November 22, 2010

Koreaceratops—A Swimming Ceratopsian?

The remains of Koreaceratops - the tip of the tail is to the upper right and the partial legs are to the lower left. From Lee et al., 2010.

Hot on the heels of a team of researchers who described Zhuchengceratops from the Cretaceous of China, paleontologists Yuong-Nam Lee, Michael J. Ryan and Yoshitsugu Kobayashi have just announced the discovery of another ceratopsian dinosaur—Koreaceratops hwaseongensis—from the 103-million-year-old rock of South Korea. It is the first dinosaur of its kind to be found in the country, though it shows some peculiar similarities to other dinosaurs found elsewhere.

Represented by a nearly complete tail, portions of the hips and partial hindlimbs, Koreaceratops was discovered on the west coast of the Korean peninsula in 2008 near Jeongok harbor. Not very much of it was left to compare to other dinosaurs, particularly since no elements of the skull were found, but the handful of distinctive characteristics in the hindlimbs and tail identified it as a ceratopsian dinosaur closely related to Archaeoceratops and Cerasinops.

The most prominent feature of Koreaceratops is its deep tail. Like Protoceratops, Montanoceratops and similar horned dinosaurs, Koreaceratops had a series of exceptionally long neural spines sticking up from its tail vertebrae which get progressively longer towards the end of the tail before becoming shorter near the tip. This would have given Koreaceratops a tall, deep tail which would have looked superficially like a paddle. Over the past century, several paleontologists have argued that ceratopsians with this tail shape may have been amphibious.

The authors of the new study approach the possibility that Koreaceratops was semi-aquatic tentatively. The paper’s abstract states that the tall neural spines of Koreaceratops, Montanaceratops and other ceratopsians may have evolved multiple times as a possibly adaptation to swimming, but in the body of the paper they state that the evidence that these dinosaurs were regular swimmers is equivocal.

I am doubtful that the deep tails of these dinosaurs can be taken as a good indicator of their swimming ability. As the authors of the new study document in the paper, the tail shapes of each of these deep-tailed ceratopsians varies significantly. Koreaceratops had a tail with taller and taller neural spines approaching the tip—making the end portion of the tail the deepest—while in Protoceratops the deepest portion is closer to the hips, being in the middle of the tail or just a bit closer to the rest of the body. If all of these dinosaurs had tails that independently evolved to allow them to propel themselves through the water, it might be expected that they would all have tails with the same shape, namely with the deepest part of the tail being near the tip as this would give them the most thrust. Instead, the different deep tail types may have been involved in display or species recognition, in which case we would expect for there to be variation in tail shape from one dinosaur to another.

Admittedly it is relatively easy to come up with hypotheses about tail function. What is more difficult is finding a way to test ideas about long-extinct organisms. In this case anatomy alone may not provide an unambiguous answer, but there may be a way to determine whether or not Koreaceratops and its kin were semi-aquatic. Paleontologists have regularly used levels of oxygen isotopes preserved in the teeth and bones of prehistoric animals to determine whether or not certain animals spent a great deal of time in the water. Earlier this year a different group of paleontologists used this technique to provide support for the idea that the predatory spinosaurs were semi-aquatic animals, and the same line of evidence could be applied in this long-running debate about ceratopsians. No single study will shut the case entirely, but the more lines of evidence we can draw upon to approach the question of swimming ceratopsians, the better.
References:

Lee, Y., Ryan, M., & Kobayashi, Y. (2010). The first ceratopsian dinosaur from South Korea Naturwissenschaften DOI: 10.1007/s00114-010-0739-y