NEW INSIGHTS INTO THE APTIAN FAUNA OF THE ARUNDEL CLAY (POTOMAC FORMATION) OF CENTRAL MARYLAND

The Aptian Stage (121-112 Ma) of the Cretaceous Period was a time characterized by profound geologic upheaval and biotic changes. Most notably, eustatic sea levels were fluctuating but generally rising, driven, at least in part, by episodic oceanic flood basalt volcanism and magmatic underplating. The first of several oceanic anoxic events (OAE’s) took place in the Aptian that correspond with marine extinctions and perturbations in the global carbon cycle as recorded by excursions in stable carbon isotope ratios of that time. Additionally, accelerated sea-floor spreading was leading toward the final severing of a Eurasian-Laurasian land connection that until then had allowed mixing of vicariant and endemic taxa on both land masses. Before this land- bridge was severed, angiosperms had gained a foothold and were ascendant among the last descendants of the Late Jurassic sauropod-allosaur dominated dinosaur fauna and the last remnants of primitive triconodont mammals, baenid turtles, and hybodont sharks. These relict taxa all coexisted with what are now regarded as ancestors of the better-known Late Cretaceous faunas that were to replace them. Therefore, identifying occurrences of Aptian age deposits of terrestrial origin and documenting their faunas and paleoecological implications are crucial to elucidating our understanding of this critical interval. Such deposits are well known from both the American Western Interior and from Western Europe but what of the land that lies between them? The fauna of the Arundel Clay, a largely fluvial-lacustrine deposit outcropping in central Maryland, is one such fauna that sits at the crossroads between two continents as well as two temporal faunal and floral regimes. Moreover, the Arundel Clay contains diverse Aptian age taxa that further yield clues to its paleoecology and is the only known site from this period on the entire eastern seaboard of North America yielding vertebrate remains. Presently, these unique remains can only be found in one last known site in Maryland. As will be shown, new fossils and data resulting from renewed and continuous fieldwork since the late 1980’s are helping us to both revise and improve our understanding of this fauna and illustrate the Arundel’s importance in gaining a more complete understanding of the Early Cretaceous world in which it existed.