Phanerozoic diversity and faunal evolution of gastropods

Gastropods are one of the most thriving benthic groups in modern oceans and have played a central role in paleo-marine ecosystems of the Phanerozoic. The shell morphology of modern-type gastropods differs markedly from that of their geological predecessors, and has been used as core evidence for the “Mesozoic Marine Revolution” and the “Late Paleozoic Marine Productivity Transformation” scenarios. However, the classification and evolutionary timeline of Phanerozoic gastropod faunas have not yet quantitatively constrained with robust data support, and the timing and dynamic of the rise of modern-type fauna have been heavily debated in past decades. In this study we compiled a dataset with over 134,037 records of gastropod fossils (4,008 genera of 475 families, belonging to 34 orders). Following mean-standard deviation normalization of the basic genus-level diversity data, the unweighted pair group method with arithmetic mean based on Bray-Curtis similarity is employed to cluster gastropod fossil faunas while the time interval was binned by period and age. The results indicate that Phanerozoic gastropod genus-level diversity exhibits an overall trend of continuous increase, with two significant diversity expansion events occurring in the Early Ordovician and the Late Cretaceous. From the Late Cretaceous, diversity shows explosive growth, far exceeding levels of the Paleozoic to middle Mesozoic stages. All the gastropod assemblages can be clearly divided into three distinct fossil faunas, i.e. the Cambrian-type (67 genera of the possible stem gastropoda, Helcionellida), Paleozoic-type (characterized by the rise and dominances of Euomphalina, Murchisoniina and Bellerophontida), and Meso-Cenozoic-type (Modern-type, featured by fast radiation of Sorbeoconcha and Neogastropoda) by using a clustering distance of 0.5 as a quantitative criterion. The transitions between these three faunas occurred at the Cambrian/Ordovician boundary (ca. 485 million years ago) and in the Middle Triassic (ca. 240 million years ago), respectively, not fully aligning with major mass extinction events (the so called “big-five” mass extinctions) and the rise of modern type fauna was significantly preceding the classic timings of “Mesozoic Marine Revolution” in Late Triassic to Early Jurassic. Comprehensive paleoenvironmental parameters such as temperature, oxygen levels, and pH of secular scale, as well as ecological data including marine productivity and the evolution of durophagy are correlated with the presented gastropod faunal changing pattern to figure out the abiotic and biotic dynamics of gastropod evolution. The results show that faunal turnovers occurred under warm climate with relatively stable oceanic oxygen levels and pH, while the modern analogues (tropical gigantism and Thorson’s rule) confirm that temperate water-mass with high productivity in tropical oceans elevated the size and ecological plasticity. The Middle Triassic emerges as a critical period for the evolution of gastropods towards modern-type forms, coinciding with the recovery of marine productivity after the Permo-Triassic mass extinction and the diversification of shell-crushing predators. Since the developments of new pelagic producers in the Meoszoic oceans accompanied with the rapid diversification of durophagy predators, co-occurred with the Paleozoic-Meoszoic (modern) faunal transition, highlighting the key role of ecological factors in the origin of Modern-type gastropod fauna. In addition, during the Middle Triassic, gastropods at the global scale had already evolved to be the Modern-type fauna, whilst the gastropod assemblage of Qingyan biota in the Nanpanjiang Basin retained a typical Paleozoic-type faunal composition feature, serving as an important refugium for relict Paleozoic gastropod species.