Ichnotaxonomy, functional morphology and paleoenvironmental context of Mesosauridae tracks from Permian of Brazil

Abstract

Subaqueous vertebrate traces are fairly common in fossil record, but there are few studies on the theme due to the difficulties in classification and interpretation. In Brazil, subaqueous tracks were recorded in rocks of Irati Formation (Lower Permian, Paraná Basin). According to morphological features and interpreta- tion, the ichnofossils were designated as a new ichnotaxon, Mesosaurichnium natans igen. nov. et isp. nov., and attributed to the activities of basal reptiles Mesosauridae. Based on faciology, the ichnofossils possibly were produced in a lower shoreface affected by storm waves and do not correspond to the original sedimentary surface, which was covered by a thin layer of crustacean shelly fragments during the production of the tracks. The tracks were produced by mesosaurids during the swimming next to the bottom with the long tail maintain- ing the propulsion. The mesosaurids were able to use the tail as the main propelling organ for fast swimming under undulatory propulsion, but with the paddle-like pedes acting as important accessory propellant during oscillatory propulsion, conferring greater maneuverability during slow swimming.