Live birth: Most mammals do it, some lizards and snakes do it, but archosaurs – a reptilian group that includes crocodiles and birds – don't... or so biologists thought.
When a long-necked, marine archosauromorph died some 245 million years ago in what is now China, she was pregnant, according to a paper published Tuesday in the journal Nature Communications. And now paleontologists are hailing this fossil as evidence that archosaurs might not have always been strict egg-layers.
"We commonly think of these aspects of animal biology as static or 'fixed' throughout evolutionary time, and cases like this demonstrate just how labile the evolution of animal form and biology can be," Dr. Nathan Smith, an associate curator at the Dinosaur Institute at the Natural History Museum of Los Angeles, who was not involved in the study of this new specimen, writes in an email to The Christian Science Monitor.
Egg-laying, or oviparity, is thought to be the ancestral reproductive strategy, with live birth, or viviparity, evolving later in some lineages. Viviparity isn't just the placenta-nourished embryonic development of mammals. It has also frequently evolved independently among lizards and snakes in a variety of forms, sometimes with babies hatching from eggs incubated inside their mothers.
So viviparity was known in mammals and lepidosaurs (the vertebrate group including lizards and snakes), explains study co-author Michael Benton, a paleontologist at the University of Bristol in Britain. But "nobody had ever discovered, in any of the living or fossil forms, any evidence that archosaurs could adopt live birth."
Finding a little version of the bigger animal in the abdominal region "is about as close as you can get in the fossil record to direct evidence of reproductive mode," Christian Sidor, a paleobiologist at the University of Washington who was not involved in the research, says in a phone interview with the Monitor.
Read the full article on The Christian Science Monitor.