The Mesozoic/Defining Disciplines: Late Nineteenth-Century Debates Over the Jurassic-Cretaceous Boundary

The last two decades of the nineteenth century were exciting times in American paleontology, with disputes over Jurassic dinosaurs between Edward Drinker Cope and Othniel Charles Marsh appearing in the press. Less well known is the dispute over defining the Mesozoic that began in 1888 when Marsh invited Lester Frank Ward, a colleague with whom he had been working on the Potomac Formation for the United States Geological Survey, to speak on the plant fossils found there. Initially agreeing with Marsh that the Potomac was a Jurassic formation, work on fossil cycads led Ward to conclude that the Potomac was Lower Cretaceous. As Ward and Marsh grappled with the question of how to determine the age and identity of Mesozoic systems, they joined other paleontologists and geologists such as William J. McGee, Albert Charles Seward, and Samuel W. Williston in a debate that often reflected scientific training and sub-specializations as much as stratigraphic principles, becoming caught up in a trans-Atlantic dispute in which their reputations were on the line as they claimed that ‘their’ fossils were key determinants of Mesozoic systems. In the end, Marsh's reputation as a paleontologist was far better established than that of Ward, who moved on to another career as a sociologist at Brown University, but cycad discoveries from Maryland, Colorado and Wyoming, and fieldwork, trumped laboratory studies—even when performed by a master systematist—as the Potomac Formation proved to be Lower Cretaceous.