By Rory Cahill

Today, the biggest animal you’re likely to encounter in Laurel, Maryland, is a deer, or maybe a large dog. But over 100 million years ago, the land that is now Prince George’s County was home to a colossal animal, possibly approaching 70 feet in length and weighing more than several elephants.

That’s how paleontologist Thomas R. Holtz describes Astrodon, Maryland’s state dinosaur. This long-necked plant eater is a sauropod, a relative of iconic dinosaurs like Brachiosaurus. It was found at Dinosaur Park near Laurel, which preserves a section of Maryland’s Arundel Clay formation for fossil finding.

And Astrodon isn’t the only dinosaur found at the site. Holtz, who teaches at the University of Maryland, discusses a wide range of fossils, from a giant armored dinosaur to one of the largest carnivores ever found in North America and several smaller — but no less fascinating — species, all found in the densely populated suburbs of central Maryland.

Prehistory

During the early Cretaceous period, Holtz says, the Arundel Clay would have been a heavily wooded, swampy environment connected to the ocean. Holtz cites the Atchafalaya Swamp in Louisiana — or, to a lesser extent, the Florida Everglades — as a good modern-day comparison.

Yet even the largest alligators in the Everglades don’t come close to the size of the Arundel Clay’s dinosaurs. Aside from Astrodon, Holtz mentioned an armored dinosaur called Priconodon, an early relative of the more well-known Ankylosaurus.

Ankylosaurus, one of the largest armored dinosaurs known, would have weighed about six or seven tons and measured about 26 feet long, according to Holtz.

Priconodon, Holtz said, “might be considerably larger. We might be talking about 10 tons or more, and easily more than 30 feet.” 

Unfortunately, as with many of the dinosaurs found at the site, the fossils found are nowhere near a complete skeleton, making it difficult to put exact numbers on Priconodon’s size. Nonetheless, Holtz said, “it’s a heckin’ chonker.”

It’s not just the plant-eating dinosaurs that grew to enormous sizes. Acrocanthosaurus was a huge theropod dinosaur — the mostly carnivorous group of dinosaurs, which includes the more famous Tyrannosaurus, Spinosaurus and Velociraptor. Growing to over 30 feet long, Holtz said, this was one of the largest meat-eating dinosaurs found in the U.S. before the emergence of Tyrannosaurus.

Acrocanthosaurus is fairly well known from fossils found in the western states, but remains in Maryland are much more sparse, primarily teeth, according to Holtz. Recently, however, the bones of a young Acrocanthosaurus found in the Arundel Clay were described by Matthew Carrano, dinosaur curator at the National Museum of Natural History in Washington, D.C.

The bones were initially found in the 1990s, Carrano said, and were initially assumed to be an ornithomimid — a group of mostly small, lightly built theropods often compared to modern-day ostriches. 

“They were working with very limited material, it is sort of a slender-looking thing, and long ago people had identified [other fossils from the Arundel Clay] as ornithomimids,” Carrano explained.

However, the fossil had never been studied in depth until Carrano decided to take a closer look. After struggling to identify which species it belonged to, Carrano eventually realized it wasn’t an ornithomimid at all, and with help from other scientists, it was eventually identified as a young Acrocanthosaurus.

There is a wide variety of other, smaller dinosaurs found in the ecosystem, according to Holtz, although the fragmentary nature of the fossils makes it difficult to pin down the exact types. However, they likely include small early ancestors of Tyrannosaurus and Triceratops, a raptor dinosaur (possibly the Velociraptor relative Deinonychus) and what Holtz described as a “beaked plant-eater” (possibly the 20-foot-long Tenontosaurus). And of course, there were many non-dinosaur animals at the site as well, including fish, turtles, crocodilians and early mammals. 

Human history

While it only became an official park in 2009, according to the park’s paleontology manager Victor Perez, its history of fossil finding goes back much further.

The first fossils at Dinosaur Park were found in 1858, said Perez. The site was originally an iron mine, and then a clay mine, but eventually the miners found more than just clay: they found the bones of a huge, extinct dinosaur. But while the Arundel Clay stretches through a large part of central Maryland, today’s Dinosaur Park only protects a tiny fraction, surrounded by heavy suburban development.

Development, Perez said, can be helpful when it unearths fossils that otherwise might never have been found. However, it can also make it more difficult to recover the fossils that are found. Many states in the west require paleontologists on site to verify that no important fossils are being destroyed by new construction, Perez said. But the eastern U.S. has no such protections for fossils.

“If construction is happening, we might get lucky enough that they contact us because they came across something interesting,” Perez said. “But in most cases, they probably would not contact us because our involvement would only delay or slow down the process.”

Featured Image: A view of the University of Maryland’s geology building on Monday. Photo by Anika Stikeleather