Dinosaurs: Beyond Cute

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At about a pound and a half, Mahakala omnogovae was certainly a cute dinosaur. But cuteness is not why paleontologists traveled to the remote ends of Mongolia to find it. It’s part of a much bigger story.

Paleontologists have known for a while now that birds evolved from one group of dinosaurs called theropods–the two-legged beasts that include the likes of Tyrannosaurus rex and Velociraptor. But precisely which lineage of theropods birds belong to has been the subject of a lot of debate. These debates, like all debates in science, are fueled by uncertainty. Paleontologists can base their arguments only on the fossils they have already pulled out of the ground. So when a new dinosaur turns up, it provides an opportunity to give these arguments a fresh look.

Today in Science, a team of paleontologists describe the newly discovered Mahakala, which lived about 80 million years ago. They survey the anatomy of this long-tailed, chicken-sized dinosaur. And then they do what paleontologists generally do these days: they compared it to a lot of other dinosaurs. They generated an evolutionary tree, which I’ve reproduced at the bottom of this post. You can get a bigger picture by clicking on it. A new species such as Mahakala sometimes allows scientists to get a clearer understanding of how species are related to one another. It can help show, for example, the order in which certain traits found in a group of species evolved.

Adding Mahakala to the mix produces a tree that shows birds sharing a common ancestry with two groups of dinosaurs–the dromeosaurids and the troodontids. (Update: on the tree below, birds are the upppermost branch, troodontids are the middle, and dromeosaurids are the lower most.) Earlier studies have pointed to those groups as close relatives of dinosaurs birds before, but this new analysis offers a more precise hypothesis: dromeosaurids and troodontids descend from a common ancestor, and that common ancestor in turns shares a common ancestor with birds. Roughly speaking, they’re siblings to each other and first cousins to birds.

Sorting out these family matters allows the scientists to learn about how the bird body plan–so different from any other living animal’s body plan today–gradually evolved. A couple months ago I wrote about how new research shows that the small genomes of birds actually got small in their non-flying dinosaur ancestors. Well, other traits seem to have gotten a head start, too. Scientists have been discovering more and more dinosaur fossils with traces of feathers or feather-like growths on their skin. In this new tree, a species known as Jinfengopteryx–which had originally been considered a bird–ends up on the troodontid branch, complete with contour feathers of the sort you can find on birds today. It’s additional evidence that the common ancestor of birds and troodontids and dromeosaurids already had fairly sophisticated feathers.

What’s particularly interesting about this new study is that the scientists looked at how big the animals all were. In recent years paleontologists have been finding some very big species of bird-like dinosaurs–see Utahraptor, for example at the bottom of the chart. The fuzzy picture of how all these dinosaurs are related to birds has made it difficult to understand how the size of bird ancestors changed. Flying birds are relatively small, although some flightless birds have evolved to big sizes. Did they evolve from some Utahraptor-sized ancestor? A lot turns on this question. If the ancestors of birds shrank down dramatically, they might have done so through an evolutionary change to their growth–stopping early, for example, or growing more slowly.

To answer that question, the scientists first estimated how much each dinosaur weighed, extrapolating from their bones. (This method works successfully when scientists test it on living birds.) They then used a series of statistical methods to estimate how much the common ancestors of these species weighed. Working their way back to the base of the tree, the scientists concluded that the ancestor of birds and their closest dinosaur relatives–a group called Paraves–was about the size of Mahakala. In other words, small.

This finding will have to be taken into consideration in future hypotheses about the origin of flight. The small size of birds was not an adaptation to flight. The reverse may have been true: cute little paravians may have already had a light enough body to experiment with gliding and flying. Their monstrous relatives got large later–on four separate occassions, in fact, marked 1 through 4 on this chart. This finding is part of a pattern that goes way beyond dinosaurs: animals get big again and again. In fact, the really interesting story may lie there, and not in the tiny size of bird ancestors. Cute, it turns out, only gets you so far.

[Illustration: © Frank Ippolito 2007]

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