ENVIRONMENTAL CONTROLS ON BODY SIZE IN THE PERMIAN AND EARLY TRIASSIC

The surprisingly small size of Early Triassic invertebrates has been recognized for more than 50 years. Shelly fossils, and trace fossils, in Induan and Olenekian marine rocks are significantly smaller than fossils from the Upper Permian or Middle Triassic. The smallest fossils occur in the immediate aftermath of the Late Permian mass extinction event, and may be interpreted as reflecting a post-extinction Lilliput effect. Strictly speaking, the Lilliput effect is a temporary ecophenotypic response by surviving species to suboptimal environmental conditions. In the Early Triassic, however, all taxa are small (survivors and newly appearing forms), and the effect lasts far longer. What environmental conditions were responsible for the dramatic fluctuations in size at this time?

Data collected from museum, literature and field studies show that, globally, the size of marine animals had been declining for 50 million years prior to the Late Permian extinction event. Following Carboniferous-Permian maxima, the body sizes of most bivalves, gastropods, brachiopods and crinoids declined through the Permian, reaching minima in the Early Triassic. Although sampling biases undoubtedly affect some intervals (e.g. where the dataset is dominated by a single facies or region), the broad pattern probably reflects biological reality. These size trends parallel an inferred decline in atmospheric oxygen levels and a rise in global temperatures. As global oceanic productivity is higher in glacial times than in warm, non-glacial times, size decrease may also have coincided with a decline in food supply. Regional variations in the size of Early Triassic Claraia may support this. Data from northern Italy demonstrate that, locally, size may also have a facies control: small size is associated with marginal, brackish settings. Finally, as changes in the burrow size of non-mineralized infauna parallel those of shelly fossils, problems with biomineralization were probably not responsible.