Viviparity may have evolved from land species

A fossil of three babies discovered in central China has shed light on the origins of live birth. Against the prevailing option, this fossil suggests that live birth may have evolved on land rather than in the sea. A team lead by Dr. Ryosuke Motani from the University of California, Davis, recently published this finding in a Plos One paper.

The chaohusaurus specimen, a giant marine reptile of the family of ichthyosaurs that lived about 248 million years ago, was found fossilized in the process of giving birth. The mother is frozen with the skeletons of three offspring: one skeleton is just under the mother’s tail, another is inside the mother and the third is in the mother’s birth canal with its skull just exiting the pelvis.

Chaohusaurus offspring were born head-first. This birth orientation is typical of humans and other land-based mammals. The offpsring of viviparous, or live-birthing, aquatic animals tend to emerge tail-first. This tail-first orientation prevents water-related suffocation of the newborn. Thus, researchers found it odd that this particular chaohusaur gave birth head-first in the water.

Researchers based in the United States, Italy and China are using this new fossil evidence to investigate the origins of viviparity. Biologists have long believed that early marine reptiles, such as the ichthyosaurs, only evolved live-birthing after they adopted an aquatic lifestyle. The discovery of this new fossil is prompting experts to shift their thoughts toward a different theory: Reptiles have inherited viviparity from their land-based ancestors.

Viviparity, the process in which embryos develop inside the mother rather than in external eggs, evolved independently at least 141 times in various vertebrate species of mammals, amphibians, bony fishes and reptiles. This process is primarily a feature of terrestrial animals. Most fish species, some amphibians and some reptiles are oviparous, meaning they lay eggs after little or no embryonic development inside the mother. Some animals, such as sea turtles, have to come ashore to lay eggs, but ichthyosaurs could not walk on land, so viviparity evolved as a more practical solution. 

Before this fossil discovery, the birthing habits of chaohusaurs were known from previously uncovered skeletons. The excavation in which this most recent fossil was discovered yielded more than 80 new skeletons of the Chaohusaurus genus and closely related members in the ichthyosaur group. The specimen with three offspring is one of only two records of ichthyosaurs giving birth.

The fossil was found while the research team was analyzing a different fish on the same slab. Analysis revealed that the discovered ichthyosaur fossils are about 10 million years older than the previous record of Mesozoic-era marine reptiles. This also leads the researchers to believe that viviparous land reptiles must have appeared much earlier than the current estimates suggest.