The early radiation of dinosaurs: competition or opportunism?

NOVAS, F. E.; EZCURRA, M. D.

Buenos Aires

Jornada; REUNIÓN ANUAL DE COMUNICACIONES DE LA ASOCIACIÓN PALEONTOLÓGICA ARGENTINA Y CONFERENCIAS: "DARWIN, LAMARCK Y LA TEORÍA DE LA EVOLUCIÓN DE LAS ESPECIES"; 2009

Museo Argentino de Ciencias Naturales

The evolutionary process explaining the early radiation of dinosaurs in Late Triassic times constitutes a subject of heateddebate. Two alternative paleoecological scenarios have been proposed: the ?competitive? versus the ?opportunistic?models. The competitive model claims that the replacement of contemporary tetrapods (e.g. rhynchosaurs, dicynodonts,traversodontids, crurotarsans) by dinosaurs was a drawn-out process which involved inter-specific competition.Conversely, the ?opportunistic? model states that the dawn of dinosaurs occurred in an ?empty ecospace? after two successivestochastic events (i.e. the end-Ischigualastian and the Triassic-Jurassic extinctions). Albeit the ?opportunistic?model received strong support in the last years, evidence amassed from late Carnian beds of Argentina, Brazil, and Indiareveals important flaws of this hypothesis. First, dinosaurs were already taxonomically diverse in rhynchosaur/traversodontid-dominated assemblages being represented by several species belonging to different clades of herbivorous/omnivorousand predatory animals. Second, the South American Late Triassic fossil record shows that carnivorous dinosaurs(e.g., herrersaurids, coelophysoids) attained large sizes alongside with crurotarsan ?rauisuchians?, and that theselarge predatory archosaurs co-existed around 30 my. Third, available evidence does not show that the end-Ischigualastian extinction affected substantially the archosaur diversity. Fourth, the morphological disparity of dinosaursseems to exhibit a steady increase during the Late Triassic. Accordingly, the early radiation of dinosaurs seems to be acomplex macroevolutionary event that can be decoupled in diachronous processes which are not consistent with an opportunisticmodel. Conversely, the long co-existence of dinosaurs with Triassic tetrapod groups which were finally replacedat the Triassic-Jurassic boundary matches better with a drawn-out process.