Box #23 in our count down is Stenopterygius…

Stenopterygius: the “Narrow Fin”

Stenopterygius was an ichthyosaur from Early Jurassic Europe. Since the original fossil (the holotype) was destroyed during World War II, it was assigned a lectotype. This fossil is a truly amazing specimen, since it preserves not only the entire animal, but also the little one inside her, almost large enough to be born!

Several Stenopterygius fossils have equally wonderful perseveration, as some come from areas with mudstone, shale, or limestone that preserve fine details like blubber and the delicate bones inside the eye. One such fossil preserves another mother that died while in labor. She has three embryos inside her, and the fourth was pushed out tail first, just like whales give birth to their young today.

But how can a reptile give birth to live young? In a nutshell, there are two ways that certain snakes and lizards can give birth to live young. Boas actually develop their young inside in a similar way mammals do, only there is no placental snack bag, and no umbilical chord to act as the straw connected to the placenta. The other method is simpler. Instead of laying eggs, the mother just holds them inside her until they are ready to hatch. Many times when this is the case, the young hatch inside of her and then are born.

Ichthyosaurs most likely did not carry eggs inside them, but truly gave live birth like modern boas. Since they breathe air and have no umbilical chord to help deliver oxygen to the young, the young must be born tail first so they do not drown. Then perhaps they were strong enough to swim quickly to the surface, or perhaps the mother helped them with their first breath, like whales. Motherly habits are difficult to say for extinct animals, especially when we have so many examples from modern reptiles to show just how complex and varied it could be from one animal to another.

Some fossils preserve the beautifully streamlined body we see in dolphins today, including dorsal fins and tail flukes. From this body plan we can assume that their lives were very similar to modern dolphins, and would have spent most of their lives out in the open sea. The shape of the tail fin is very similar to the crescent shape we see in the fastest of ocean fish today, which made them well suited for swimming after fish and squid.

The preservation of blubber is another important discovery, because blubber shows that they had a steady internal body temperature. Not only was it steady, but they generated their own body heat, and had blubber to help stay warm in cold water and maintain a high metabolism. Like wearing a diver’s wet suit.