Maryland State Dinosaur: Astrodon johnstoni

In 1858, workers digging the iron ore pits near Muirkirk in Prince George's County, Maryland, unearthed bone and tooth material from the clay deposits. These pits, part of the Arundel Formation, had been worked for iron ore for years — the Cretaceous clays contained nodules of siderite (iron carbonate) that were commercially valuable. The bones and teeth were noticed as curiosities.

Christopher Johnston, a prominent Baltimore physician and dental surgeon, examined the teeth and recognized their unusual anatomy. Cross-sectioning a tooth revealed a star-shaped internal pattern of enamel — a feature Johnston found remarkable enough to name. He described the teeth in 1859, proposing the genus name Astrodon for 'star tooth.' The species name johnstoni was later applied in his honor.

Joseph Leidy, the leading American vertebrate paleontologist of the era, subsequently examined Astrodon material and provided additional context for the animal. Leidy formally established the species in 1865. The teeth Johnston had described belonged to a large sauropod dinosaur — one of the long-necked plant-eaters — but the fragmentary nature of the material meant decades would pass before the full picture of Astrodon's anatomy could be assembled.

The Arundel Formation is an Early Cretaceous deposit in the Maryland Piedmont, primarily in Prince George's County. It formed roughly 112–110 million years ago in a low-lying river floodplain — a warm, densely forested swampland fed by rivers draining eastward toward a retreating sea. The modern suburb of Muirkirk sits on top of what was once coastal lowland vegetation and river mud.

The iron ore operations near Muirkirk, which ran from the 18th century through the mid-20th century, exposed the Arundel clays and inadvertently created a major paleontological resource. Workers regularly encountered bones and teeth while digging, and significant material was collected during the 19th century and studied by paleontologists including Leidy and later Othniel Charles Marsh.

Muirkirk is no longer mined. Dinosaur Park in Laurel — a few miles north, in the same Arundel Formation — is the closest public access point to this geological unit. The Arundel Formation's fauna included the predatory dinosaur Acrocanthosaurus (or an early relative of it), smaller ornithopods, and abundant plant life. The formation has never been systematically surveyed across its full extent; most of what's known came out of 19th-century mining operations that weren't looking for fossils.

For much of the twentieth century, the Texas sauropod material now known as Paluxysaurus was referred to as Pleurocoelus — a genus Marsh named from Arundel Formation material that was thought to overlap with Astrodon. This created a tangled nomenclatural situation: was 'Pleurocoelus' the same animal as Astrodon, just named differently? Were the Texas materials actually the same genus as the Maryland materials? Researchers argued these questions for decades.

The Astrodon/Pleurocoelus confusion arose because both names were based on fragmentary material from the same formation, and neither was complete enough to make an unambiguous comparison. Marsh named Pleurocoelus in 1888 from Arundel Formation material; the name was widely applied across the country to sauropod material from Early Cretaceous contexts in both Maryland and Texas.

Current taxonomy generally treats Astrodon johnstoni as a valid genus and species distinct from Pleurocoelus, and the Texas material that was once called Pleurocoelus has since been renamed Paluxysaurus jonesi. This resolution took most of the twentieth century and multiple revisionary analyses. Maryland's Astrodon has come through the process with its name intact — a 170-year-old genus name that has survived both competition and taxonomic revision.

The choice of Astrodon was not contested. It is the only named dinosaur genus with clear Maryland provenance, and its scientific history — predating the Civil War and involving major figures in early American paleontology — gave the designation a historical weight that younger state dinosaur campaigns don't usually have. The argument for Astrodon wasn't 'this is a cool dinosaur'; it was 'this is the oldest chapter in North American dinosaur paleontology, and it happened here.'