Maryland holds a prominent place in North American paleontology, containing some of the continent’s earliest evidence of dinosaurs, with initial discoveries dating back to the mid-19th century. These finds established a record of life from the Early Cretaceous period when the eastern seaboard was a low-lying, subtropical landscape. The fossils offer a unique window into terrestrial ecosystems not well-represented elsewhere in the region. Studying these remains helps scientists construct a more complete picture of dinosaur distribution and evolution across the continent before the formation of the Western Interior Seaway later divided the landmass.
The Geological Setting of Maryland’s Dinosaurs
The presence of dinosaur fossils in Maryland is directly tied to the Arundel Clay formation, a specific geological unit within the larger Potomac Group. This sequence of sedimentary rocks runs along the Atlantic coastal plain and dates primarily to the Early Cretaceous period (roughly 112 to 130 million years ago). At that time, the area was an extensive inland fluvial environment characterized by slow-moving rivers and oxbow swamps. This ancient ecosystem was a broad, low-relief floodplain covered in dense vegetation. The soft, clay-rich sediments and iron-ore deposits acted as an exceptional preservation medium for terrestrial animal remains. The Arundel Clay formation is significant because it represents the only major source for Early Cretaceous terrestrial vertebrate fossils on the eastern seaboard. These deposits formed in depressions within the underlying Patuxent Formation, creating lenses of dark, carbonaceous clay and mudstone.
Maryland’s Giant Herbivores
The most famous dinosaur from the state is the immense sauropod, Astrodon johnstoni, a long-necked herbivore that defined the Early Cretaceous fauna. Adults reached lengths of 50 to 60 feet and possibly weighed up to 20 tons. Its morphology suggests it belonged to the Brachiosauridae family, characterized by longer forelimbs than hindlimbs, giving it a giraffe-like posture.
The initial discovery occurred in 1858 when Philip Tyson recovered two teeth from an iron mine near Muirkirk in Prince George’s County. One tooth was examined by Dr. Christopher Johnston, who observed a distinct star-shaped pattern, leading to the name Astrodon, meaning “star tooth.” This finding was one of the earliest dinosaur discoveries in the United States and the first sauropod formally described from North America. Muirkirk and the surrounding areas, including Dinosaur Park, have yielded the most significant fossil evidence. While much of the material is fragmentary, a 1990s find included a femur nearly two meters long, proving the dinosaur’s immense scale. Astrodon johnstoni was designated the official State Dinosaur of Maryland in 1998.
Other Dinosaur Residents of the Cretaceous Coastline
While sauropods were the largest inhabitants, the ancient Maryland ecosystem was also populated by diverse groups, including carnivores, bipedal herbivores, and armored species. Evidence for these residents is often fragmentary, consisting of isolated teeth or small bone fragments washed into the river systems.
Carnivores (Theropods)
Recent discoveries point to the presence of large carnivorous dinosaurs, possibly related to Acrocanthosaurus or other large carnosaurian predators. A three-foot-long shin bone, the largest theropod fossil found in eastern North America, was recovered from a bone bed in the Arundel Clay. Smaller carnivores, such as Dromaeosaurs (the group that includes Deinonychus), are suggested by distinctive fossil teeth, indicating swift, feathered hunters were part of the fauna.
Bipedal Herbivores (Ornithopods)
The bipedal, plant-eating Ornithopods were represented by early Hadrosauroids, the ancestors of the duck-billed dinosaurs. Although definitive bone material is rare, a possible caudal vertebra and tooth fragments suggest the presence of species like Tenontosaurus, a mid-sized herbivore. These animals used their beaks and grinding teeth to process the tough, fibrous plants of the low-lying swamps.
Armored Dinosaurs (Nodosaurids)
Armored dinosaurs, specifically members of the Nodosauridae family such as Priconodon crassus, also roamed the region. These four-legged herbivores were covered in thick, bony plates and spikes for defense. Historically known only from teeth, more complete material, including a gigantic tail vertebra, has recently been unearthed, confirming the variety of dinosaurs that once called Cretaceous Maryland home.