Dinosaur Park

Sauropods Before Astrodon

4 Digs

Image: An outdated illustration of Astrodon from the early 20th century.

Astrodon johnstoni was the first dinosaur discovered in Maryland and the first North American sauropod to be scientifically described. Nevertheless, the word “sauropod” (meaning “lizard foot”) did not appear in print until 1878, when it was coined by the 19th century paleontologist O.C. Marsh. This begs the question: how much did the scientists who studied and described the first Astrodon fossils truly know about this animal?

Giant bones that probably belonged to sauropods have been known for thousands of years by ancient peoples of North America, China, and elsewhere. The first sauropod fossil to be studied in the modern scientific tradition was a heart-shaped tooth from southern England. In 1841 Richard Owen, an influential British anatomist, gave it the name Cardiodon. At the time, Owen did not know anything about the tooth other than its age (Jurassic) and that it came from a large reptile. The real start of sauropod studies happened later that year, when Owen acquired a set of fossilized bones from several localities around England. The fossils, which included a neck vertebra and several incomplete limb bones, again did not give Owen much to work with. Still, their porous texture reminded him of whale bones. Owen imagined the owner of the fossils as a whale-sized marine crocodile, which he named Cetiosaurus (whale lizard).

In 1842, Owen coined the name “Dinosauria” to refer to advanced, straight-legged reptiles like Megalosaurus and Iguanodon. Still believing Cetiosaurus to be an ocean-going crocodile, he excluded it from the dinosaur club. This assessment would not last, however. As more “cetiosaur” bones trickled in, other British scientists began to suspect that these animals stood upright and walked on land. In 1867 Thomas Huxley concluded that “cetiosaurs” were, in fact, especially large dinosaurs.

On the other side of the Atlantic, Christopher Johnston, a professor at the Baltimore Dental College, came into possession of a pair of teeth found in a Prince George’s County clay pit. In 1859, he published a short article in which he named them Astrodon – what we now know to be America’s first sauropod. Given the timing of his publication, however, it is unlikely that Johnston made a direct connection between these teeth and the “cetiosaurs” of England. He referred to the owner of the Maryland teeth in only the vaguest of terms, calling them “thecodont* saurians.”

Indeed, it took until the 1870s for a proper understanding of sauropods to take shape. The western frontier yielded spectacularly complete skeletons of now-famous dinosaurs like Brontosaurus, Apatosaurus, and Diplodocus, clarifying the long-necked, long-tailed shape of sauropod dinosaurs. In 1888, a trove of fossils from Maryland, including jaws with Astrodon-type teeth, would finally reveal what sort of animal our state’s first dinosaur actually was.

*The word “thecodont” literally means socket tooth, referring to teeth that fit loosely in sockets rather than being attached to the jaw. However, “thecodont” was historically used as a catch-all term referring to the ancestors of dinosaurs, pterosaurs, and crocodiles. It is not clear which usage Johnston intended.

References

Hallett, M. and Wedel, M. 2016. The Sauropod Dinosaurs: Life in the Age of Giants. Baltimore, MD: Johns Hopkins University Press.

Johnston, C. 1859. Note on Odontography. American Journal of Dental Science 9:337-343.

Kranz, P. 2016. Personal communication.

Taylor, M.P. 2010. Sauropod Dinosaur Research: A Historical Review. Geological Society of London, Special Publications 343:361-386.