Plesiosaurs’ Live Birth

Written by on August 28, 2011 in Marine Life

Not scientifically accurate. Interpretive rendering by Heinrich Harder, 1916

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.

The findings appear in Science, a publication of the AAAS.


Copyright ©  2011 by Marine Science Today, a publication of OceanLines LLC

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  1. Steven McDermott says:

    This is interesting stuff. We should all be reading Mr. Workman’s articles regularly.

  2. Gillian Stokes says:

    This story confirms my observation from years ago. I am not a specialist, but there was a story (on UK TV or newspaper, I forget) with a picture of a marine fossil which defied explanations as to its cause of death. I could instantly see this was a breech birth and the creature had died in the birth process. I was so convinced of this, and amazed that experts (mostly men?) had not also seen this as obviously as I (a woman) could that Isent a message to the scientific organisation that had publisized the picture of the fossil with my explanation. This was years ago, and I forget if my messsage was even acknowledged, but this new finding (a different fossil from the one I commented on) just confirms what I spotted way back. It might have been the Natural History Museum or Science Museum in London that I wrote to, but I can’t remember now and that may mislead. I am so excited to see my ‘discovery’ vindicated after all these years. Maybe I can find the letter I wrote (if it was an email) but I suspect it was a paper letter as it was so long ago.

  3. Henry says:

    Indeed, just because this is the first definitive conclusion made about plesiosaurs’ birth doesn’t mean there weren’t indications in previous fossil specimens. The one used in this research has been around since the 80s; only the analysis is new. This study may spark new interest in other fossils, such as the one you saw. Thank you for commenting.