Maryland's State Dinosaur (Astrodon johnstoni)
Nearly 140 years after its discovery as Maryland's first known dinosaur, Astrodon johnstoni has been officially named the Maryland State Dinosaur (Fig. 1). In 1998, the Maryland General Assembly passed House Bill 1170 and Senate Bill 520 by wide margins. The designation becomes effective October 1, 1998. In the forefront of efforts to get Astrodon named State Dinosaur was Dr. Peter Kranz, local paleontologist, dinosaur hunter and educator.
In November 1858, Philip Tyson, then State Agricultural Chemist for Maryland, discovered two teeth in an open-pit iron mine in the Arundel Clay on the property of John D. Latchford near Muirkirk, Prince George's County. He turned the teeth over to Dr. Christopher Johnston, a local physician and dentist, who sliced one very thinly for microscopic examination. Dr. Johnston described the tooth in the American Journal of Dental Science in 1859 and named it Astrodon, or "star tooth," a name descriptive of the appearance of the dentine in cross section. The teeth currently are housed at the Peabody Museum of Yale University.
In 1865, famous pioneer of American paleontology, Dr. Joseph Leidy, formally described the new species and named it Astrodon johnstoni, making it the first sauropod dinosaur described from North America (Fig. 2). The Astrodon discovery was almost simultaneous with Leidy's most famous find, Hadrosaurus foulkii, dug up in New Jersey and considered to be the first dinosaur found in North America.
In late 1887, renewed exploration at iron pits in the Arundel Clay between Washington and Baltimore was carried out under the auspices of Othniel C. Marsh, first professor of paleontology in the U.S. and director of the Peabody Museum at Yale University in Connecticut. Collecting was done mainly by John Bell Hatcher, a former student of Marsh and a geologist with the U.S. Geological Survey. Hatcher soon found what would become the richest dinosaur fossil site ever found in the Lower Cretaceous of the East Coast -- the Arundel Clay in the area between Beltsville and Muirkirk in Prince George's County. The outcrop belt of the Arundel Clay between Washington and Baltimore became known as "dinosaur alley." Under adverse weather conditions during the winter of 1887-1888, Hatcher collected many different fossils, shipping them to Marsh at Yale University. Today, the collection resides at the Smithsonian Institution in Washington, D.C.
Not only were the bones that Hatcher collected disarticulated, which means they were not connected to each other, but they were not from a single individual. That made it very difficult to know how they should be reassembled to form a skeleton. Such fragmentary, disarticulated remains naturally led to confusion and controversy. There are hundreds of Astrodon bones in the collections of the Smithsonian Institution. In spite of that, exactly what kind of dinosaur Astrodon was and what it looked like has been debated for many years. O. C. Marsh had earlier called it a morosaur, and Charles W. Gilmore, curator of vertebrate paleontology at the Smithsonian in the 1940s, called it a brontosaur on one occasion. It is currently believed to be a brachiosaurid (Fig. 1).
Apparently no more teeth or other fossil remains of Astrodon were found until Arthur B. Bibbins, a graduate student at the Johns Hopkins University and museum curator and instructor of geology and biology at the Woman's College of Baltimore, found several invertebrate and vertebrate fossils at Muirkirk in 1894. Among his finds was one Astrodon tooth.
A few 20th century discoveries are noteworthy. Workers found a very large thigh bone from Astrodon while building the McMillan Water Filtration Plant at First and Channing Streets, N.W. in Washington, D.C. in 1942. Charles W. Gilmore, curator of vertebrate paleontology at the Smithsonian, described the bone as belonging to a sauropod about 10 feet (3 meters) high at the hips, 50 or 60 feet (15 to 18 meters) long, and weighing about 10 tons.
In August 1989, Robert Eberle unearthed another partial femur from the Arundel Clay near Arbutus, just south of Baltimore.
Two years later, in May 1991, Arnold Norden and his children, John and Heather, were hunting dinosaur bones in the Cherokee Sanford clay pit (also known as the Maryland Clay Products pit) near Muirkirk. (Dinosaur and other fossil vertebrates and invertebrates are common in a layer of dense clay charged with lignite, a layer known to the old miners as "blue charcoal clay.") Mr. Norden noticed some bone fragments and reported his find to the Smithsonian Institution. The excavation by a Smithsonian crew the following day retrieved part of an Astrodon femur. Although the proximal (upper) end was missing, its length was more than 3 feet (1 meter) and its weight approximately 90 pounds (40 kilograms). When whole, the femur may have been twice that size.
It is not always easy to differentiate the skeleton of a juvenile reptile from an adult. For a time, this led to confusion about the true size of Astrodon. Today, however, there is agreement that Astrodon johnstoni did grow to a length of 60 feet or more.
Astrodon was definitely vegetarian, but as with all dinosaurs it is difficult to be specific about the exact diet. It probably browsed conifers, cycads and low-growing plants. It probably was a forest dweller. Although its bones have been recovered from river deposits, if current ideas are correct, it did not spend its time in the water.
There has been debate about the taxonomic classification of Astrodon. It has been known in the past as Pleurocoelus, and some paleontologists have even suggested that it is synonymous with Brachiosaurus. At present, it is thought that Astrodon and Pleurocoelus are synonymous, with the name Astrodon having precedence because it was used first. Although it is probably a brachiosaurid, it is not Brachiosaurus. Where else it is found and how many species exist are areas of greater uncertainty. It may have been found in Texas, Montana, England, and Spain, and possibly Australia. There are three species recognized in Maryland: Astrodon nanus, Astrodon altus, and Astrodon johnstoni (nanus means "dwarf," and altus means "tall"). The latter two, however, are known from very limited material. These three possibly represent juvenile through old age specimens of the same species.
As uncertain and limited as the Astrodon material is, Astrodon is far more abundant and is better known than any other dinosaur in the Potomac Group. Because Astrodon was the first dinosaur found in the Maryland and the first sauropod described from North America, it is fitting that it be acknowledged officially as the "Maryland State Dinosaur."
______, 1996, Notes on the sedimentary iron ores of Maryland and their dinosaurian fauna, in Brezinski, D. K. and Reger, J. P. (eds.), Studies in Maryland Geology: Maryland Geological Survey, Special Publication No. 3, Maryland Geological Survey, pp. 87-115.
Weishampel, David B. and Young, Luther, 1996, Dinosaurs of the East Coast: The Johns Hopkins University Press, Baltimore, 275 p.
Compiled by the Maryland Geological
Survey, 2300 St. Paul Street, Baltimore, MD 21218
This electronic version of "Fact Sheet No.15 " was prepared by R.D. Conkwright, Division of Coastal and Estuarine Geology, Maryland Geological Survey. Please send comments on this page to Dale Shelton (firstname.lastname@example.org)