COMPARATIVE SIZE EVOLUTION OF MARINE CLADES FROM THE LATE PERMIAN THROUGH MIDDLE TRIASSIC

The small size of Early Triassic marine organisms has important implications for the ecological and environmental pressures operating during and after the end-Permian mass extinction. However, this ‘Lilliput effect’ has only been documented quantitatively in a few invertebrate clades. Moreover, the discovery of Early Triassic gastropod specimens larger than any previously known has called the extent and duration of the Early Triassic size reduction into question. Here, we document and compare Permian-Triassic body size trends globally in eight marine clades (gastropods, bivalves, calcitic and phosphatic brachiopods, ammonoids, ostracods, conodonts, and foraminifera). Our database contains maximum size measurements for species present in each stage from the Late Permian through the Middle to Late Triassic. The Permian-Triassic boundary shows more significant size reduction than any other interval. For most taxa, maximum and median size decreased dramatically from the latest Permian (Changhsingian) to the earliest Triassic (Induan), and then increased during Olenekian (late Early Triassic) and Anisian (early Middle Triassic) time. During the Induan, the only taxon with a size larger than its long-term mean was the ammonoids; they increased significantly in maximum and median size across the Permian-Triassic boundary, a size shift perhaps related to their comparatively rapid diversity recovery after the end-Permian extinction. The loss of large species across the Permian-Triassic boundary resulted from both selective extinction of larger species and evolution of surviving lineages toward smaller sizes. This within-lineage component of size decrease suggests that only part of the size decrease can be related to the end-Permian kill mechanism; in addition, Early Triassic environmental conditions or ecological pressures continued to favor small body size. After the end-Permian extinction, there was widespread size decrease across ecologically and physiologically disparate clades, but this Lilliput effect was primarily an Induan phenomenon. Nektonic habitat or low sensitivity to anoxia may explain the contrast of Early Triassic size increase and diversification in ammonoids versus size reduction and slow recovery in benthic clades.