Meeting Abstract
The recent description of a pregnant specimen of Polycotylus demonstrates that plesiosaurs were viviparous, but much of their life history remains unknown. To address this, we gathered morphological and histological data on a growth series of polycotylids from the Pierre Shale of South Dakota. We examined two partial skeletons (an adult and small juvenile) of Dolichorhynchops bonneri and an isolated humerus referable only to Polycotylidae. The juvenile skeleton is 40% adult length, close to the birth size estimated for Polycotylus, so we predict it is a neonate. The isolated humerus is 28% adult length, and predicted to be a pre-term fetus. Our size-based predictions of relative development are consistent with morphology (bone surface texture, epiphyseal ossification). We also histologically sampled the humeral diaphyses of all three specimens. The adult cortex is thick, dense, and completely remodeled. It grades into a cancellous endosteum and lacks a marrow cavity. The fetal endosteum is similar, but the cortex is thin and consists of unremodeled, radially vascularized, woven-fibered bone, suggesting very rapid deposition. The neonatal endosteum is identical in size and histology to the fetus. Its cortex is thicker and has a birth line, but otherwise is similar histologically. The birth line is not a line of arrested growth, but rather a sudden change in vascular angle, decrease in canal diameter, and increase in bone density, suggesting a slowed growth rate, possibly in response to changes in hydrodynamic forces or diet after birth. The proximity of the birth line to the bone surface indicates that the neonate would have been just under 40% maternal length when born. Our histological data are further evidence that polycotylids were viviparous and birth size was large, and demonstrates that fetal growth rates were very high.
