WHERE SAUROPODS ROAMED: USING GASTROLITHS TO INFER PALAEOGEOGRAPHY AND PALAEOECOLOGY

The Early Cretaceous Wessex Formation, Wealden Group, of the Isle of Wight, UK, is a series of fluvial sandstones and floodplain mudstones, that yield a high diversity of vertebrate remains, including dinosaurs. Commonly found amongst these remains and throughout the succession, are well-rounded exotic stones, exhibiting a high degree of surface polish that identifies them as gastroliths. High concentrations of these stones have also been found in association with sauropod remains. An analysis of 388 gastroliths, all discovered in situ within the Wessex Formation, revealed a wide diversity of lithologies including chert, silicified limestone and wood, sandstone and metamorphic rocks. The sources of these gastroliths were investigated using petrography and fossil content and revealed two major provenances: 1. relatively locally derived (within 50 km) silicified Jurassic limestones and cherts, and 2. orthoquartzite and metamorphic clasts derived from Triassic conglomerates in Devon, some 150-200 km to the south west. Other than in gastroliths these lithologies do not occur in the Wessex Formation of the Isle of Wight and are not represented in Brittany and France to the south and east.

These results indicate that dinosaurs, most probably sauropods were responsible for their transport and that their acquisition involved travel over considerable distances, to access two individual source areas. Although the primary reasons for visiting these sites are unknown, and may be linked to seasonal migratory routes or topographical constraints, it implies that the sauropod’s choice of gastroliths may have been highly selective and even indicative of complex patterns of learnt behaviour

Experiments to replicate the polish using a rock tumbler and a mix of Equisetum, pine needles, water and pebbles with a lithology typical of the study gastroliths, demonstrated an effective maceration process. Contrary to previous studies, the diagnostic polish of gastroliths could be achieved in a relatively short time.

This study provides new evidence that gastroliths from the Early Cretaceous sediments of the Wessex Formation were transported by dinosaurs and in this case were acquired from two specific localities.