Head of The University of Queensland's School of Earth and Environmental Sciences and co-author Professor Jonathan Aitchison said the fossil unexpectedly provided the first evidence for live birth in an animal group previously thought to exclusively lay eggs.
"Live birth is well known in mammals, where the mother has a placenta to nourish the developing embryo," Professor Aitchison said.
"Live birth is also very common among lizards and snakes, where the babies sometimes 'hatch' inside their mother and emerge without a shelled egg."
Until recently it was thought the third major group of living land vertebrates, the crocodiles and birds (part of the wider group Archosauromorpha) only laid eggs.
"Indeed, egg-laying is the primitive state, seen at the base of reptiles, and in their ancestors such as amphibians and fishes," Professor Aitchison said.
He said the new fossil was an unusual, long-necked marine animal called an archosauromorph that flourished in shallow seas of South China in the Middle Triassic Period.
Super cool animal, check out this artist’s reproduction. Awesome find to show that live births are not exclusive to mammals and some lizards and snakes. The lizards and snakes are described to hatch inside of their mothers, which sounds distinctly different to me than what occurs with most mammals.
"Some reptiles today, such as crocodiles, determine the sex of their offspring by the temperature inside the nest," he said.
"We identified that Dinocephalosaurus, a distant ancestor of crocodiles, determined the sex of its babies genetically, like mammals and birds.
"This combination of live birth and genotypic sex determination seems to have been necessary for animals such as Dinocephalosaurus to become aquatic," he said.
Buried at the bottom of the article it seems almost an after thought in the conversation with the authoring Professor. To me, this seems like the most relevant detail. Egg laying would be impossible for a creature whose respiratory system depends on air sacs, similar to birds. Unless the young underwent an amphibious style transformation from gills to air sacs, there’s no way a creature in an egg would be able to get enough oxygen in a submerged egg.
Conversely, everything about birds has been completely redesigned to allow flight. Things like feathers, bone structure, the aforementioned air sacs and beaks are all unique genetic adaptations which drastically lower weight and allow for flight. Laying eggs is drastically more useful for organisms trying to shed weight as an embryo is just one more thing that doesn’t need to be taking up weight inside the body,
We talk about eggs as a more primitive state, but for flying animals in particular, I’d argue that it’s superior to a live birth. We know that animals frequently evolve traits and lose them as they adapt to new environments. As an example, flightless birds like penguins actually have solid bones, losing an adaptation which was beneficial for flight but is limiting when diving in water for the aquatic penguins.
In order to survive in the oceans, it was more practical to adapt live births. It makes sense that the creatures that thrived in water evolved these traits. If conditions lined up properly, and these animals slowly adapted to an aerial life, it would seem likely the ancestors would slowly favored eggs as they would be most successful.
When it comes to adaptations, I think primitive should be reserved for scenarios where something was clearly evolving away from a trait as it is less useful in their current environment. Thus, I’d argue some snakes and lizards have primitive legs as in their burrowing lifestyle the legs are a hinderance and not an advantage.