Adaptations that make tetrapods efficient, fast swimmers make it difficult for them to move on land. This can pose a problem for marine reptiles, which must return to land to lay eggs. Ichthyosaurs are known to have given birth to live offspring. Mosasaurs (now extinct, and related to snakes and lizards) are believed to have also given live birth because (1) their limbs are short and weak, (2) their bony ribs do not enclose the entire chest, and (3) their pelvis is wider than necessary for passage of an egg. The evidence was circumstantial until a recent discovery of the first mosasaur embryos. A skeleton of Plioplatecarpus primaevus (from the Pierre Shale of South Dakota) contains the bones of up to four tiny mosasaurs in the posterior pelvic area. The lack of pitting or corrosion on the tiny bones suggests that these are not stomach contents but embryos. The bone microstructure indicates that these are very immature embryos. The “bone” is calcified cartilage, not true bone.