Eggs With Trilobite Larvae in a Cambrian Community. Modeling the Hydrodynamics

Background: Reproductive behaviors include mating, pregnancy, birth, and ulterior brood care. Fossil records (eggs, nests, etc.) provide a glimpse into the reproductive sequences of ancient animals. Here, we show putative trilobite egg clusters from the middle Cambrian (Miaolingian, Wuliuan Stage) that were found in the Kaili Formation, South China. The arrangement of these egg clusters would ensure efficient oxygen diffusion from the water into cells. Results: : We applied a computational fluid mechanics simulation method to understand the behavior of Kaili trilobite eggs in Cambrian seawater under different flow patterns. The drag force and lift force load on the eggs increased rapidly when current speeds exceeded 0.2 m/s. Moreover, the drag coefficient of the single egg model was 1.8–2 times larger than that of each egg in the egg cluster model. The trilobite larva within the egg cluster (ca. 0.5 mm in size) are smaller than the eggs (ca. 0.6 mm), which indicates that they had recently hatched. Conclusions: : We suggest that not all trilobites had an unmineralized preliminary stage in their ontogeny and that some protaspis shield may have been formed before hatching. Moreover, we hypothesize that aquatic animals can use egg clustering to effectively reduce the effects of drag and lift forces from the surrounding water.