Diversity through time and space in the Upper Jurassic Morrison Formation, western U.S.A.

Understanding how biodiversity has changed through time and space is a central aim of paleobiology. To elucidate accurate biodiversity patterns in deep time, regional case studies, where sampling biases can be minimized, are needed. The Upper Jurassic Morrison Formation of the western U.S.A. crops out over 1.2 million km² and covers 12° of latitude. It was deposited over a ∼9-million-year time period, and was home to some of the most iconic dinosaurs. Utilizing a new, high resolution chronostratigraphic framework for the formation, tetrapod occurrences from the Paleobiology Database were temporally and spatially mapped to examine patterns of diversity change through time and space, and the geographic ranges of taxa were examined to shed light on niche partitioning. Latitudinally, diversity was found to peak in the center of the basin, perhaps due to the availability of water resources. Diversity increased over time in the Morrison Formation, and there is no evidence to indicate a decline in diversity prior to the extinction of the fauna at the end of the Jurassic. There appears to be some degree of geographic separation of faunas in the Morrison basin, with a southeastern and northwestern fauna, albeit with a number of overlapping taxa. High resolution climate models paired with detailed sedimentological analysis could help to elucidate the drivers of the patterns observed here.