Early dinosaur radiation: testing macroevolutionary models through diversification rate shift analyses of an early Mesozoic amniotan supertree

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

La Plata

Congreso; X Congreso Argentino de Paleontología y Bioestratigrafía VII Congreso Latinoamericano de Paleontología; 2010

The early radiation of dinosaurs has been traditionally explained through a ?competitive model?, which attributesdinosaur success to their superiority in a drawn-out process involving inter-specific competition, or a more accepted?opportunistic model?, which states that dinosaur radiation occurred in an empty ecospace cleared by two successiveLate Triassic extinctions. Following the ?opportunistic model?, it would be expected that dinosaurs acquireddiversification shifts only after the Late Triassic extinctions. Conversely, in the ?competitive model? diversificationshifts would be expected during the evanescence of their competitors. With the aim to test how these models fit tocurrent phylogenetic reconstructions, a diversification rate analysis was performed on an amniotan supertree of 690taxa. We found that the early dinosaur phylogeny significantly departs from a stochastic branching model (p<0.001)and a significant branching shift is located at the base of Dinosauria (p<0.05). This result is in agreement with thehigh diversity of early dinosaurs observed in the Ischigualasto Formation (early Late Triassic), clearly supportingthat the dinosaur radiation started before the end-Ischigualastian extinction (middle Late Triassic). Ischigualastiancontinental assemblages were dominated by rhynchosaurs and traversodontids, and crurotarsans were common faunistic elements, indicating that the early radiation of dinosaurs firstly took place into a crowded-ecospace.However, branching patterns are not homogenous within Dinosauria: diversification shifts for Theropoda occurredat Ischigualastian and Hettangian-Sinemurian times (Early Jurassic), but in the case of Sauropodormorpha andOrnithischia the shifts took place in the Coloradian (latest Late Triassic), immediately after the end-Ischigualastianextinction. Together with herbivorous dinosaurs another six significant/conspicuous branching shifts are alsorecorded among Coloradian amniotan clades (e.g., mammaliamorphs, crocodylomorphs, pterosaurs), indicating thatthis stage of diversification was also experienced by another amniotan lineages. Accordingly, the early radiation ofdinosaurs started during the Ischigualastian in a crowded-ecospace, showing different patterns within the grouplater in the Triassic, within a macroevolutionary scenario that does not completely fit with none of the two traditionalmodels.