Ankylosaur Reef

Dinosaurs created temporary reefs. At least, the ones whose bodies floated out to sea did.

Even though there were no aquatic dinosaurs, dead dinosaurs sometimes washed down rivers to the coast. When their bodies settled on the ocean bottom, scavengers of various sorts and sizes glommed onto the dinosaurs and formed short-lived communities with their own ecological tempo—perhaps similar to what happens to the carcasses of modern whales. The Cretaceous dinosaur bones found in my home state of New Jersey are the result of this kind of transport and marine breakdown, and other examples have been found at sites around the world.

Even bodies of the heavily armored ankylosaurs were sometimes swept out to sea. They must have been quite a sight—a bloated, belly-up ankylosaur, drifting for as long as the gases inside its body could keep it afloat. One of these dinosaurs, found miles from the closest land at that time, was recently discovered in the oilsands of Alberta, Canada, but this wandering ankylosaur isn’t the only one we know of. When I visited the San Diego Natural History Museum last month, I saw another.

Hung on the wall, the creature was less than half the dinosaur it used to be. Even though additional parts of the dinosaur were recovered when it was excavated during the construction of the Palomar-McClellan Airport in 1987, the articulated hindlimbs and adjoining hip material is what museum visitors are greeted with. (The rest sits in the collections.) At first glance, the specimen doesn’t look like much. But what makes this fossil so strange is the group of associated creatures. Embedded on and around the dinosaur bones were shells from marine bivalves and at least one shark’s tooth. This ankylosaur had settled and been buried in the sea off the coast of Cretaceous California.

Tracy Ford and James Kirkland described the ankylosaur in a 2001 paper included in The Armored Dinosaurs. Previously, the specimen didn’t have a proper scientific name. The dinosaur was simply referred to as the Carlsbad ankylosaur. And the details of the dinosaur’s armor, especially over the hips, seemed to be quite similar to that of another dinosaur called Stegopelta. This would make the Carlsbad ankylosaur a nodosaurid, a group of ankylosaurs that typically have large shoulder spikes but lack a tail club.

After reexamining the specimen, though, Ford and Kirkland came to a different conclusion. The dinosaur’s armor identified it as an ankylosaurid, the armored dinosaur subgroup that carried hefty, bony tail clubs. The club itself was not discovered, but the rest of the dinosaur’s anatomy fit the ankylosaurid profile. And the dinosaur was different enough from others to warrant a new name. Ford and Kirkland called the ankylosaur Aletopelta coombsi. The genus name, meaning “wandering shield,” is a tribute to the fact that the movements of geologic plates had carried the dinosaur’s skeleton northward over the past 75 million years.

We may never know exactly what happened to this Aletopelta. Detailed geological context is essential for figuring out how a skeleton came to rest in a particular spot, and that information was destroyed with the excavation of the skeleton. Still, paleontologists have put together a general outline of what happened to this dinosaur. The unfortunate ankylosaurid died somewhere along the coast, and its carcass was washed out to the sea by a river, local flood, or similar watery mode of transport. Aletopelta settled belly-up and was exposed for long enough to become a food source and even home for various organisms. Sharks and other larger scavengers tore at the carcass, but various encrusting invertebrates also settled on the skeleton. Fortunately for paleontologists, the skeleton was sturdy enough to survive all this and eventually be buried. Even though dinosaurs never lived in the marine realm, their deaths certainly enriched the sea.

References:

Ford, T., Kirkland, J. 2001. Carlsbad ankylosaur (Ornithischia: Ankylosauria): An ankylosaurid and not a nodosaurid. pp. 239-260 in Carpenter, K., ed. The Armored Dinosaurs. Bloomington: Indiana University Press.

Hilton, R.P. 2003. Dinosaurs and Other Mesozoic Reptiles of California. Berkeley: University of California Press. pp.39-40

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Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.