The origin and early evolution of dinosaurs

Area of Science:

• Paleontology
• Evolutionary Biology
• Geology

Background:

• The earliest definitive dinosaur fossils (Dinosauria) originate from Late Triassic rocks, approximately 230 million years ago, found in rift basins of southwestern Pangea.
• While no undisputed dinosaur body fossils predate the Late Triassic, the Middle Triassic origin of the lineage is suggested by footprint records and its relationship to basal dinosauromorphs.

Purpose of the Study:

• To review the current understanding of early dinosaur phylogeny and evolutionary history.
• To discuss the fossil evidence, phylogenetic analyses, and key adaptations related to dinosaur origins and radiation.

Main Methods:

• Analysis of fossil records from the Ischigualasto and Santa Maria Formations.
• Review of cladistic analyses and phylogenetic definitions of Dinosauria.
• Examination of anatomical traits, particularly pelvic girdle and limb structures, related to bipedalism.

Main Results:

• The oldest dinosaur fossils are from the Late Triassic, with key early genera identified in Argentina and Brazil.
• Phylogenetic analyses place Pisanosaurus mertii as a basal ornithischian and Herrerasaurus, Eoraptor, and Staurikosaurus within Saurischia.
• Controversies remain regarding the precise placement of certain basal taxa and the identification of definitive dinosaur apomorphies due to incomplete remains.

Conclusions:

• Dinosauria is defined by derived traits, including those facilitating a fully erect bipedal gait, a key adaptation for their radiation.
• Dinosaur radiation occurred in at least three major phases during the Triassic and Early Jurassic, interspersed with extinction events.
• Early dinosaurs, originating from bipedal ancestors, diversified from a restricted South Pangean distribution to a nearly global presence by the Late Triassic.