Gigantic gastropods from the Middle Triassic Qingyan biota of Guizhou

Body sizes of organisms underpin their ecological functions and evolutionary trajectories. Previous studies have shown that significant reduction in body size — the Lilliput effect — occurred in the aftermath of the Permian–Triassic mass extinction (PTME) and that body size did not rebound until to the late Middle Triassic (Ladinian). Here we describe three new gastropod specimens, identified as Toxoconcha sp., with unprecedented large shell sizes among Triassic gastropods, from the Qingyan biota of Guizhou, China. Meanwhile, a new dataset on the gastropod size of Lopingian (Late Permian) to Middle Triassic is presented. The largest specimen we found is ∼350 mm in height while its shell volume is estimated to be 1358661 (= 6.13 log10) mm3. As the largest record of the Triassic gastropod shells, these new fossils from Qingyan suggest that gastropod body size rebounded from the post-PTME Lilliput effect as early as the Anisian. Rapid gigantism has been reported in Middle Triassic marine reptiles, and the occurrence of giant gastropods in Qingyan suggests a convergent morphological response to the unique ecological and environmental conditions of the Middle Triassic.