High fecundity in an early-diverging ichthyosaur with over ten fetuses.

The mode of reproduction is a key aspect for an organism to adapt to its environment. However, understanding the reproductive strategies in extinct animals from deep time remains challenging, due to the scarcity of fossils providing such information. Although viviparity is well-documented across various ichthyosaur lineages, our understanding of the reproductive characteristics of early-diverging Triassic taxa is still limited, particularly regarding their litter sizes. Here, we describe an exceptionally preserved gravid specimen of the Middle Triassic ichthyosaur Mixosaurus from southwestern China, containing at least eleven fetuses. This represents the largest litter sizes ever recorded in a Triassic ichthyosaur, and even one of the highest numbers of fetuses known in all ichthyosaurs. The well-articulated fetal skeletons exhibit consistent developmental stages and advanced ossification, indicating a late prenatal or perinatal state. Their variable body orientations further suggest the absence of a fixed head-first or tail-first birth posture. The presence of over ten fetuses in Mixosaurus demonstrates that high fecundity, potentially indicative of a more r- than K-selection reproductive strategy, had evolved early in ichthyosaur evolutionary history. This reproductive trait may have been a significant factor contributing to the success of this ichthyosaur group with enormous populations and widespread distribution across Northern Hemisphere during the Triassic.