Did Plants Make Feathered Dinos Fat?

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?)

Kids, come eat your dino, I mean turkey, wait no, dinosaur… never mind.

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. 

The biggest members of three feathered dinosaur groups Oviraptorosauria, Ornithomimosauria, and Therizinosauria topped out at an whopping 7,000-13,000 lbs, rivaling the mass of the largest predators of the day.

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.

The largest ornithomimosaur, Deinocheirus (shown here with paleontologist Altangerel Perle) is known from the latest Cretaceous. This same time and place preserves the most massive therizinosaurs.  Image © Louie Psihoyos/Corbis

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.

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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.

An unknown journal turns up!

I needed some background information on the fieldworkers who accompanied the Captain Marshall Field Paleontological Expedition to Argentina and Bolivia (CMFPE), including George F. Sternberg, John B. Abbott and others. I asked one of my volunteers, Dick Webb, an avid genealogist, to search for documentation about these men on-line. He found a site that he claimed was chock full of information about Abbott. When I followed the link he sent me, I found many useful documents and pictures. At the bottom of the page, I found a name and contact information for the person who had posted the pictures – a distant relative of Abbott’s. I wrote to compliment her page. I mentioned my project involving her relative and the part he played on the CMFPE. She wrote back right away with some unexpected good news: she had a diary Abbott kept in Argentina. “Would I like it?” she asked!

The journal arrives.
The journal arrives.

A few days later a package arrived at the museum. Inside I found the original journal in perfect condition. I have been so busy with other things that I’ve hardly had any time to read it over. Every now and then, though, I take it out and read a few pages.

The journal with its original cover.
The journal with its original cover.

From what little I know I can tell readers this: the Abbott journal is now one of the best sources I have on the CMFPE. It’s a first-person, daily account of expedition activities. Many entries are rich with details about the weather, their itinerary, the fossils they discovered, etc.

A random journal entry.
A random journal entry.

I look forward to the opportunity to transcribe this journal and incorporate its contents into my book.

Stay tuned.