May 7, 2012
Ankylosaur Reef
A full-size restoration of what Aletopelta might have looked like, at the San Diego Natural History Museum. Photo by the author.
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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Strangely, the most common dinosaur in the Niobrara chalk is also a nodosaur. There might be something to the hypothesis that their armored skin helps them “stay together” longer, enabling them to “bloat and float” for greater distances (they wind up on the ocean floor up to 200 miles from the nearest shoreline). This may also be the reason we get the occasional (but exceptopnally rare) gar that far out too.
@ Anthony: interesting. I have heard here and there that perhaps the reason armored dinosaurs are more common in marine strata is because they may have favored habitats around rivers, thus increasing the chance that they could get swept out to sea.
Horner’s first article on dinosaurs from the bearpaw shale indicated that, at the time, there were more nodosaurids known from marine deposits than terrestrial, and they constituted the most abundantly known dinosaur taxon preserved within marine strata.
Interestingly – the carlsbad ankylosaur is not only encrusted by serpulid worms and oysters, but also has hollowed-out limb bones, which the original authors (Coombs and Demere 1996) argued was taphonomic rather than natural, as no other ankylosaurs are known to possess hollow limb bones.
Although one can certainly view this as a case of information loss – it is a far more interesting case, taphonomically speaking. We know that after skeletonization, the bones were colonized – and that the preserved portion of the skeleton was not substantially disarticulated during this period of time, which is surprising given that it was exposed on the seafloor long enough to host relatively large encrusting bivalves. This subject is of keen interest for me, as I just submitted a manuscript over the weekend documenting barnacle colonization of fossil sea lion bones.
Lastly – a friend of mine who was a former SDSU student affectionately referred to this specimen as the “ankylosaur ass”.