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Late Cretaceous sauropods of North America

Once thought of as a time when sauropod diversity was all but lost, the Late Cretaceous is now recognized as a time when one sauropod clade, Titanosauria, radiated into a multitude of forms occupying every continent. One reason this radiation long went unrecognized is that only a single species of titanosaur has ever been described from North America. That species, Alamosaurus sanjuanensis, appears in Maastrichtian-aged rocks following a lengthy interval from which no sauropods are known, a time dubbed the "sauropod hiatus". I am interested in understanding what factors caused sauropods to disappear from North America and how titanosaurs finally came to colonize the continent.Alamosaurus reconstruction

Composite reconstructions of adult and juvenile Alamosaurus sanjuanensis (Lehman and Coulson, 2002)

At present, no fossil evidence records the dispersal route taken by titanosaurs into North America. Asia and South America have both been proposed as possible continents of origin for Alamosaurus, each with different implications for how Maastrichtian faunas in North America achieved their composition. With no direct evidence available, establishing the phylogenetic position of Alamosaurus holds the key to determining the origin of the species. The species-level relationships within Titanosauria have yet to be determined. Alamosaurus is one of the best-known taxa within the clade, but the utility of the information available is limited by uncertainty surrounding the taxonomic status of much of the material collected to date.

My previous work on Alamosaurus has focused on evaluating new and previous collected titanosaur specimens from Texas and comparing them to material from the type locality in New Mexico and from Utah. Although it was not possible to demonstrate whether all the material pertained to a single species, the new information was a step toward that goal. Comparison of the new specimens to other titanosaur species showed greater similiarities to titanosaurs from Late Cretaceous Brazil, providing support for the hypothesis of a South American immigration event.