• Category Archives Extinct Reptiles
  • Articles about extinct reptiles

  • The return of the scaly T. rex to modern paleo-art

    Tyrannosaurus rex walking towards camera. Art by John Sibbick.

    [NOTE: Post has been updated to include a section on scale size]

    This has certainly been an interesting year. Two papers dropped in the past three months that have put the brakes on a recent trend in paleo-art. That trend? Why the feather-coated T. rex of course.

    First, in March, we saw the release of a paper detailing a new species of Daspletosaurus and its relationship to D. torosus.

    Carr, T.D., Varricchio, D.J., Sedlmayr, J.C., Roberts, E.M., Moore, J.R. 2017. New Tyrannosaur with Evidence for Anagenesis and Crocodile-Like Facial Sensory System. Scientific Reports. 7(44942):1–11.

    In this paper, Carr et al. argue for the designation of a new Daspletosaurus species, D. horneri. The authors argue, based on skull shape and chronostratigraphic position, that D. horneri was the direct ancestor to D. torosus. I thought that the authors put forth a compelling argument for this anagenic event and backed up their position well. Interestingly, this part of the paper should have been the most controversial. As anyone who has read anything from Horner and Scanella over the past eight years can attest, arguing for a direct ancestor-descendant relationship for dinosaurs is difficult to do and even harder to win over others in the field. So it is somewhat surprising to see a case for anagenesis in Daspletosaurus taken so well by the palontological community. All the more so given that it involves a tyrannosaur, the poster children for “cool guy” dinosaurs.

    Instead, the most controversial part of the paper wound up being their soft-tissue reconstruction of the face for D. horneri. The author responsible for the soft-tissue reconstruction was Jayc Sedlmayr of Louisiana State University. Sedlmayr did his doctorate on osteological correlates for vasculature in extant archosaurs (birds & crocs). He is the seminal alumnus of the WitmerLab and thus is well within his wheelhouse for this type of soft-tissue reconstruction. Sedlmayr borrowed heavily from the work of another WitmerLab alumnus, Tobin Hieronymus, whose PhD work involved osteological correlates for integument on the skulls of animals. Although the skin is often well away from the underlying bones on most of the body, there are exceptions when it comes to the skull. There, areas that are not heavily muscled, tend to show intimate connections between the skin and the underlying bone. Hieronymus used these connections to determine how different integumentary appendages (scales, hair, feathers) affect the underlying bone (Hieronymus & Witmer 2007; Hieronymus et al. 2009). The authors found that the surface texture along the skull of D. horneri was “hummocky”. That is, it was covered in lots of closely packed ridges. According to Hieronymus & Witmer (2007), this texture correlates to scales as the overlying integumentary appendage. Thus, according to the authors, D. horneri had a scaly face (this is grossly oversimplified as the authors were able to piece together a variety of different integument variants along the skull, but you get the idea).

    Scaly tyrannosaur cannonball one had been shot.

    Then two weeks ago, we saw the release of another paper on tyrannosaur integument. However, unlike the previous paper, this one was specifically dedicated to integumentary types in tyrannosaurids.

    Bell, P.R., Campione, N.E., Persons, W.S., Currie, P.J., Larson, P.L., Tanke, D.H., Bakker, R.T. 2017. Tyrannosauroid Integument Reveals Conflicting Patterns of Gigantism and Feather Evolution. Biology Letters. 13:20170092

    In this paper, the authors set out to survey all known instances of “skin” impressions for tyrannosaurids. Their list of taxa included Albertosaurus, Tarbosaurus, Daspletosaurus, and Gorgosaurus. Their results pretty definitively indicated that scales were the predominant integumentary appendage on tyrannosaurids. The authors then went on to speculate why that would be if earlier tyrannosauroids had filamentous integument. They performed an ancestral character state reconstruction based on Parsimony and Bayesian-based trees from Brussatte and Carr 2016. Their results found that filaments came out strongly as the ancestral character for tyrannosauroids, but by no later than Tyrannosauridae proper, a reversion to scales had taken effect. The authors attributed this to body size evolution. Namely, larger tyrannosauroids reverted to scales over protofeathers.

    Cannonball number 2 had just been shot.

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  • New study shreds the dinosaur family tree (and exposes double-standards in Phylogenetic Nomenclature)

    Figurative illustration of the new phylogeny by Baron et al. 2017

    Most folks who visit my site by now have seen the big dinosaur news that has hit the interwebs. A new study from Matthew Baron, David Norman and Paul Barrett from University of Cambridge and the Natural History Museum of London, has seriously challenged the classic interpretation of dinosaur phylogeny.

    Baron, M.G., Norman, D.B., Barrett, P.M. 2017. A New Hypothesis of Dinosaur Relationships and Early Dinosaur Evolution. Nature. 543:501–512.

    Classical dinosaur phylogenetics

    Although originally thought of as two unrelated branches of Reptilia that grew to immense size during the Mesozoic (e.g., Charig et al. 1965), for the last 43 years the group, Dinosauria, has been considered monophyletic (i.e., sharing a single origin) with the subgroups, Saurischia & Ornithischia, forming the first major branches within the group (Bakker et al. 1974). Saurischians, or “reptile hips” were aligned together by their similar hip shapes, skull characters (e.g., open antorbital fenestrae), and inferred soft tissues (e.g., air sacs). Ornithischians, or “bird hips” shared a hip structure that was superficially similar to that of birds, with a pubis that pointed caudally rather than rostrally, along with a variety of unique skull characters such as a neomorphic bone known as the predentary.

    Study after study showed that this relationship was sound, and so it stayed that way. The problem with getting the same answer over and over again is that one eventually stops questioning it. Consistent results become  common knowledge, and may even graduate to dogma. That’s not so bad if that common knowledge is true, but all too often many of these “obvious” cases wind up being just so stories upon closer inspection.

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  • Modern-day paleo myths: Dinosaurs as lizards

    Paleomyths

    In this day and age there are no shortage of books, websites, and videos dedicated to debunking classic paleo myths. The majority of this mythbusting focuses on myths about dinosaurs. As the poster children for paleontology, this isn’t that surprising. With so many takes on this subject it comes as no surprise that all of the classic dinosaur myths have long since been debunked, such as dinosaurs as low-energy tail draggers, walking around like Godzilla, being evolutionary failures, inferiority to mammals, being pee brained monsters, etc.

    However, as quickly as these classic dinosaur myths have been eradicated, new ones have come and taken their place. These myths/misconceptions are routinely cited today without any question despite being just as erroneous as the myths that preceded them.

    This is the start of a new series I want to cover on the site: dispelling modern myths in vertebrate paleontology. Given the bent of my website, these myths/misconceptions will largely stay focused on reptile-related animals, though I am open to taking the occasional foray into other animal groups if the myths are egregious enough (which is to say that suggestions are welcomed).

    The seminal installment for this series is one that I see mentioned time and again:

    “Dinosaurs were once thought of as big lizards.”

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  • T-U-R-T-L-E Power Part 4: The little-known paleobiology of the world’s largest tortoise

    Megalochelys_atlas
    Megalochelys atlas skeleton on display at the AMNH. Photo by Clair Houck (Wikipedia)

    Today, the largest turtle alive is easily the leatherback turtle (Dermochelys coriacea), at a whopping 916 kg (2,015 lbs, Eckert & Luginbuhl 1988). On land, the largest turtle goes to Chelonoidis nigra (Galápagos tortoise) which has been reliably recorded as reaching up to 417 kg (919 lbs) in weight (Guinness World Records). However, both extant turtles are dwarfed in size by an immense land tortoise that lived as little as 1.7 million years ago, in the Pleistocene.

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  • Jurassic World Review

    It's here!
    It’s here!

    I figured if I was going to do a Jurassic World-related post on Stegosaurus I might as well follow it up with a review for the film. I grossly underestimated the draw of dinosaurs to the cinema. Despite 22 years of Jurassic Park, Walking with Dinosaurs (BBC version, not the Disney thing), and so on, people never seem to be burnt out on dinosaurs. That’s good news for paleontology (yay!), and also for movies seeing as how Jurassic World just raked in a record-breaking $208.8 million domestic in its opening weekend.

    So what did I think?  In short: I liked it and found it to be a worthy successor to the franchise.

    If you’d like the longer, spoiler-ridden version click on the jump.
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  • Jurassic World and the case of the droopy-tailed Stegosaurus

    As I write this the US premiere of Jurassic World is just around the corner. I had gone back and forth regarding this post given that we currently know very little about the film and as such the interpretations written about here and elsewhere may well be pointless by the time the film premieres.

    Ultimately I decided to post this anyway since the overall thrust of the article should remain true regardless of how the film pans out.


    Now there has been a lot of buzz around Jurassic World since it was first announced last year. The buzz has been mixed, but fairly positive. I suspect this was, in part, because everyone was happy to hear that the godawful military dinosaur idea was shelved in favour of a more “traditional” JP franchise storyline. Nonetheless the movie has still drawn its fair share of detractors, including myself. Most of the people who are unhappy with the film are either paleontologists, or hardcore dinophiles. Many of the problems leveled at the film have to do with the portrayal of the extinct animals. The problems are actually myriad ranging from pterosaurs capable of picking up humans using grasping feet, mosasaurs that are twice the size of blue whales, sauropods covered in elephant skin rather than scales (a problem not unique to Jurassic World), everything about Velociraptor, and of course Indominus rex.  My biggest beef with the film is that the dinosaurs are not being shown as dinosaurs so much as monsters. However, after The Lost World: Jurassic Park came out it became pretty evident that Spielberg’s original vision of portraying dinosaurs as animals had been shelved in favour of the more entertainment-friendly movie monster approach. However, for what seems like a majority of the detractors, the biggest gripe with the film has to do with a lack of  feathers on pretty much all the dinosaurs. This seems to be a common theme these days with a particularly vocal group of dinophiles and paleontologists strongly pushing for the feathering of every dinosaur in sight and insisting that all media that portrays scaly (erroneously called: “naked”) dinosaurs is inaccurate. Never mind the fact that a feathered, pack-hunting, 2 meter tall Velociraptor mongoliensis is still every bit as inaccurate as a scaly one.

    Anyway, I digress. Dealing with the overwhelming amount of internet drama surrounding Jurassic World (and the media depiction of dinosaurs in general) is a topic for another day. My reason for writing this post is centered around one particular criticism that popped up a few weeks ago.

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  • Tall spines and sailed backs: A survey of sailbacks across time

    One of the quintessential depictions of prehistoric times is that of an ancient, often volcano ridden, landscape full of animals bearing large showy sails of skin stretched over their backs. Sailbacked animals are rather rare in our modern day and age, but back in the Mesozoic and Paleozoic there were sails a plenty.

    By far the most popular sailbacked taxa of all time would be the pelycosaurs in the genus Dimetrodon. These were some of the largest predators of the Permian (up to 4.6 meters [15 feet] long in the largest species). Dimetrodon lived alongside other sailbacked pelycosaurs including the genus Edaphosaurus. These were large herbivores (~3.5 m [11.5 ft] in length) that evolved their sails independently from Dimetrodon. The Permian saw many species of sphenacodontids and edaphosaurids, many of which sported these showy sails (Fig. 1. [1–8]).

    SailbackRoster
    Fig. 1. A brief survey of the sailbacks of prehistory. Permian sailbacks, the sphenacodontids: Dimetrodon(1), Sphenacodon(2), Secodontosaurus(3), and Ctenospondylus(4). The edaphosaurids: Edaphosaurus(5), Ianthasaurus (6), Echinerpeton(7), Lupeosaurus(8). The temnospondyl: Platyhystrix(9). Triassic sailbacks, the rauisuchians: Arizonasaurus(10), Ctenosauriscus(11), Lotosaurus(12), and Xilousuchus(13). Cretaceous sailbacks, the theropods: Spinosaurus(14), Suchomimus (15), Acrocanthosaurus (16), and Concavenator (17). The ornithopod: Ouranosaurus (18), and the sauropod: Amargasaurus (19). Image credits: Dmitry Bogdanov (1–2, 8, 14–15), Arthur Weaseley (5, 19), Smokeybjb (7), Nobu Tamura (3–4, 6, 8–9, 10–12), Sterling Nesbitt (13), Laurel D. Austin (16), Steven O’Connor (17), Sergio Pérez (18).

    However sails were hardly a pelycosaur novelty. The contemporaneous temnospondyl Platyhystrix rugosus (Fig. 1 [9]) also adorned a showy sail.

    Fast forward 47 million years into the Triassic and we find the rauisuchians Arizonasaurus babbitti, Lotosaurus adentus, Xilousuchus sapingensis, and Ctenosauriscus koeneniall bearing showing sails on their backs (Fig. 1 [10–13]). Much like in the Permian, many of these taxa were contemporaneous and, while related, many likely evolved their sails separately from one another.

    There are currently no fossils of sailbacked tetrapods in the Jurassic (as far as I know. Feel free to chime in in the comments if you know of some examples). However the Early Cretaceous gave  us a preponderance of sailbacked dinosaurs (Fig. 1 [14–19]) including the cinematically famous theropod Spinosaurus aegyptiacus, the contemporaneous hadrosaur Ouranosaurus nigeriensis, the gharial-mimic Suchomimus tenerensis, the potentially dual sailed sauropod Amargasaurus cazaui, as well as the allosauroids Acrocanthosaurus atokensis, and Concavenator corcovatus. Lastly, the discovery announced last year (and just now coming to light in the news) of better remains for the giant ornithomimid Deinocheirus mirificus have revealed that it too may have sported a small sail along its back.

    Once again we find a group of related, largely contemporaneous, animals, most of which probably evolved their sails separately.

    Such a huge collection of sailbacked animals all living around the same time (and sometimes the same place) has begged for some type of functional explanation. The usual go-to for large, showy surfaces like these or the plates of Stegosaurus has been thermoregulation. The thinking being that blood pumped through a large surface area like this, when exposed to the sun, has the ability to warm up faster than other areas of the body. Conversely when the sail is placed crosswise to a wind stream, or parallel to the orientation of the sun, heat will radiate out into the environment faster than other areas of the body. That most sailbacked dinosaurs were “localized” to equatorial areas, coupled with the large sizes of all the taxa (1-10 tonnes depending in species) has favoured a cooling mechanism function for dinosaur sails. Whereas a heating function has been presumed to be the primary function for sails in Dimetrodon and Edaphosaurus. No real function has been ascribed to the sails in rauisuchians or Platyhystrix, though this is probably due to a lack of knowledge/interest in these groups.

    Alternate functions proposed for these sails have included a self-righting mechanism for swimming, sexual signaling and other presumed display functions. In certain cases, namely Spinosaurus aegyptiacus and Ouranosaurus nigeriensis, it has even been argued that the enlarged spines did not support a sail, but rather were supports for a large, fatty hump akin to that of camels or bison (Bailey 1996, 1997).

    Given the wealth of hypotheses for potential sail functions it would be beneficial to first understand what extant sailbacked taxa use their sails for. Unfortunately—though unsurprisingly—there are few if any scientific studies on sail use in extant sailbacked animals. This has lead to the apparent assumption that there are no extant vertebrates with sailbacks.

    There are, in fact, quite a few sailbacked animals alive today. These include various fish, amphibians and even reptile species. Learning what these taxa use their sails for may offer us a glimpse at what extinct animals were doing with their sails.
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  • “Feathers” on the big, “feathers” on the small, but “feathers” for dinosaurs one and all?

     

    Yutyrannus artwork by Brian Choo. Sciurumimus artwork by Arkady Rose

    This year has seen the discovery of two big deal dinosaur specimens. At least they are a big deal in regards to dinosaur integument and, possibly, metabolism.

    First off from a few months ago we had the announcement the theropod Yutyrannus hauli, the “beautiful feathered tyrant.”

    Xu, X., Kebai, W., Ke, Z., Qingyu, M., Lida, X., Sullivan, C., Dongyu, H., Shuqing, C., Shuo, W. 2012. A Gigantic Feathered Dinosaur from the Lower Cretaceous of China. Nature. Vol.484:92-95

    This was not just a single fossil, but a collection of three fossils (one might be tempted to call it a family group, but that would only be speculation). As with all other dinosaur fossils that have been found to have filamentous integument, these guys come from Liaoning, China. They are suspected to have come from the Jehol Group in the Yixian formation. I say suspected because the complete three specimen set was a purchase from a fossil dealer, an all too common occurrence for Chinese fossils. As such the provenance information is unknown. A lot of Chinese fossil dealers don’t like to give away the location of their find due to the potential loss of other profitable specimens. This current trend in China is a good example of what happens when capitalism comes into play with fossil collecting (something that the U.S. has been mostly, but not entirely, able to avoid). So it is currently uncertain whether these fossils are from the Yixian. However given that all the others guys are too it is probably a good bet. Given the sketchy nature in which many Yixian fossils are collected, coupled with the possibly large consequences of the find, one should naturally be skeptical of the fossil. Had it been one individual on multiple slabs I would question its validity as a real thing. However since Y.huali is known from three individuals, and the filaments seem to follow a consistent pattern around the body (compare that to the helter-skelter nature of Tianyulong‘s preservation), forgery seems unlikely. These guys are probably the real deal. This has some potentially far reaching consequences to interpretations of Late Cretaceous coelurosaurs and the Jehol Biota itself (more on this in a bit).

    The second announcement came just a few weeks ago. This was the discovery of a potentially new, miniscule theropod from Bavaria Germany.

    Rauhut, O.W.M., Foth, C., Tischlinger, H., Norell, M.A. 2012. Exceptionally Preserved Juvenile Megalosauroid Theropod Dinosaur with Filamentous Integument from the Late Jurassic of Germany. PNAS Early Edition:1203238109v1-201203238.

    The specimen is exceptionally well preserved. So well preserved in fact that it actually looks like a plastic toy. While this degree of preservation warrants importance all its own, the main interest behind this new guy—dubbed: Sciurumimus albersdoerferi (Albersdörfer’s squirrel mimic)—is the apparent presence of filamentous integument on the body coupled with its apparent placement among much more basal theropods. This discovery has far reaching consequences for theropod integument interpretations. Note: As with Y.hauli, Sciurumimus albersdoerferi was also purchased from a private collector. I don’t suspect forgery here either as this was in Germany, where fossil dealing is neither a big problem nor a lucrative business. The exceptional detail on the specimen would also require a substantial amount of theropod knowledge to pull off. Anyone having that amount of knowledge is more likely to be a real paleontologist than a get rich quick forger.

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  • Bad-ass shield crocs, or: Another weird Mesozoic crocodyliform

    Aegisuchus witmeri goes to town on a Mesozoic lungfish. Illustration by the talented Henry Tsai

    Oh hey look, the blog has come to life again, if just for a bit. As has been typical these few years, things IRL have taken up much of my time and the website has suffered because of it. I still have a few posts that I have been sitting on as I try to find the time to finish them. Until then small updates like this will have to do.

    Just announced today in the journal PLoS ONE:

    Holliday, C.M. and Gardner, N.M. 2012. A New Eusuchian Crocodyliform with Novel Cranial Integument and Its Significance for the Origin and Evolution of Crocodylia. PLoS ONE 7(1): e30471. doi:10.1371/journal.pone.0030471

    Congratulations to the internet’s own Nick Gardnerfor helping get this guy published.

    Stomatosuchus was the quintessential "duck faced" croc. Illustration by Dmitry Bogdanov

    The croc in question Aegisuchus witmeri was a member of the Aegyptosuchids. They were a strange group of eusuchians that are known mostly for their weird, flat “duck faces.” As there are no living crocodylians that even come close to these guys in skull shape, it is difficult to imagine what these guys were doing with these flattened rostra. One hypothesis was that, given their numerous small teeth, these guys were filter feeders.

    Holliday and Gardner describe a preserved braincase and compare it to other published data on Aegyptosuchids. Results suggest that this guy was huge by modern croc standards (~9 meters) and no slouch for a Mesozoic croc. Muscle scars indicate the presence of strong jaw opening abilities in this taxa, which would go well for a possible filter, or suction feeder.

    Probably the most interesting feature of this guy, and the one likely to spark the most controversy, was the presence of an enlarged boss on the top of the skull. Inferred vasculature to this region suggest that Aegisuchus witmeri was using this part of its skull for something. That thing might have been a display structure such as an “eyespot” or just a particularly bright patch of skin. Though speculative, there are reasons to consider this possibility, including the fact that extant crocodylians use their heads in all manner of displays.

    All in all this was a pretty cool critter. The species epithet was named in honour of professor Lawrence Witmer, PhD, prolific paleontologist, comparative anatomist and even blogger. He is my mentor and was Dr. Holliday’s back in his PhD days. It might not be Archaeopteryx, but getting named after a bad-ass ancient crocodile isn’t half bad.

    ~Jura

     


  • 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