Diversity of continental tetrapods and plants in the Triassic of the Southern Alps

Fabio Massimo Petti, Massimo Bernardi, Evelyn Kustatscher, Silvio Renesto & Marco Avanzini

We review the Triassic record of terrestrial plants and animals from the Southern Alps at more than 60 sites. The Triassic tetrapod track record seems to mirror the global body fossil record of diapsids (with the exception of the Olenekian record), but is almost completely devoid of synapsids and amphibians. During the Bithynian-Pelsonian interval (early-late Anisian, Middle Triassic) the ichnoassociation documents a diverse tetrapod fauna composed of lepidosauromorphs, basal amniotes, archosauriformes (“rauisuchians”) and possible dinosauromorphs, whereas the body fossil fauna is represented only by small lepidosauromorphs and archosauriformes (“rauisuchians”). The rich flora is one of the best examples of early recovery of ecosystems after the Permo-Triassic event. The Illyrian (late Anisian, Middle Triassic) ichnoassociation reflects a faunal composition of lepidosauromorphs, archosauriformes and dinosauromorphs, while the body fossil assemblage is only represented by a few remains of archosauromorphs. Tetrapod footprints are missing from the Fassanian (early Ladinian, Middle Triassic), while scattered skeletal remains document the presence of large and small archosauromorphs. Fassanian plant fossils record a flora dominated by conifers and seed ferns. The Longobardian body fossil record (late Ladinian, Middle Triassic) documents the presence of archosauromorphs and mammal-like-reptiles, whereas tracks are completely absent. During this period, floras still show some typical Early-Middle Triassic elements and are dominated by conifers. The Longobardian fossil record is biased by taphonomic selection, as these specimens are found only in basinal successions. Middle and early Late Triassic rich plant fossil associations record some interesting evolutionary trends, such as the radiation of modern fern families and the appearance of entire new groups (e.g., the bennettitaleans). During the Carnian (Late Triassic) the flora reaches its highest diversity and shows the first occurrence of unequivocal bennettitaleans and the first putative cheirolepidiacean conifers. From the latest Carnian to the end of the Norian (Late Triassic), faunal associations are dominated by dinosaurs, while archosauriformes, drepanosaurids, protorosaurs, pterosaurs, lepidosauromorphs, “rauisuchians” and dinosauromorphs are only minor components of the fauna. Norian floras are poorly known but seem to be dominated by conifers, with rare fragments of Bennettitales. The numerous tetrapod track associations provide a series of snapshots trough time that show a huge increase in variability reflecting the morphological diversity spanning from a stem-amphibian to a crurotarsan to a dinosaur foot. The nearly-complete absence of non-diapsid tetrapods appears to be possibly correlated with the paucity of fluvial-lacustrine environments, while the dominance of dinosaurs in the Norian has to be understood in the context of a monotonous, harsh, carbonate-platform environment. The Norian sites document the presence of a variety of archosaurs, prolacertiforms and lepidosauromorphs whose diversification may have been the result of strong evolutionary pressures triggered by the unstable environment.

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