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