title: A basal dinosaur from the dawn of the dinosaur era in southwestern Pangaea.
abstract: Upper Triassic rocks in northwestern Argentina preserve the most complete record of dinosaurs before their rise to dominance in the Early Jurassic. Here, we describe a previously unidentified basal theropod, reassess its contemporary Eoraptor as a basal sauropodomorph, divide the faunal record of the Ischigualasto Formation with biozones, and bracket the formation with (40)Ar/(39)Ar ages. Some 230 million years ago in the Late Triassic (mid Carnian), the earliest dinosaurs were the dominant terrestrial carnivores and small herbivores in southwestern Pangaea. The extinction of nondinosaurian herbivores is sequential and is not linked to an increase in dinosaurian diversity, which weakens the predominant scenario for dinosaurian ascendancy as opportunistic replacement.
authors.affiliation: Instituto y Museo de Ciencias Naturales, Universidad Nacional de San Juan, San Juan 5400, Argentina.
authors.forename: Ricardo N
authors.initials: RN
authors.lastname: Martinez
authors.forename: Paul C
authors.initials: PC
authors.lastname: Sereno
authors.forename: Oscar A
authors.initials: OA
authors.lastname: Alcober
authors.forename: Carina E
authors.initials: CE
authors.lastname: Colombi
authors.forename: Paul R
authors.initials: PR
authors.lastname: Renne
authors.forename: Isabel P
authors.initials: IP
authors.lastname: Montañez
authors.forename: Brian S
authors.initials: BS
authors.lastname: Currie
mesh_terms: D000818:Animals; D001118:Argentina; D005075:Biological Evolution; D001842:Bone and Bones / Q000033:anatomy & histology; D025061:Dinosaurs / Q000033:anatomy & histology* / Q000145:classification*; D053476:Extinction, Biological; D005269:Femur / Q000033:anatomy & histology; D005580:Fossils; D010802:Phylogeny; D012886:Skull / Q000033:anatomy & histology; D013131:Spine / Q000033:anatomy & histology