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Aquilops, the little dinosaur that could

Today, several colleagues and I named a really cute little dinosaurAquilops americanus. At around 106 million years old, Aquilops turns out to be the oldest “horned” dinosaur (the lineage including Triceratops) named from North America, besting the previous record by nearly 20 million years. Even more interesting is the fact that Aquilops is not at all closely related to later horned dinosaurs from North America, but is mostly closely related to forms that lived in Asia around the same time. This is in line with a growing body of evidence showing an exchange of animals between the two continents at that time.

Aquilops in life, by Brian Engh. CC-BY.
Aquilops in life, by Brian Engh. CC-BY.

To learn the major details, you can of course check out the paper in PLOS ONE or read any number of news articles and blog posts on the find. For the rest of this post (and probably one or two other posts), I instead want to talk about some things that weren’t necessarily discussed in the paper or the press. [note: I should definitely add that the fossil is housed at the Sam Noble Oklahoma Museum of Natural History; Matt Wedel has more over at his blog, and I’ll add some on the history of the project in an upcoming post]

What’s with that bump on the front of the beak?
The rostral bone, which forms the upper beak and is a feature that unambiguously identifies Aquilops as a horned dinosaur, has a funky bump on the front of it. I spent a lot of time up-close with the specimen in hand and under a microscope, trying to convince myself that it wasn’t just some piece of bone displaced by crushing. In the end, I’m pretty certain that it’s a “real” feature; i.e., something the animal had while it was alive. Perhaps it’s pathological (abnormal) bone, but once again the texture doesn’t look ugly enough for that to be my preferred explanation.

At any rate, this animal had a funky bump on the front of its face. No idea what it was for (fighting? digging? something else?), but it sure was cool.

Skull and lower jaw of Aquilops, in my hand for scale. CC-BY.
Skull and lower jaw of Aquilops, in my hand for scale. CC-BY.

That animal is pretty small! How do you know it’s not just a baby?
It is indeed small! Based on Matt Wedel’s estimates, it was probably about the size of a large raven and body mass of a bunny rabbit (we only have a skull, so this was based off scaling from more complete skeletons of closely related species, such as Archaeoceratops [body mass statement corrected after initial post]).

As you can see from the picture, the skull could fit pretty neatly into your hand. The overall head is a little less than two-thirds the size of the largest published skulls of its closest relatives (Liaoceratops and Archaeoceratops), so based on size alone I suspect it’s not fully grown. The bone texture is also consistent with a young-ish animal.

But…other features (such as the development of the teeth and the comparatively close fusion between some bones) suggest that although the animal was young, it probably wasn’t a baby (or even a kid). I imagine it something like the Aquilops equivalent of an adolescent or maybe a teenager.

Can’t you just slice up the bone to determine the age?
Unfortunately, we don’t have any limb bones, which are the gold standard for determining dinosaur age. Skull bones are a possibility (and have been used for some dinosaurs, at least to determine relative age), but…the growth and changes in skull bone are very poorly characterized (especially relative to calendar age), and virtually nothing is known about skull bone growth in early ceratopsians. Thus, microanatomy of skull bones probably wouldn’t be that informative.

Why name a new species if it the animal is not fully grown?
It is no secret that dinosaurs change–sometimes radically–as they grow up. So, this can raise some concerns about whether or not species should be established based on young specimens. However, I am pretty comfortable naming Aquilops as a new species, even if the skull is from a young individual. Several features of the skull–including the unique shape of the beak as well as some combinations of features in the skull–distinguish Aquilops from other animals. Many of these features seem to be fairly stable through the life of an animal in species where we have large samples (e.g., beak shape). Thus, there is little to suggest that we wouldn’t recognize a larger and older Aquilops when and if we find one.

Aquilops to scale next to a human. They look like they would make cute pets, but don't let that fool you--those beaks would have been sharp! Illustration by Brian Engh, CC-BY.
Aquilops to scale next to a human. A dinosaur at this size looks like it would make a cute pet, but don’t let that fool you–those beaks would have been sharp! Illustration by Brian Engh, CC-BY.

If you only have a skull, how do you know what the rest of the dinosaur looked like?
Even though we only had the skull, we did want to attach that head to the rest of the body for the artwork that is being used to publicize this find. The body plan of early horned dinosaurs such as Aquilops was fairly conservative–large head, long hindlimbs, shorter forelimbs, mostly bipedal, long tail. Decent skeletons are known for close relatives of Aquilops (such as Archaeoceratops) and quite fine skeletons are known for slightly more distant relatives (such as Psittacosaurus. Thus, although some details may be shown to be slightly off if a complete skeleton of Aquilops is found, we are pretty confident in the overall reconstruction.

Coming up…more behind the scenes tidbits
Update: Want to learn more? Check out this blog post from co-author Matt Wedel and a behind-the-scenes look at the artwork from Brian Engh.

Farke, A. A., W. D. Maxwell, R. L. Cifelli, and M. J. Wedel. 2014. A ceratopsian dinosaur from the Lower Cretaceous of western North America, and the biogeography of Neoceratopsia. PLOS ONE 9(12):e112055. [read the paper – open access!]

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