• Tag Archives plesiosaurs
  • Turns out that plesiosaurs gave birth to live young. It’s about damned time.

    _Polycotylus latippinus_ mother giving birth to young in a very cetacean-like fashion. Illustration by: S. Abramowicz

    Just announced today in Science, researchers at the Marshall University and the Los Angeles County Museum described the presence of fossil young inside the body of the plesiosaur: Polycotylus latippinus. The results of their find seem to confirm what has been suspected for quite some time now, that plesiosaurs were viviparous animals.

    O’Keefe, F.R., Chiappe, L.M. 2011. Viviparity and K-Selected Life History in a Mesozoic Marine Plesiosaur (Reptilia, Sauropterygia). Science. Vol.333(6044):870-873

    The evidence had been mounting for some time now. While plesiosaurs came in numerous shapes and sizes, most of those sizes were in the large to giant range measuring in at multiple tonnes (e.g. Liopleurodon and Kronosaurus). That is a lot of weight to attempt to drag up on a beach for egg laying. Further, though the rib cage is well braced ventrally, the limb girdles are not braced against the vertebral column. This would make it very hard for a large landlubbing plesiosaur to make any kind of headway as the limbs would have no leverage against the body for dragging itself on land.

    Lastly, and perhaps most importantly, we have known of at least one plesiosaur fossil that had embryos in it. This has been known for at least five years now (I learned of it four years ago, and it has been hinted at before [Smith 2008]). Sadly this specimen still remains unpublished. This new paper by O’Keefe and Chiappe goes on to mention the relatively large size of the young, estimated at 1.5 meters when born. This was much larger than the young of other large extinct and extant marine reptiles. The authors (cautiously) suggest that this might hint at a different life history for plesiosaurs vs. other marine reptiles. They posit that plesiosaurs might have nurtured a small amount of relatively large young, which in turn might have meant that they were more social than previously thought.

    Naturally this has resulted in the inevitable comparison to whales. While a “pod of plesiosaurs” does sound interesting, we have far too little evidence to say if such a thing ever happened (and the authors state this too). What we do know is that young plesiosaurs have been found in shallow marine settings. These have been posited to have been “nurseries” where young could stay out of sight from predators while reaching adult size (Martin et al. 2007). Whether, or not adults stayed around, or if they joined a “pod” later (if at all) is all unknown. Still, it is nice to see some validation to what seemed almost necessary for so long.

    Admittedly not everyone is convinced (a good thing to see in science). Dr. Ken Carpenter of the Utah State Museum offered Science magazine a dissenting view, suggesting that the position of the young could still indicate that these were juveniles that had been eaten. The O’Keefe and Chiappe considered this in the paper and pointed out that the skeletons lacked any signs of acid etching, as well as showed numerous skeletal bones that did not appear fully ossified. Further analysis could shed more light on this. Publishing on that other plesiosaur could really help things out too.

    Viviparity - could these guys be next? Image from the Nature Museum in Stuttgart.

    Assuming that we are looking at viviparous plesiosaurs, that just leaves two other large marine reptile groups of the Mesozoic. Turtles and Crocodylomorphs. In both cases we have extant animals that are obligate oviparous animals, but there might still be reason to think that live birth might have evolved in these groups too.  Again, much like with the plesiosaurs, the groups in question (protostegid sea turtles and the podocnemid Stupdendemys, as well as metriorhynchid crocodylomorphs) have members that grew extremely large. While Protostega gigas may have been able to haul itself out on land as extant leatherbacks (Dermochelys coriacea) do, it seems harder to justify that in the much larger Archelon ischyros; an animal that has been estimated to tip the scales at 2 tonnes. Given the amount of effort it takes a large female leatherback  (~1 tonne) to haul herself up and down a beach (not to mention the damage it causes to the animals in the short term), it would be all the more amazing if A.ischyros was able to pull off such a feat. The same would go for the metriorhynchids, who had adapted completely to a marine lifestyle (i.e. they had flippers and a tailfin). If a 5 meter Gavialis gangeticus can barely move around on land, I’d hate to see what a 5 meter Dakosaurus would look like. To date we have no evidence one way, or the other for these last two groups. There is a bit more resistance to the idea of viviparity in these groups as no extant members exhibit viviparity. This has lead some to wonder if the calcified eggs of archosaurs (and many chelonians) might prove a phylogenetic constraint on live bearing (the young absorb calcium from the shell, which could mess up calcium absorption in a taxon evolving along the lines of viviparity). The chelonian shell — in turn — may also have been constraining on the size of young that can be held in the body cavity. Still, to date, there are no nests, eggs, or embryos for any of these taxa, thus leaving the matter open for debate. It is interesting that neither protostegids, nor metriorhynchids got to the huge sizes of mosasaurs, ichthyosaurs and plesiosaurs, but that could have been for any number of reasons including the simple lack of finding the larger taxa yet.  Until then the physics vs. phylogeny argument remains unresolved.

    Anyway, compelling evidence for live bearing in at least some plesiosaurs. Woohoo!

    ~Jura

    References

    Martin, J., Sawyer, F., Reguero, M. Case, J.A. 2007. Occurrence of a Young Elasmosaurid Plesiosaur Skeleton from the Late Cretaceous (Maastrichtian) of Antarctica. 10th Int.Symp.Antarctic Earth Sciences.
    O’Keefe, F.R., Chiappe, L.M. 2011. Viviparity and K-Selected Life History in a Mesozoic Marine Plesiosaur (Reptilia, Sauropterygia). Science. Vol.333(6044):870-873
    Smith, A.S. 2008. Fossils Explained 54: Plesiosaurs. Geol.Today. Vol.24(2):71-75

     


  • Scientists discover a huge pliosaur

    Pliosaur picture

    Just announced today on the BBC news website, scientists have unearthed the remains of a giant plesiosaur from the Arctic island of Svalbard.Coming in at 15 meters (50ft), it ranks as one of the largest known specimens of ancient marine reptile. Since there are other plesiosaur and mosasaur specimens that are known to approach 50ft in length, I’m assuming the reason that this species is given the term “monster” is because of its overall size. No species name has been given yet. Judging from past plesiosaur behemoths, I’m thinking that it might be Liopleurodon. Folks who saw the BBC series: Walking with Dinosaurs, might remember the Liopleurodon in one of the episodes. That one was a whopping 21m (70ft) in length. So, to date, it is still an unrealistic size.That said, there sure were a lot of giant marine reptiles swimming around the Mesozoic seas.

    It is interesting to note that there has yet to be a fossilized sea animal (or any animal) that approaches the monstrous blue whale (Balaenoptera musculus) in terms of overall size (giant jellyfish [Cyanea capillata] grow longer, but have nowhere near the mass). Off the top of my head, the largest prehistoric fish was the Jurassic giant: Leedsichthys problematicus, which has had length estimates very close to blue whales (~30m, or 98ft). There was one ichthyosaur that exceeded “the monster” in length, but might have fallen “short” in terms of overall mass (Shonisaurus sikanniensis). All in all, B.musculus stands alone in terms of oceanic giants. What was imposing this size limit on all the Pre-Tertiary marine fauna? One possibility that I’ve heard tossed about (and the only one that I happen to think is on the right track) is that the reason for the size of blue whales, is due to the prevalence of krill (order: Euphausiacea).

    Leedsichthys
    Liopleurodon swims by Leedsichthys; weighing its options.

    Krill are way down at the near bottom of the food chain. They eat plankton and are, in turn, eaten by a huge chunk of the marine ecosystem. Krill are global in their distribution, and their biomass is astronomical (500 million tonnes, according to Wikipedia). So why is this important?Well, if we look back at all the large marine reptiles, or most of the large prehistoric marine vertebrates; all of them were large predators. “The Monster,” Kronosaurus, Liopleurodon and most of the others, all had a taste for meat. The problem with this, is that a large meat eater requires lots of meat in order to survive. This imposes a size limit right away. Either a large marine animal is going to eat shoals and shoals of small fish (which may reproduce rapidly, but probably not rapidly enough to maintain a viable population of large carnivorous marine vertebrates), or it is going to eat any large animal that it can take down. If shoals of fish can’t maintain a viable population of marine behemoths, then anything bigger will certainly not. Large marine animals with big appetites, need something that can take the hit and keep on going.Krill and plankton provide the only real option for giant marine animals. And it just so happens that we are currently living in a time period where most of the oceans are temperate.This is important. Temperate waters mean that there is a section of ocean that is very cold and a section that is warm. This results in upwelling, or the pushing of nutrients from the bottom of the sea, up to the top where it can be used by other life forms (namely: plankton, squid and krill) to make energy. The importance of thermohaline circulation for all of this cannot be discounted either. Both result in the necessary conveyor belt like mixing of oceanic nutrients.

    So more nutrients results in greater biomass of krill, which allows for the evolution of large marine animals beyond the 15 meter / 30 tonne mark. Today that niche happens to be filled by mammals. Why?

    Is there something special about their physiology that allows only them to grow to this size?

    No…not really. Most likely, mammals just so happened to be big enough at the right moment in time.

    If Antarctica, or Australia had moved closer to the South Pole and iced over, then we would have had a thermohaline circulation in the Mesozoic, and most likely, giant planktivorous marine reptiles. It didn’t so the best we got was the smaller, yet equally impressive: Leedsichthys problematicus. Incidentally, it probably was a planktivore; though it probably relied on the less productive tropical plankton spawns (not much choice, given the time period).

    Nevertheless, these were all truly awesome animals. I look forward to seeing what else these Arctic islands are going to give up.

    ~Jura