By Henry Workman
Marine Science Today Writer
There is a tendency to identify viviparity, that is, giving birth rather than laying an egg, as a trait exclusively characteristic of mammals. However, just as there are examples of egg laying mammals (the platypus, to name one), cases of live birth can be seen across multiple classes of animals.
One viviparous reptile, it is now known, is the plesiosaur, a group of water-dwelling species that appeared in the late Triassic period and died out approximately 65 million years ago. Though these carnivores could reach lengths of up to twenty meters long and thrived throughout much of the Mesozoic Era, they are not classified as dinosaurs, of which strictly aquatic varieties have never been found.
It was previously speculated that plesiosaurs made their way onto dry land to lay eggs, in a similar fashion to modern sea turtles. New analysis of a fossil at the Natural History Museum of Los Angeles County indicates that their reproduction process was in fact closer to that of whales. The fossil, LACM 129639, was originally discovered in Kansas.
The highly fragmented and crude remains were found to be those of two, Late Cretaceous Polycotylus latippinus, a species with a shorter neck and larger head than the image “plesiosaur” tends to conjure up. One was a 4.7-meter-long mother and the other was determined to be her offspring. Despite being quite large, by all indications the baby was still in its fetal stages, with portions of the fossil revealing it to be inside the parent. The possibility that it had simply been eaten had to be ruled out, since this had been shown to be the case with past fossils thought to bear proof of live birth for their respective species.
The fact that the fetus was found without others is revelatory for the re-creation of the plesiosaur’s habits. This would indicate that a parent expended a great deal of energy and commitment to one offspring. Whereas many comparable species rely on the principal of safety in numbers, mother plesiosaur left the responsibility of genetic perpetuation to a single individual at a time. This paints a picture of a nurturing and protective parent, in contrast to the brutal upbringings many dinosaurs and coexisting reptiles are thought to have had.
This particular reproductive strategy is a relative novelty in the spectrum of reptiles throughout the eras. It is still unknown why many modern reptiles and birds never evolved the particular advantages that are associated with birthing live young. It comes as little surprise, though, that a large predator with few natural enemies could survive using this method.
Copyright © 2011 by Marine Science Today, a publication of OceanLines LLC