Unusual locomotion behaviour preserved within a crocodyliform trackway from the Upper Cretaceous Bayanshiree Formation of Mongolia and its palaeobiological implications

摘要

Crocodyliform tracks are reported from the Upper Cretaceous (?Cenomanian-Santonian) Bayanshiree Formation in southeastern Mongolia. Ten tracks are preserved as natural casts, forming a trackway with a quadrupedal gait pattern with a tail trail. All tracks are short and wide, and dominated by toe traces without plantar impressions. Pes tracks are characterized by four deep claw impressions and push-back marks behind them. Manus tracks have shallow claw marks and long, sub-parallel scratch marks behind. The preferential association of the scratch marks with only the distal digit impressions and irregular pattern of footfalls suggests that this trackway was made by a bottom walking and punting crocodyliform under water. This trackway represents the first crocodyliform "swim tracks" in the Late Cretaceous of Asia and the first evidence for punting behaviour of a fossil crocodyliform. The "swim tracks" can be divided into two categories such as bottom walking tracks with punting for moving somewhat more quickly and subaqueous walking tracks without punting to be associated with slower underwater speeds. The tracks show that crocodylians had adopted a bottom walking behaviour similar to extant crocodylians by Cretaceous times.