Critters Abroad: Aletopelta

Hey there every peoples!

California is not a good place for dinosaurs. When I was growing, I was obsessed with dinosaurs (that was before I discovered the joys of mammals) but was hard pressed to find any. Sure Los Angeles had some fantastic fossils, and the California Academy of Sciences had some too, they just weren’t in any kind of abundance. That’s because for most of the age of dinosaurs, California was underwater, with a tropical sea lapping at the feet of steep coastal mountains. As you should know, dinosaurs didn’t live in water and mountain environments don’t preserve fossils very well. But every now and then the odd dinosaur fossil pops up, usually in marine sediments. And one of these rare dinosaur fossils really stands out from the others.

Its name is Aletopelta coombsi. Aletopelta was found back in 1987 during a construction project (as many, many fossils are found in California) in Carlsbad, near San Diego. The front end was scraped away by a backhoe, but a substantial amount of the skeleton was recovered, allowing a description of the material.

Holotype specimen of Aletopelta coombsi, from the late cretaceous of California

Not terribly impressive, but for dinosaur poor California that’s pretty good. Aletopelta means “wandering shield”, because when Aletopelta was deposited, it was living just across the border in Mexico. Tectonic forces have since moved the remains into what is now California (and as we’ll see, “wandering” is apt for this particular specimen). The species name honors Walter P. Coombs, Jr., and expert on ankylosaurs. Aletopelta was originally thought to be a nodosaur. But later work argued, based on plate and tooth morphology as well as limb proportions, that it is an ankylosaur. Thus Aletopelta is the only ankylosaur known from the west coast. It is also the only dinosaur named from California. But what makes specimen interesting to me is how it was deposited.

Fleshed out model of Aletopelta coombsi at the San Diego Natural History Museum

As I alluded to above, this animal’s name can be applied for another reason besides plate tectonics. Aletopelta was found in 75 million year old marine sediments. But how did it get there? Heavy armor plating and a stiffened tail ending in a club hardly qualify as adequate swimming gear. It is thought that ankylosaurs lived along the banks of rivers and estuaries, since their teeth seem adapted to eating the kinds of soft vegetation that grow along their edges. Because Aletopelta was found in the ocean, it is likely that the animal fell into a river where it drowned. Its carcass was washed out to sea, where it probably bloated from decomposition and floated around for a few days. A shark tooth was found among the remains which suggests sharks may have scavenged the carcass. So after a while the animal’s body fell to the ocean floor. The remains rested on the sea bed long enough for a colony of oysters and clams to grow on them. Slowly the dinosaur and mollusks were buried by sediments. Geologic uplift brought the sediments out of the sea where millions of years of erosion and one piece of construction equipment with good timing exposed them for the first time in 75 million years. That’s quite a journey to undertake!

 

Aletopelta and Lambeosaurus in prehistoric California (illustration by Ken Kirkland)

How lucky are we that a freeway needed to be expanded so that a backhoe could uncover one of the most complete dinosaurs on the west coast? And one that could tell such a story nonetheless! It may not be as impressive as other finds made in more dinosaur rich areas, but it’s an animal found nowhere else (and isn’t just another hadrosaur) and adds a splash of the age of dinosaurs to the Golden State.

Til next time!