formation (part of the Newark Supergroup) in Massachusetts. The age and environment of the Portland Formation are similar to that of the Moenave Formation in which these tracks were found here in Utah, but the Massachusetts tracks don’t have hand prints like ours do. Ours also don’t show anything about potential skin the way the Massachusetts tracks do (and those have been heavily debated, to the point where, presently, I think most paleontologists don’t consider them “protofeather” impressions).
Well, not quite — they most certainly had a wrist, complete with wrist bones (though probably fewer than most mammals, like humans, have). The ability to turn the hands either palms-up or palms-down, though, doesn’t have anything to do with the wrist in humans and other mammals. Instead, that ability comes directly from the lower arm bones — the two (ulna and radius) between the elbow and wrist. (The ulna is the bigger, thicker bone and the one that sticks out at the elbow end to make that bony lump in the elbow; the radius is the smaller, skinnier one.) In humans and some other mammals, the elbow end of the radius has a little roller surface on it, and there’s a corresponding surface on the elbow. When this end of the radius rolls across this surface, the rest of the radius crosses over the ulna, and it is this motion that enables turning the hand either palms-up or palms-down — you can see this in your own arms and hands: when you turn your hand palms-up, you are also rotating the lower arm so that the “bottom” of the arm is up when the palms are (and you can feel the movement just below the elbow if you gently use your other hand to feel this region while turning your arm palms-up and -down). There are good pictures of this here and here, for two examples. Theropod dinosaurs — and most other vertebrates, actually, even ones that walk on all fours and have their hands more or less locked palms-down — lack this ability. Movement in the wrist is limited to the “waving goodbye” motion and a tiny bit of side-to-side motion in humans; in theropod dinosaur wrists (including birds), there seems to be more of the side-to-side motion and less of the “waving goodbye” motion.
I hope this all helps!
It does, thanks
[...] this week a new paper in the journal PLoS One reported a set of fossilized impressions that showed how theropod dinosaurs held their hands. Scientists were able to confirm that [...]
[...] are time and again shown with their hands in a palms-down position, something that would have been anatomically impossible (at least without moving the arms to which those wrists were attached to). This does not mean that [...]