Early evolution of dinosaurs: indications from the Triassic footprint record

Hendrik Klein¹ & Spencer G. Lucas^2
1Saurierwelt Paläontologisches Museum, Alte Richt, D-92318 Neumarkt, Germany.
²New Mexico Museum of Natural History, 1801 Mountain Rd. NW, Albuquerque, NM 87104, USA.
Corresponding author e-mail: Hendrik.Klein@saurierwelt.de

The Triassic footprint record not only has abundant chirothere and other archosaur tracks, but also gives insight into the early evolution of dinosauromorphs and dinosaurs. Their roots can be followed back to the Olenekian when small agile forms, the makers of Rotodactylus, left their footprints and trackways in dense accumulations distributed globally in deposits of Olenekian-Ladinian age. Some researchers see a perfect match of these footprints with small Dinosauromorpha such as some lagerpetonids known from Middle-Upper Triassic strata of Argentina. Considering the “dinosaurian” evolution of the tridactyl mesaxonic pes, increasing reduction of pedal digits I and V has been demonstrated in Middle Triassic Chirotherium and interpreted by some researchers as an early evolutionary trend. Furthermore, some trackways of tridactyls from Ladinian deposits clearly show evidence of facultative bipedal progression. Tridactyl footprints of the Atreipus-Grallator plexus that can be attributed to dinosauriforms-dinosaurs are abundant and have a global distribution, beginning in the late Ladinian. Trackways of early sauropodomorph dinosaurs (Evazoum, Pseudotetrasauropus, Tetrasauropus, Eosauropus) are known from Late Triassic (Carnian-Norian-Rhaetian) strata, essentially from the western USA, Europe and southern Africa. The biostratigraphic utility of Triassic dinosaur footprints is limited because of the poor ichnotaxonomy of these footprints. Nevertheless, evolutionary developments and trends in the locomotor apparatus of early Dinosauromorpha and Dinosauria are documented impressively in their trackways and footprints and make these a valuable additional database for further analyses.

tracks, theropods, sauropodomorphs, Grallator, Eosauropus.