Live Birth in Aquatic Reptiles!

This week, a [study] was published that unveiled an amazing new fossil from the Middle Triassic (247-237 million years ago) of South China. This fossil is of a pregnant archosauromorph. An archosauromorph is a reptile that’s closely related to archosaurs (crocodiles, birds, their ancestor and all of its descendants). So it’s not quite an archosaur, but it’s not more closely related to any other reptile. This particular archosauromorph is a very long necked Dinocephalosaurus.

The Archosauromorpha family tree. Pictures from various online coloring books, Benton (1983) and Liu et al. (2017).

This fossil is the first archosauromorph to show pregnancy. How do we know it was pregnant? Great question, and one that the authors had to answer. We know the baby is the same species as the adult because of the shape of the bones and the crazy long neck found in this species. We know the adult didn’t just die on top of a baby and get fossilized like that because the baby is completely inside the body outline of the adult. We know that the adult didn’t eat the baby because aquatic animals usually eat fish head first (as seen by a partially digested fish in the gut of the adult), and the baby is oriented the other way. The authors also note that the baby is curled up in a typical way for developing babies. Lastly, we know that this adult wasn’t on her way to lay an egg containing the baby because usually eggs are laid with a much less developed baby in them. There’s no evidence of eggshell around the baby either.

Figure 3 from the paper showing the fossil on the left and the interpretation on the right. The baby is drawn in pink on the right.

We know that crocodiles and birds (archosaurs) lay hard eggs and there aren’t any crocs or birds that give live birth. We have fossils of other reptiles (like ichthyosaurs, plesiosaurs, and mosasaurs), along with several living snakes and lizards that give live birth. This fossil is the first example of a close relative of archosaurs that do this.

Figure 3c from the paper showing a reconstruction of the pregnant Dinocephalosaurus.

The Plant Eating Hammerhead

Last week in May (2016) an [article] was published describing a new marine reptile that lived during the Mesozoic, specifically during the Middle Triassic (274-242 millions of years ago). Even though we had known about this animal from previous studies, this new study described the shape of the skull based on 2 new fossils found in Yunnan, China.

Fig 1a

Figure 1 from the paper showing the two new fossils.

This animal is called Atopodentatus unicus (Atopo means ‘strange’, dentatus means ‘tooth’ and unicus means ‘unique’). The name comes from its crazy shaped skull. The bones that make up the front of the snout (maxilla and premaxilla) are shaped into a ‘T’ like a hammerhead! All of the teeth in the front of the snout are small and shaped like pegs. The teeth in the back of the mouth are normal, so this animal has 2 completely perpendicular (meeting at a right angle) sets of teeth.

Fig1B

Figure 2 from the paper showing the bones in each of the new fossils. “m” is maxilla and “pm” is premaxilla.

Before these fossils were found, paleontologists thought the front of the face was turned downwards. These fossils give us a much clearer idea of what their faces looked like.

What were they doing with their hammerheads? Probably scraping aquatic plants and algae off of rocks, meaning this marine reptile was an herbivore. They may have even fed like baleen whales do: taking in a lot of water with their food and pushing the water through their teeth to filter the water out and keep the food in. Because it lived in the Middle Triassic, Atopodentatus was the earliest marine reptile that was an herbivore that we’ve found!

Fig3

Figure 3 from the paper showing Atopodentatus eating. Drawn by Y. Chen.