Wednesday, June 2, 2010

Sinoceratops and the Ceratopsian Family Tree

I think I'll keep the ceratopsian train chugging along here. First, a correction. Yesterday I mentioned that the Sinoceratops description in the Chinese Science Bulletin puts forth the idea that the ceratops-ians may have originated in Asia. Actually, Xu Xing and his coauthors write this of the ceratops-ids. Those couple letters make a big difference. My lovely brain has a nasty habit of fudging subtle details like those suffixes. Maybe laying out the context into which these new dinosaurs fit will be a sort of mental strength training.

When I began writing today, I figured I'd talk about Yinlong, an important basal ceratopsian from China. But I think it may be better to draw the big picture, the ceratopsian family tree, if you will, and save Yinlong for tomorrow. This will dip into the often maddening world of taxonomy, the classification of living things, which is an even more difficult undertaking when dealing with a source of data as fragmentary as the fossil record. While I'm not looking to get granular, I'll preface this by saying that names of groups and the members therein may change as paleontologists find new fossils and debate how the old ones relate to each other.

The ceratopsians are a member of the largely herbivorous clan of dinosaurs called the ornithischians, which I discussed a bit in a post from last August. Most of the ceratopsians, save for some of the most primitive, are quadrupedal, and the group ranges widely in size, from as big as a dog to larger than an elephant.

The pelvic differences between saurischians and ornithischians. By yours truly, a derivative work based on separate diagrams by wikimedia user Frederik.

The ceratopsians all share a completely unique bone called the rostrum, which forms the top half of their beaks. Find one of these, and you've got a ceratopsian for sure.

There are many families under the ceratopsian umbrella, but I'm not going to get into the details of all of them. As this review is spurred by the Sinoceratops discovery, I'll instead discuss its family, the ceratopsids: the largest ceratopsians, distinguished from each other by a variety of ornaments on their skull. The basics are the bony frill extending from the back of their heads and the horns, bumps, and knobs on their faces - working from these basic elements, the variety is astounding.

Based on these ornaments, the ceratopsids are then divided into two main posses: the centrosaurinae, which generally bear larger nose horns with smaller frills and brow horns, and the chasmosaurinae, which have the opposite arrangement. Looking at those horns is a good way to get a rough idea of what kind of ceratopsid you're dealing with - just as you might look at the shape of a beak to begin to identify a strange bird in your yard. Some prominent chasmosaurines are Triceratops, Chasmosaurus, and the new Medusaceratops; the centrosaurines include Styracosaurus, Einosaurus, and Centrosaurus. Sinoceratops is notable for the blend of centrosaurine and chasmosaurine characteristics in its skull, which is the basis for Xing's idea of an Asian origin for the ceratopsids.

Centrosaurines, by Nobu Tamura, via Wikimedia Commons

It used to be a given that if a new ceratopsid was discovered, it would be from western North America in Cretaceous rock. Sinoceratops naturally flips the geographical part of that on its head, and raises questions about how wide the range of this classically North American group could have been. Xing floats two guesses as to why ceratopsids are mostly found in North America. First, there may have been factors that limited the ceratopsids geographically, and what we see is what we get. This is certainly plausible, but a bit shaky considering how few windows to the Cretaceous the rocks give us. He favors the second option, which is that we simply haven't found everything. Sinoceratops is a tantalizing look at what may be there waiting to be unearthed.

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