This post-Thanksgiving week, as you try to erase the memory of Uncle Henry indulging in his third slice of pumpkin pie, you may be delighted at the prospect of a distraction. In that case, try this one on for size. If your holiday meal included the traditional turkey, than you’ve just fed your family on the roasted carcass of a small dinosaur. Yes, that’s what I said, a small dinosaur. Feel better? Yeah… I didn’t think so.
Turkeys, like all birds, are members of a fascinating group of dinosaurs called theropods. Paleontologists love theropods, in part, because they were the Harry Houdini of the Cretaceous Period, managing a great escape when all other dinosaurs bit the big one 65 million years ago. This small factoid means that rather than always having to marvel at extinct animals, paleontologists can also study a small group of living dinosaurs in the flesh (how cool is that?!). And while living birds are cool, theropod dinosaurs on the whole were even cooler back in their heyday. Think: if birds are Maxwell Smart, than Cretaceous theropods are James Bond. This is because Cretaceous theropods also included animals like T. rex and Velociraptor, meaning that birds could have hosted some wicked and weird relatives at their holiday meals. (Can you imagine if cousin Utahraptor dropped by for Christmas dinner?)
Thankfully, a lot of Cretaceous theropods were more into veggies than flesh, including some very odd-looking and unusually large critters whose evolutionary history is just beginning to unfold. And while the benefit to your waistline that comes from choosing more vegetables over meat this holiday is clear, the same cannot necessarily be said for these veggie-loving theropod dinosaurs.
Scientists have long considered larger body size to be advantageous to vegetarian animals. Larger guts can fit larger digestive tracts allowing animals to get more energy from food with a lot of fiber and few calories. For that reason, scientists tend to think that bigger is better when it comes to plant eating animals. Interestingly enough, as you get closest to birds on the theropod family tree, the biggest species are also the ones we think ate plants not prey. Could this mean that feathered theropods fit the same pattern? A colleague of mine and I were dying to find out.
We began by estimating body mass for 47 species of feathered theropods representing three major groups that abandoned a strictly meat-eating diet–ornithomimosaurs (“bird-mimics”), oviraptorosaurs (“egg-thieves”), and the bizarre therizinosaurs (“scythe-lizards”). Our results showed that all three groups had members of gigantic proportions (up to 100 times more massive than an average person). The largest oviraptorosaur weighed over 7,000 pounds, and the biggest ornithomimosaurs and therizinosaurs topped out at over 13,000 pounds! rivaling the size of the great T-rex.
Nonetheless, just because some feathered dinosaurs got big, doesn’t mean that large size was an evolutionary advantage. To test whether these feathered dinosaurs were being driven to large body size by natural selection, we fitted a variety of evolutionary models to the data, looking to see which model best described the pattern. Turns out that plant eating theropods experimented with larger and smaller body sizes as they evolved and there was no clear drive to get big, no simple, overwhelming advantage to reach gigantic proportions. However, we did find one interesting pattern… different species from the same time and place tended to be about the same relative size.
Although we were a bit disappointed not to find a trend toward large body size, in a way this latter discovery was much more interesting. It suggests that changing environments during the evolution of these animals played a bigger role in body size evolution. Different climates, range sizes, resource abundance all exerted a stronger influence on the evolution of size than the simple relationship between eating plants and being big.
Of course, there are two other possibilities… first, these theropods might have been eating a high proportion of caloric plant material like fruits, nuts, and seeds instead of low fiber foods to begin with or, (and this one is always the elephant in the room for paleontologists), bias in the fossil record is messing with our data.
As in all good science, this research raised more questions for us than it answered. Back to the drawing board.
You can read more about this research in the November 28th online edition of the journal Proceedings of the Royal Society B. Thanks to my colleague Peter Makovicky of the Field Museum for his contribution to this blog post.