Resumen de Isochirotherium trackways, their possible trackmakers (Arizonasaurus): intercontinental giant archosaur migrations in the Middle Triassic tsunami-influenced carbonate intertidal mud flats of the European Germanic Basin
A remarkable Middle Triassic (Pelsonian, Anisian) fossil track site at Bernburg in Central Germany permits reconstruction of a tetrapod fauna at the point of recovery when early poposaur quadruped dinosaurs diversificated globally. The site has yielded numerous tetrapod and thousands of arthropod horseshoecrab tracks from several levels in intertidal biolaminites with intercalated storm and seismic-influenced carbonates. The tetrapod tracks, Procolophonichnium, Rhynchosauroides, Chirotherium, and Isochirotherium, are assigned to smaller Archosauromorpha and large Archosauria. The largest chirotheroid tracks Isochirotherium herculis (Egerton in Proc Geol Soc London 3:14–15, 1838) reach up to 350-mm-long pes sizes and 120-mm-long manus imprints. Those were most probably made by a 5-m-long poposauroid archosaur like Arizonasaurus, based on matching skeleton anatomy and trackways including a new three-dimensional model of those giants in locomotion. A first German poposauroid bone record is added herein with a scapula from shallow marine carbonates of similar Pelsonian ages. These crurotarsan archosaurs were at the top of their food chains globally, and fed most probably on smaller tetrapods (Hescheleria, Macrocnemus), the latter also represented well by long behavioural escaping Rhynchosauroides trackways. The smaller reptiles may have fed especially on “Millions” of horseshoecrab eggs in those arthropod reproduction seasons in the intertidal carbonate mud flats. Such tidal flats and sabkhas extended over hundreds of square kilometres surrounding the Germanic Basin in Central Europe in the Middle Triassic, representing a drastic change in the palaeoenvironment of central Pangaea, and a possible trigger for continuing diversification of archosauromorphs and eventually dinosaurs. The horseshoecrab reproduction seasons seem to have caused a food chain reaction along the southern Pangaean (northern Tethys) coast in carbonate mud flats between Northern America, Africa, Europe and China, with possibly long-distance migrations of poposauroids such as Arizonasaurus, whose bones were found in terrestrial to shallow marine carbonate deposits in terrest to coastal context
