Hesperonychus elizabethae

Hesperonychus elizabethae is a tiny carnivorous dinosaur; in fact, it's the smallest carnivorous dinosaur known from North America, and the smallest dinosaur of any sort known from the Cretaceous of North America. It comes from the late Campanian (75 million years ago) of Alberta, Canada, where it inhabited the lush coastal forests that grew along the margins of the Western Interior Seaway. 

My first hint of the existence of this animal came when I started finding tiny bones from carnivorous dinosaurs; really tiny bones. The photo below is of a claw from the foot; it's easily small enough to fit on a quarter. It was pretty clear that these bones came from dromaeosaurs; dromaeosaurs are small to medium sized carnivorous dinosaurs bearing a sickle-shaped claw on the second toe. The most famous dromaeosaur is Velociraptor, but more recent discoveries have shown that some Chinese dromaeosaurs, such as Sinornithosaurus and Microraptor were small, cat-sized or even smaller. It stood to reason that North America could have had similar small dinosaurs, if anything their absence was rather puzzling. It made sense that small carnivorous dinosaurs would be in North America, but I need to prove that these weren't just hatchlings of a big dromaeosaur, like Saurornitholestes langstoni. It was necessary to prove that they were the adults of a small species. 

This involved a lot of rummaging through collections. For several years, any time I was in a collection, I would sort through the fossils, quickly skimming through boxes, drawers, and cabinets full of bones, trying to find something that might go to the animal. I found a fair amount- toe bones, mostly, a few vertebrae from the tail- but these scraps weren't really enough to do anything with; they didn't establish that these weren't just juveniles, or allow us to make sense of the animal.

I'd largely given up on ever finding more of the animal, but I kept in the habit of searching through collections, just because you never know what you might find in that next cabinet. So one day, I was in the collections at the University of Alberta. I was looking through a cabinet down at the end of the rows, looking in the top drawer of the top cabinet at the very back. And even standing on a ladder, I couldn't really see what was in the back of the drawer. I'm not a terribly tall guy, so I ended up literally climbing up the collections- bracing one foot against the cabinet on one side, and the other foot against the cabinet on the other side- and then reaching up as far as I could, I reached in and grabbed a fossil from the very back of the very top drawer of the top cabinet of the last row.

It was not much to see. Just a lump of sandstone with a weird little bone in it, sitting in an old carboard box with a scrap of paper and a number on it. No other information. No identification, no provenance data. Nothing. 

This is the kind of thing that papers are made of.

My first thought was that it was one of the little bones that stick off the underside of the tail in duckbill dinosaurs. But this didn't quite seem right. Then I had one of these flashes of recognition. I'd seen this before, I remembered. It looked suspiciously like the hip bones of one of those small Chinese dinosaurs.

After some comparisons, I was starting to feel more confident that it was, in fact, part of the hips of a small dromaeosaur. But it was actually a pretty disappointing find. I'd been hoping for a nice piece of the jaws, maybe a maxilla with all the little dagger-like teeth still in their sockets. But this was just the hips, and in pretty bad shape. It didn't seem likely that they'd tell us much we didn't already know, or really be the piece of the puzzle that would lock everything together, so I was inclined to sort of write them off.

Oh well, I figured. It looks like we'll never get enough of this animal to really decipher it. Still, I figured it might be worth a shot to prepare the fossil out of the rocks, and see what we have. This, as it turned out, was a really good move.

Preparation of fossils is usually a slow, tedious, business. The bones are often delicate, and the rock is often hard, so it can take weeks of work to get the fossil to where it's possible to study and describe it in detail. In this case, we were lucky: the bone was well-preserved, the sandstone was weakly held together and easily prepared away, and the bone and the sand separated easily and cleanly from each other. It took a day to do the prep work, which ended up costing pabout $100. When I saw the specimen, I was pretty blown away.

We had an almost perfect set of hip bones, beautifully preserved. It's not quite as exciting as a skull, but they preserved a lot of features that clearly linked this animal to the little Chinese dinosaurs- the weird flanges sticking off the sides of the bones, the strongly hooked J shape of the pubic bones and the spatula-shaped end of the pubic bones. Critically, the bones were fused together: the animal had stopped growing. It wasn't a juvenile, it was a full grown adult, and probably about the size of a small cat.

We did an analysis to figure out what it was related to; and it came out with a group of small, Chinese dinosaurs. The Chinese things are known from around 120 million years ago, and Hesperonychus is about 75 million years old, so the discovery of Hesperonychus extended their range by about 45 million years- a damn long time. There are fossils from Hesperonychus (or something similar), from the Hell Creek Formation in Montana, and in the Lance Formation of Wyoming. Those rocks are around 65 million years ago, laid down just before the asteroid that wiped out the dinosaur, so these dinosaurs survived right up to the end.

My guess is that Hesperonychus was probably an arboreal animal. I can't prove it, but the toe bones suggest fairly long, slender toes of the sort that would be good for gripping branches; and the small size is consistent with this. Another curious detail is that the hips are actually broken; there's a lump on one of the pubic bones that looks like a bone that's broken, set, and started healing. Tree-climbing animals tend to break their bones a lot, given that they take a lot of falls. The relatives of Hesperonychus- Microraptor and Sinornithosaurus- also seem to have been tree-climbing animals. So I'd envision these animals as being a bit like cats or raccoons, climbing fairly frequently.

We did some fairly crude calculations and figured out that Hesperonychus probably weighed around five pounds. It was far and away the smallest thing in the community, and filled the role of a small, warm-blooded carnivore- something that nothing else at the time really seemed to fill. I had recently named Albertonykus as "the smallest North American dinosaur" but Hesperonychus was even smaller. So after naming one smallest dinosaur, I named another a few months later. I didn't really plan it that way, it just sort of happened. Intriguingly, nobody really caught me on this... the media has a pretty short attention span.

All in all it was a fun project, and just goes to show what you can do if you're persistent and keep your eyes open.

You can find the paper here  http://www.pnas.org/content/106/13/5002.full