The rarity of gastroliths in sauropod dinosaurs - a case study in the Late Jurassic Morrison Formation, western USA

Occurrences of suspected sauropod geo-gastroliths and exoliths (exotic clasts) are compared with authentic finds of stomach stones in the sauropods Diplodocus, Cedarosaurus, and Camarasaurus. Sedimentological and taphonomical evidence from classic sauropod dinosaur localities in the Late Jurassic Morrison Formation (Cleveland-Lloyd Dinosaur Quarry, Dry Mesa Dinosaur Quarry, Carnegie Quarry/Dinosaur National Monument, Howe Quarry, Como Bluff, and Bone Cabin Quarry) reveals very few sauropod finds with unambiguous gastroliths. The scarcity of clasts in the fine-grained sediments of most of the localities suggests that only a small number of sauropods possessed gastroliths. The occurrence of a hypothetical avian-style gastric mill in sauropods is not supported by taphonomical evidence. Exoliths that are abundant in the Early Cretaceous of the western USA are nearly absent in Late Jurassic sediments. Without an association with fossil bone, there is no convincing evidence that such clasts represent former gastroliths. It is more plausible that most exoliths have been transported in hyperclastic flows or that surface-collected stones are weathering relicts of former conglomerate layers.