The Beginning of the Age of Dinosaurs

Abstract

The record of life on land has been a principal concern of historical biology not only because of our fascination with our own past (and with giants, dragons, and other ancient monsters) but because of special opportunities and challenges for development of methods, principles, and concepts of explanation. The Beginning of the Age of Dinosaurs treats an intriguing phase of that history, one that included the first appearance of dinosaurs, and mammals, the extinction or near extinction of many clades of vertebrates, and extensive changes in plant associations. Further, the patterns of change (and of stasis) raise general questions about macroecologic and macroevolutionary processes and factors and even about the roles of chance and determination in biological history. Although the book was published initially in 1986 (and was based on a 1984 symposium sponsored by the Society of Vertebrate Paleontologists), its content remains current and its release in paperback form (for $34.50 rather than $75.00 for the hardcover version) justifies a review even at this late date. The Introduction and the Summary and Prospectus, written by the editor, Kevin Padian, demonstrate the significance of the interval from mid-Triassic to early Jurassic-particularly for vertebrates on land. Advanced mammal-like reptiles (therapsids) dominate lower Triassic assemblages in abundance, taxonomic diversity, and ecological variety; non-therapsids (mostly archosaurs) are rare elements and apparently of little ecological importance. In upper Triassic and lower Jurassic assemblages the situation is reversed, therapsids rare with limited diversity and variety but archosaurs abundant, diverse and varied. The archosaur expansion starts in middle of the succession; pterodactyls, crocodylomorphs, and dinosaurs appear (as archosaur subclades) in approximate coincidence with a marked decline in therapsids. Mammals (at least 3 subclades) occur along with two other subclades of very mammal-like therapsids very close to the top. In the upper Triassic two relatively sharp breaks in faunal composition appear, one relatively low, in the top of the Carnian and base of the Norian stages (around 225 Ma), and one higher, at the top of the Norian (around 215 Ma). These breaks, if real and not a consequence of miscorrelations or gaps in sampling, suggest high rates of taxonomic extinction and origination and have been interpreted as intervals of catastrophic extinction. These changes coincide more or less with some in the flora (except that the latter seem continuous rather than stepped) and thus with overall changes in terrestrial ecosystems. Radically different explanations have been offered for these patterns, at one extreme a deterministic argument from the competitive superiority of dinosaurs to the other, an opportunistic one based on chance differences in survival through episodes of mass extinction. This book can be viewed (and reviewed) as an extended example of analysis and interpretation in historical biology. The concerns of the discipline are twofold, chronicle and narrative (the concepts those of O'Hara, 1988). Chronicle comprises when, what, and where; narrative, how. A chronicle extends of course beyond description and chronologic ordering of fossils to paleobiogeographic, paleoecologic, and phylogenetic reconstructions. The latter derive from patterns in form and occurrence of fossils as analyzed in terms of taphonomic, constructional, functional, and phylogenetic processes and factors (viz Seilacher, 1970) and of stratigraphic and geographic distribution. Each reconstruction represents a particular state, and stratigraphic analysis arranges these reconstructions into a chronicle. Narrative, in contrast, involves explanation of the patterns (temporal, geographic, ecologic and phyletic) in the chronicle by a sequence of biological and physical circumstances and by evolutionary processes and factors (genetic, phylogenetic, and ecological). Of the 26 papers in this volume, 24 focus primarily on the chronicle and are dominated by consideration of what-when, i.e., the stratigraphic distribution of various groups of fossils, and of what-how, i.e., the phylogenetic and functional analyses. Among those in the what-when group are papers by Colbert on historical aspects of upper Triassic-lower Jurassic stratigraphy, by Ash on fossil plants, by Olsen and Baird on the ichnogenus Atreipus, by Chatterjee and by Parrish and Carpenter on vertebrates of the Dockum Group (Texas and New Mexico), and by Long and Padian on biostratigraphy of the Chinle Formation (Arizona). Also best included here are the studies by McCune and Schaeffer on Triassic and Jurassic fishes, Gaffney on turtles, Clemens on mammals, Olson and Padian on crocodylomorph ichnogenera, Sun and Cui on saurishians from the lower Lufeng (China), Clark and Fastovsky on the vertebrates of the Glen Canyon Group (Arizona), Haubold on archosaur trackways, Sigogneau-Russell, Frank, and Hemmerle on a new family of Triassic