Patterns of body size evolution in fossil crinoids

Crinoids were abundant and highly diversified marine benthic invertebrates, whose fossil record extends back to the Ordovician. However, little is known about their body size evolution at a global scale. Here, we compiled a comprehensive database on calyx biovolumes of holotypes for type species throughout the Paleozoic and Mesozoic, and evaluated temporal patterns of their body size evolution using the 'paleoTS' package. Our results showed that crinoids exhibited a heterogenous body size trajectory in the Paleozoic and early Mesozoic that is best fit by the unbiased random walk followed by a long period of stasis for the remaining Mesozoic times. However, the support for each model is strongly related to taxonomic level and temporal duration of sequences. In the Paleozoic, two major declines in body size at around the late Ordovician and the late Devonian extinction events are observed. In the Mesozoic, major declines are noted in the Early Jurassic and mid-Cretaceous, which roughly coincide with some of the anoxic events. Although the role of the end-Permian mass extinction on crinoid body size evolution is difficult to explore because of the limited data points from this time period (only a single taxon is thought to survive into the early Mesozoic), we note that Mesozoic calyx biovolumes are much smaller than in the Paleozoic taxa (in terms of maximum, mean and minimum biovolumes).