Taking a quick break from the ‘origins of our genome’ series, because…
Today, Drs. Luis Chiappe and Robin O’Keefe publish a paper about an exciting fossil find from the Cretaceous oceans of Kansas, 78 million years ago (yes, Kansas was then covered by an ocean of water, not crops). It describes the spectacularly preserved and articulated remains of a Polycotylus latippinus (a short necked plesiosaur, a type of marine reptile with four flippers), but the most exciting aspect of the find is that the animal was pregnant when it died, and the embryo is preserved within the mother’s skeleton .
Why do we suspect it wasn’t the animal’s last meal? The shapes of the bones are consistent with those of other Polycotylus individuals, and were neither fully formed, nor fully ossified (turned from cartilage to bone), which is characteristic of embryos.
Most excitingly, the young creature was over 30% of the length of the adult (6 versus 15 feet, or 1.8 versus 5 m), offering the first strong evidence that plesiosaurs gave birth to live young, since this size is far too large to have fit in a reasonable sized egg. Plesiosaurs had occasionally been restored as emerging onto the shore to lay eggs like sea turtles, but evidence to the contrary had been lacking until now.
I have illustrated what a diver might have seen, had one been around in the Cretaceous, during the live birth of the Polycotylus, had the embryo been carried to term (image above). Notice the relatively large size of the young compared to the mother. Such big babies are characteristic of many social animals, leading to the hypothesis that plesiosaurs may have formed social groups, much as modern dolphins travel in pods. Indeed, plesiosaurs are great examples of convergent evolution (i.e., the phenomenon of relatively distantly related organisms acquiring similar shapes or life styles due to experiencing similar types of selection pressures); not only do they have a body plan reminiscent of cetaceans (whales and dolphins), but their gestation patterns and (possible) prosociality also make them resemble marine mammals more than reptiles.
Other extinct marine reptiles, such as ichthyosaurs, have been known to give birth to live young. For example, fossils of Stenopterygius quadriscissus have been found containing the bones of embryos. In this way, some of the groups of extinct marine reptiles differ from most modern reptiles, including sea turtles. Although we usually associate live birth with mammals (which I discussed briefly in my last post), the bearing of live young (viviparity) appears to have evolved multiple times in other groups of animals. This includes reptiles, most species of which lay eggs (a condition known as oviparity). Retention of embryos in the mother increases the probability of survival of young compared to the vulnerable state of development in eggs, which are unable to escape and are more likely to become damaged.
Still, Polycotylus is not the earliest pregnant vertebrate animal known from the fossil record. Amazingly, two species of early fish from the Devonian Period (members of an extinct group known as placoderms) have been found with young within their bodies [2, 3]. The 380-million-year-old fossil of Materpiscis attenboroughi even contained the preserved remains of the umbilical cord .
Meanwhile, Polycotylus is not in Kansas anymore. The beautifully prepared specimen now resides in the new Dinosaur Hall of the Natural History Museum of Los Angeles County. Readers of this blog may recognize this museum as one whose opening I featured in a previous blog, “The New Face of Museums“.
1. F. R. O’Keefe, L. M. Chiappe. Viviparity and K-Selected Life History in a Mesozoic Marine Plesiosaur (Reptilia, Sauropterygia). Science, 2011; 333 (6044): 870 DOI:10.1126/science.1205689
2. Long et al. Live birth in the Devonian period. Nature, 2008; 453 (7195): 650 DOI: 10.1038/nature06966
3. John A. Long, Kate Trinajstic, Zerina Johanson. Devonian arthrodire embryos and the origin of internal fertilization in vertebrates. Nature, 2009; 457 (7233): 1124 DOI: 10.1038/nature07732