ORIGIN OF DINOSAURS How are dinosaurs classified? Primarily by hip structure (Harry Seeley, 1887) He also believed that dinosaurs were polyphyletic. Richard Owen considered dinosaurs monophyletic based on fused sacral vertebrae, some later scientists disagreed arguing that saurischians and ornithischians originated separately; lately tide has turned again Who and where? Who: Contenders for earliest dinosaur: Herrerasaurus, Eoraptor and Pisanosaurus (Argentina) = ornithischian prosauropods (Madagascar) Staurikosaurus = 2m long, bipedal, theropod (Brazil) Herrerasaurus= 3m long, bipedal, theropod vs. Eoraptor = small theropod Madagascar prosauropods: cf. Azendosaurus (Carnian, Morocco) ***Oldest North American Dinosaur: Coelophysis (225 Ma) ***Did dinosaur evolve in South America? (SA sites more productive than any others + lagosuchids last found in SA) Basic anatomy: most early dinosaurs were small, bipedal, unspecialized meat- eating saurischians Herrerasaurus has an ancestral-looking hip bone, so looks most primitive Environment? Dinosaur faunas similar in Late Triassic in different parts of the world. Climate varied largely with latitude and distance from ocean. When? Between 228 and 230 million years ago, at the beginning of the Carnian Age of the Late Triassic How? Two competing hypotheses related to the Late Triassic mass extinction(s): a) Competitive wedge: dinosaurs outcompeted preexisting forms because they had certain evolutionary advantages, such as upright posture, etc. e.g. dinosaur biologically superior to earlier forms problems: carnivorous synapsids fully erect, some mammal-like reptiles probably warm-blooded, etc... b) Lucky survivor: dinos luck out that other critters don’t survive. TAKE HOME POINTS BY FIRST FEW MILLION YEAR AFTER DINOSAUR 1ST APPEARED: All major body type blueprints established, though first dinos apparently small bipedals Dinosaurs got their feet in the door, stage was set for 140 million years of dominance 1 .