Dinosaur Embryo Frozen Mid-Birth Gives Paleontologists Rare Snapshot Of Ancient Reptile Baby-Making

By Ben Wolford on February 15, 2014 1:43 PM EST

This fossil shows the fully developed embryo of an ancient lizard mid-birth, a treat for paleontologists wondering whether live births happened on land or under water. (Photo: PLOS ONE)

This fossil shows the fully developed embryo of an ancient reptile mid-birth, a treat for paleontologists wondering whether live births happened on land or under water. (Photo: PLOS ONE)

A newly described fossil is almost of portrait quality: its subject is a Chaohusaurus fetus nose-diving out of its mother's pelvis. Scientists who studied it called it an "exceptional specimen" and one that resolves a paleontological mystery of how a 248-million-year-old sea creature gave birth. More importantly, it yields fresh clues about the evolutionary transition between marine animals and their land-dwelling ancestors.

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But first, how did such a graphic fossil come about? "The reason for this animal dying is likely difficulty in labor," Ryosuke Motani, a paleobiologist at the University of California, Davis, told The Huffington Post. "Obviously, the mother had some complications." The fossil actually shows two fully developed embryos. The clearest one is already nearly out of the mother, one sibling is close behind, and another is nearby. Motani — the lead author of a new research paper on the fossil, published Wednesday in PLOS One — believes the one frozen in the pelvis was already dead.

Among land animals that give live births (as opposed to laying eggs), head-first birth is typical. In marine animals, however, it's common for babies to be jettisoned tail first, according to the BBC, probably so that they don't suffocate. That's why Motani and his crew were so surprised to see this perfectly preserved birth-in-progress etched into a rock in Anhui Province, China — and the baby coming the "wrong" way out of a marine animal.

The findings show that Chaohusaurus gave birth in a way that was different from its more advanced relative, the Ichthyosaurus, which was strictly a marine animal and resembled more closely modern dolphins. The Ichthyosaurus, being more adapted for sea life, gave birth tail first. What does this mean? Well, scientists had always figured that live birth in marine reptiles developed in the ocean. The new evidence suggests live births were inherited from land-dwelling reptilian ancestors. "Being reptiles, their ancestors lived on land. What happened during the transition from land to the sea is not well understood, and Chaohusaurus holds a key to [unlock] the mystery," Motani told the BBC.

One other thing that's interesting, is that live birth among land reptiles likely developed much earlier than previously thought. Around 250 million years ago, the greatest mass extinction the world has ever known took place. Nearly all of the planet's species died out. This study suggests an impressive rebound in which land animals were giving live births relatively soon after the extinction event, known as the end-Permian mass extinction.

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