CONTRASTING TRENDS IN THE EARLY TRIASSIC ECOLOGICAL RECOVERY OF BENTHIC AND PELAGIC FAUNA

The Early Triassic recovery from the end-Permian mass extinction exhibits contrasting trends in taxonomic recovery for benthic and pelagic fauna. We use two models to explore ecological consequences of diversity patterns and suggest that the environmental conditions of the Early Triassic had disparate effects on these two groups.

A four step model (Twitchett et al. 2004) was adapted to address benthic ecological recovery. Combining diversity, tiering, body size, and ichnofaunal ecology we can assign a recovery stage 1 (low) through 4 (high). Across the globe the Griesbachian and Dienerian often represent Stage 1 dominated by disaster bivalves and shallow bioturbation. Stage 3 recovery develops incrementally through increased taxonomic diversity, body size, depth and complexity of bioturbation, and the reappearance of crinoids by the Smithian or Spathian.

Westermann morphospace (Ritterbush and Bottjer 2012) uses experimental data and shell measurements to estimate extinct ammonoid swimming capabilities, swimming vs. non-swimming. Early Triassic ammonoids found in the Boreal Zone from the type and stratigraphic collections of Tozer (1994) show constant turnover of genus level diversity at the high resolution of stratigraphic subzones (n=14). In contrast, the ammonoid fauna occupy a remarkably consistent ratio of ecological morphotypes, both swimmers and non-swimmers, from the earliest Griesbachian through the rest of the Early Triassic. Constrictions in diversity at stage boundaries have no significant selective effect on ammonoid morphology or hypothetical swimming ecology.

Step-like increases in diversity and delayed ecological recovery for the benthic fauna are observed across the globe. In contrast ammonoids show consistent generic turnover and ecological space occupation in the Boreal Sea. These results imply that the characteristics of the Early Triassic marine environment had very different effects on these two broad groups. The benthic taxa were negatively affected by anoxic conditions and their survival was often controlled by environments that provided increased oxygenation through moderate wave activity. The ammonoid fauna inhabited a wide variety of niche space implying that Early Triassic environmental conditions did not limit occupation of disparate pelagic life modes.