THE ASCENT OF THE DINOSAURS: ECOLOGICAL RELEASE AFTER THE TRIASSIC-JURASSIC MASS EXTINCTION IN CONTINENTAL ENVIRONMENTS

New data from early Mesozoic rift strata correlated by Milankovitch cyclostratigraphy and magnetostratigraphy allow high-resolution analysis of the Triassic-Jurassic (TJ) continental transition. Tetrapods are represented by an extraordinary ichnological record (>10E^4 specimens) and many new skeletal discoveries.

The Late Triassic saw an increase in tetrapod diversity, but also high turnover rates with as many dinosaurian as non-dinosaurian extinctions. Theropod (?ceratosaurian) dinosaurs increased in relative abundance and maximum size through this time. End-Carnian tetrapod extinctions (correlated to the marine magnetostratigraphically) are insignificant, although there was an accelerated turnover within the Carnian. Large scale zonal faunal and floral provinciality continued through the Triassic. The Rhaetian relative overabundance of theropod compared to herbivore tracks suggests ceratosaurians(?) were opportunistic carnivores, with diets dominated by aquatic or semiaquatic prey.

Abundant and diverse ichnofaunules containing typical Triassic forms occur ~10 ky below the palynologically-identified TJ boundary. A >50% extinction occurred at the boundary, which also features an Ir anomaly and fern spike. A similar pattern is known at coarser levels for osseous taxa.

Earliest Jurassic (< 20 ky post-boundary) ichnological assemblages have extraordinarily low diversity (3 ichnogenera in ~10E^3 specimens) with no herbivores; theropod track abundance soars together with a 20% increase in maximum size marked by the appearance of Eubrontes giganteus. In eastern North America, three new ichnogenera (including two dinosaurian) appear within 400 ky, but there is virtually no change in the succeeding 2 my. Herbivore footprints are numerically subordinate to theropods, although less so with time and increasing latitude. Globally, Early Jurassic osseous and ichnological faunas are nearly homogeneous, even at the generic level.

We propose that the remarkable and relatively abrupt change in maximum size of theropod tracks was a consequence of ecological release upon extinction of competitors at the TJ boundary, and that the earliest Jurassic theropod communities were part of an aquatic-based, post-catastrophe ecosystem. Diversity did not recover to pre-boundary levels for more than 10 my.