Dinosaur Park

Cretaceous Roll Call: Maryland's Giant Carnivore

2 Digs

Illustration by Dean Cole.

There are no Tyrannosaurus rex fossils at Dinosaur Park – the Potomac Group rocks are about 45 million years too old – but Maryland did have a giant meat-eating dinosaur of its own. Unfortunately, its remains are vanishingly rare, and the exact identity of this prehistoric predator remains a mystery.

Theropods are two-legged, mostly meat-eating dinosaurs like T. rex and Velociraptor. Modern birds are also theropods. The first Maryland theropod fossils to be identified were discovered by the 19th century fossil hunter John Bell Hatcher in 1887. In the winter of that year, paleontologist O.C. Marsh sent Hatcher to survey the fossil-producing iron mines in Prince George’s County. Hatcher recovered hundreds of dinosaur bones, most of which belonged to the plant-eating Astrodon. Nevertheless, Marsh recognized a handful of bones as being very similar to the theropod Allosaurus fragilis, which he had named two decades earlier based on fossils from Colorado and Wyoming. In fact, Marsh thought the Maryland fossils were so similar that he declared them to be a second species of Allosaurus, which he called Allosaurus medius.

However, Marsh was working under the assumption that the Maryland fossils were Jurassic in age. Later researchers used fossil pollen and other stratigraphic indicators to determine that the Arundel Clay was actually deposited during the Early Cretaceous. This meant that the Maryland theropod clearly wasn’t Allosaurus – the two animals were separated in time by 35 million years. Several other names for this predator have been proposed over the years, including “Dryptosaurus” and “Creosaurus.” For now, these names only serve as unofficial placeholders. With so few fossils to work with, it’s impossible to properly define the Maryland theropod. We recognize that this assortment of limb bones and vertebrae are similar to large theropod bones found elsewhere in the world, but without good, diagnostic remains, such as a skull, we can’t be more specific. We can’t even say with certainty that there was only one kind of big theropod in Cretaceous Maryland – perhaps there were several!

The most compelling identification came from Thomas Lipka in 1998. The best-preserved Arundel theropod fossils are isolated teeth, probably because teeth are covered in enamel and are therefore more durable than bone. Lipka compared the Maryland teeth to other theropods, and concluded that they strongly resembled the teeth of Acrocanthosaurus, a dinosaur primarily known from Texas and Oklahoma. In particular, the number of denticles (bumps that make up a serrated edge) per 5 millimeters was nearly the same, as was the size of the denticles relative to the length of the tooth. Like the Arundel fossils, Acrocanthosaurus comes from the Aptian Age of the Cretaceous Period, so it seems reasonable that this species (or a close relative) made it as far east as Maryland.

Still, many paleontologists are hesitant to commit to an identification based solely on teeth. It’s also important to note that in 1998, Acrocanthosaurus was the only big theropod known from reasonably complete remains found in early Cretaceous North America. Many fragmentary fossils were attributed to it simply because it was the only available taxonomic container. What’s more, J.D. Harris made the case that a giant theropod vertebra found in Washington, DC was definitely not Acrocanthosaurus. This means that if the Dinosaur Park teeth did belong to Acrocanthosaurus, it was not the only big carnivorous dinosaur around.

What we need now are more fossils – diagnostic remains that can tell us the affinities of this animal with more certainty. Last summer, two young visitors each found half of a new big theropod tooth. With so few fossils available, every little bit is immensely important. Fortunately, we’re continuously searching for fossils at Dinosaur Park alongside sharp-eyed members of the public. If there’s more of the Maryland theropod to be found, we should find it sooner or later.

References

Carpenter, K., DiCroce, T., Gilpin, D., Kinneer, B., Sanders, F., Shaw, A., and Tidwell, V. 2002. Origins of the Early and “Middle” Cretaceous Dinosaurs of North America: Implications for Plate Tectonics. Proceedings of the International Symposium on New Concepts in Global Tectonics.

Harris, J.D. 1998. Large, Early Cretaceous Theropods in North America. New Mexico Museum of Natural History and Science Bulliten. No. 14 pp. 225-228.

Lipka, T.R. 1998. The Affinities of the Enigmatic Theropods of the Arundel Clay Facies (Aptian), Potomac Formation, Atlantic Coastal Plain of Maryland. New Mexico Museum of Natural History and Science Bulliten. No. 14 pp. 229-234.

Marsh, O.C. 1888. Notice of a New Genus of Sauropoda and Other Dinosaurs from the Potomac Formation. American Journal of Science, Third Series. Vol. 35, pp. 89-94.