Unusual bone bed reveals a vertebrate community with pterosaurs and turtles in equatorial Pangaea before the end-Triassic extinction

Abstract

Temporally constrained microvertebrate bone beds are powerful tools for understanding continent-scale biotic change. Such sites are rare globally in nonmarine settings during the 12 million years (Ma) preceding the end-Triassic extinction (ETE; ~201.5 Ma), obscuring patterns of faunal change across this interval. A vertebrate assemblage from Arizona, USA, provides unique insights into community composition and ecology prior to the ETE. PFV 393 is a macro- and microvertebrate bone bed preserved in a volcaniclastic fluvial channel-fill with a high-precision U-Pb zircon age of 209.187 ± 0.083 Ma. The fossil assemblage consists of three-dimensionally preserved, delicate, and small skeletal elements of known and new taxa that document a local paleocommunity including hybodontiformes, actinopterygians, actinistians, metoposaurids, salientians, synapsids, lepidosaurs, testudinatans, trilophosaurids, Vancleavea , doswelliids, Revueltosaurus , loricatans, phytosaurs, and pterosaurs. The new early-diverging pterosaur is one of the few Triassic pterosaurs found outside of Europe and the only one with a documented precise radioisotopic age. The testudinatan material shows the rapid dispersal of terrestrial stem-turtles across the Pangaean supercontinent in the Norian and refines temporal constraints on the origin of the turtle shell. The presence of vertebrate lineages endemic to the Triassic highlights their persistence in a mesic, fluvial paleocommunity through a prolonged phase of environmental change preceding the ETE. These lineages coexisted with frogs, lepidosaurs, turtles, and pterosaurs- all key elements of post–Triassic Mesozoic communities. The arrival of turtles and pterosaurs in west-central Pangaea therefore may have been driven by the northward drift of Laurentia from humid equatorial conditions into more arid subtropical latitudes.