Late Jurassic Dinosaurs on the Move, Gastroliths and Long-Distance Migration

The Jurassic Morrison Formation of the Rocky Mountain and Colorado Plateau regions is famous for its dinosaur fossils. The Morrison Formation (150-157 Ma) is comprised mostly of sandstone and mudstone that was deposited in a terrestrial deposition system that included fluvial, paludal, and lacustrine environments. Paleocurrent data indicates that Morrison sediment was transported to the north and east. Within the Morrison Formation, we find exotic pebble and cobble-size durable clasts of quartzite, chert and vein quartz weathering out of the mudstone paleosols. We interpret these exotic clasts as gastroliths, carried within the gastric mills of dinosaurs. For this study, we collected red and pink quartzites in the Bighorn Basin of Wyoming and in northeastern Utah. We determined the provenance of these red quartzite gastroliths using detrital zircon U-Pb geochronology and LA-ICPMS methods at the University of Arizona Laserchron Center. Ten individual gastroliths were analyzed, six from the Bighorn Basin and four from Utah. Zircons were separated from five gastroliths collected from the Morrison Formation in the northeastern Bighorn Basin, Wyoming (n=36, 68, 66, 41, 29). Four of the gastroliths have Yavapai and Penokean maximum depositional ages, and peak ages that are Penokean and Archean. We interpret these four gastroliths were derived from Yavapai (Baraboo Interval) quartzites exposed more than 1000 km to the east. The other two gastroliths contain early Paleozoic and Grenville age zircons in addition to populations of Paleoproterozoic and Archean grains. We interpret that these gastroliths were derived from Pennsylvanian strata that were exposed several hundred km to the west of the sampling locality. The gastrolith detrital zircon age spectra are statistically distinct from overlying and underlying Morrison Formation sandstones. Four gastroliths from Utah (n=22, 25, 90, 21) are dominated by Mesoproterozoic zircons, and are interpreted to have been derived from Neoproterozoic strata. The fourth gastrolith (n=21) includes a population of early Paleozoic zircons, which indicates that it is likely Carboniferous in age. These three red quartzite gastroliths are different in age from those in the Bighorn Basin of Wyoming, which were derived from Yavapai “Baraboo Interval” quartzites of the Great Lakes region. Both the Bighorn Basin and Uinta Mountain areas contain gastroliths of Carboniferous age. Our data indicates that Neoproterozoic quartzites, perhaps of the Neoproterozoic Jacobsville Sandstone of the Lake Superior Basin, were the source of the 5 gastroliths from Utah. These gastroliths were ingested by dinosaurs and then transported in the belly of a dinosaur to the Bighorn Basin and northeastern Utah