EVOLUTIONARY DYNAMICS OF GASTROPOD SIZE ACROSS THE END-PERMIAN EXTINCTION AND THROUGH THE TRIASSIC RECOVERY INTERVAL

Abundant, small gastropods have been reported from numerous, widespread Early Triassic localities and have been interpreted as opportunists that flourished in the aftermath of the end-Permian mass extinction. However, many surviving genera range through the Early Triassic but have never been observed within it. Here I evaluate the global temporal and evolutionary dynamics of gastropod size across the end-Permian extinction and through the Middle Triassic recovery using a literature database. The most unusual feature of the Early Triassic gastropod record is the absence of large gastropods. No specimens of unambiguous Early Triassic age larger than 2.5 cm have been reported, in contrast to common 5-10 cm specimens of both Permian and Middle Triassic age. The loss of large gastropods is abrupt even at a fine scale of stratigraphic resolution, while the return of larger individuals in the Middle Triassic appears gradual when finely resolved. Taphonomic and sampling biases do not adequately explain the absence of large Early Triassic gastropods. Examination of size trends by genus demonstrates that the size decrease across the Permian-Triassic boundary is compatible with both size-selective extinction at the species level and anagenetic size change within lineages. Size increase in the Middle Triassic resulted from the origination of large species within genera that have Early Triassic fossil records and the occurrence of new genera containing large species during the Middle Triassic. Genera recorded from the Permian and Middle Triassic but not the Early Triassic (“Lazarus taxa”) do not contribute significantly to the size increase in the Middle Triassic. Furthermore, the small size and low species richness of Lazarus genera when they reappear in the Middle Triassic indicate that they were more severely affected by the end-Permian extinction, likely surviving as small, rare forms rather than existing at large sizes in Early Triassic refugia. The absence of large Early Triassic gastropods may reflect environmental and/or ecological inhibitions on recovery that persisted throughout the Early Triassic.