Hydrodynamics as a hidden abiotic factor constraining Ordovician chitinozoan morphological evolution

The Great Ordovician Biodiversification Event (GOBE) was one of the most important macro-evolutionary events of the early Paleozoic, and involved diversification of several planktic groups. Although hydrodynamics must have affected the evolution of these groups, it is difficult to establish causality with confidence. Chitinozoans, mysterious fossils with simple morphology and biostratigraphic utility, underwent great morphological innovations and adaptions through the Ordovician Period, and thus are especially suitable for exploring the effects of hydrodynamics on morphological diversification. We constructed simplified computer models of 92 Ordovician chitinozoan species from major paleoplates to assess the long-term changes in hydrodynamic properties (outer-wall pressure fields and velocity fields) by using computational fluid dynamics. The pressure difference between both ends of the chitinozoan vesicle is suggested to represent the floating ability of the chitinozoan. During the Ordovician, the pressure difference exhibited diachronous fluctuations in different families and on different paleoplates. A continuous increasing passive floating ability was achieved by the Desmochitinidae since the beginning of Ordovician. This evolutionary tendency is consistent with the previously suggested planktonic diversity increase during the GOBE, which initiated since the earliest Ordovician and indicates that the GOBE involved both long-termed taxonomic and morphological radiations. The low-velocity region at the antiapertural part of the vesicle in both individual and chain-structured chitinozoans supports the hypothesis that chitinozoans are independent protists, rather than metazoan eggs. We also analyzed how the evolution of structural innovations, such as chamber shape, neck length, carina length, and ornamentation arrangement, were influenced by hydrodynamics within individual lineages. The results show that several chitinozoan lineages evolved toward increased stability and better floating ability, indicating that convergent evolution in chitinozoans occurred as a result of hydrodynamic pressure. This study highlights the importance of hydrodynamic constraints for the evolution of Ordovician microplankton.