Dinosaur origins: evidence from the footprint record

Abstract

Triassic tracks and trackways assigned to dinosaur trackmakers or closest relatives have been mentioned from several Middle to the latest Triassic successions from both northern and southern Pangea. At present, the earliest gondwanan records are those from the Middle Triassic Los Rastros Formation in west-central Argentina. A reanalysis of Los Rastros ichnites at the Ischichuca area, including new material, has revealed the presence of a more diverse ichnofauna than previously suspected. The ichnocoenosis includes several tracks and trackways of bipeds with functionally tridactyl digitigrade pes, well developed claws, and a parasagittal posture of the hindlimbs. Previously, some large tridactyl footprints from the Ischichuca area were allied to theropod dinosaurs, although no synapomophies are preserved in the three-toed footprints that might discriminate among theropods, basal saurischians and basal ornithischian groups as their possible trackmakers. If the Ischichuca trackmakers are referred to a dinosaur taxon and/or to a close dinosaur sister-taxon, their presence in the Los Rastros levels suggests that derived dinosauriforms (including dinosaurs) had diverged and acquired their characteristic functionally tridactyl pes by at least the Middle Triassic, something that the body-fossil record has failed to document to date.

The precise temporal calibration of dinosaur origins

“…The transition to communities containing the first dinosaurs occurred in less than a 5-million year interval, based on unambiguous dinosaur body fossils dated to 231.4 Ma from the lower part of the Ischigualasto Formation (21,25). Possible early dinosaur footprints from the intervening Los Rastros Formation suggest that the origin of dinosaurs could have been even older (34). Footprints attributed to early dinosauromorphs from the Early-Middle Triassic of Poland were used to pull the origin of the group to the beginning of the Triassic (35).…”

Tetrapod tracks in a marginal lacustrine setting (Middle Triassic, Argentina): Taphonomy and significance

“…Previous analysis suggested that they were produced by medium-sized (hip height approx. 75 to 115 cm) basal dinosaurs and/ or dinosauriform trackmakers (Marsicano et al, 2007).…”

“…The tetrapod record of the succession is almost exclusively composed of tracks and trackways, as the whole column is virtually devoid of tetrapod bone remains. In all cases, the track-bearing strata were interpreted as having been deposited along paleo-lake shorelines and proximal deltaic settings (Arcucci et al, 1995;Marsicano et al, , 2007.…”

Mesozoic dinosaurs from Brazil and their biogeographic implications

“…No osteological remains from other parts of Pangaea suggest the presence of dinosaurs in the Ladinian, but possible footprints from Europe and Argentina (Haubold and Klein 2002, Marsicano and Barredo 2004, Diedrich 2009) may represent earliest dinosaurs in the fossil record. Although the attribution of several of these Triassic footprints has been disputed (King and Benton 1996), the issue is still unresolved (Melchor and de Valais 2006, Marsicano et al 2007, Silva et al 2008.…”