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Prediction confirmed: plesiosaurs were viviparous


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Regular readers will know that I often avoid discussing new palaeontological discoveries at Tet Zoo, the exceptions being those in which I was personally involved (hmm). But I have to break that soft rule on occasion. Yesterday, Robin O’Keefe and Luis Chiappe published their excellent Science paper on the polycotylid plesiosaur specimen LACM 129639, collected in the Campanian Pierre Shale of Kansas in 1987.

Reconstruction of a polycotylid giving birth, by S. Abramowicz.

Close-up of the foetus, adjacent to bones of the mother. (c) LACM.

Identified as Polycotylus latipinnus, the specimen is remarkable in being a pregnant female, with the well preserved (but largely disarticulated) remains of a single foetus positioned adjacent to her belly region (O’Keefe & Chiappe’s 2011). As the authors explain, several features mean that the juvenile has to be identified as an in-situ foetus: it exhibits diagnostic characters of P. latipinnus, was preserved with some of its bones tightly adhering to the inside surface of the mother’s pelvis, lacks signs of maceration or consumption, and exhibits the poor ossification and so on that you expect for a foetus.

Finally we have confirmation that at least some plesiosaurs were definitely viviparous. Informal banter on plesiosaur reproduction has been going on for years within the palaeontological community. Some people have suggested that plesiosaurs actually hauled up onto beaches to lay eggs turtle-style, while others (like me) have argued that a committed aquatic existence makes viviparity far more likely.

Pregnant specimens of the pachypleurosaur Keichousaurus, from Cheng et al. (2004). The babies aren't obvious at this resolution, sorry, but they're there, I promise.

My opinion has been that plesiosaurs grew to such enormous sizes (way exceeding those of even the biggest sea-turtles) and became so specialised for a life in water (enormous, mostly ventrally located pectoral and pelvic girdles, wing-like, claw-less paddles, pachyostotic bones etc.) that the presence of viviparity is more likely than the persistence of an egg-laying habit, especially when we know that diapsid reptiles of several lineages evolved viviparity on numerous separate occasions. And supporting, pre-2011 evidence for this contention comes from the fact that we now know for sure that pachypleurosaurs – distant cousins of plesiosaurs within Sauropterygia – practised viviparity (Cheng et al. 2004). Foetuses of the nothosaur Lariosaurus – preserved in close association and without eggshells – also indicate the presence of viviparity in the sauropterygian clade Nothosauridae (Renesto et al. 2003). In fact, the presence of viviparity in pachypleurosaurs and nothosaurs suggests (as I said at a conference in 2004) that viviparity evolved early on within Sauropterygia, first appearing in Triassic taxa that were amphibious and small (body lengths = ancestrally less than 1 m).

The caring, sharing, motherly plesiosaur

There’s more to O’Keefe & Chiappe’s (2011) study than confirmation of a viviparous habit in plesiosaurs, however. The fact that this plesiosaur was carrying a single, very large foetus (estimated to have been about 35% of the mother’s 4.7 m length when at full term) shows that this plesiosaur, at least, was K-selected: that is, a lot of maternal investment went into the production of a very small number (n = 1) of expensive babies.

Polycotylus adult to scale with full-term baby, from O'Keefe & Chiappe (2011).

What might this mean for parental care and social behaviour in these animals? We don’t really know of course, but [note caveats!] it at least hints at the possibility that there was some sort of complex social stuff going on in these animals. Live birth, low reproduction rates and substantial investment in proportionally big babies isn’t just for mammals – it’s also seen in some lizard lineages, specifically various skinks (well, and in other animals too… caecilians, some sharks etc.). As O’Keefe & Chiappe (2011) note, these lizards exhibit parental care, kin recognition and the formation of social groups and social bonding. It’s clear that we don’t have enough information on plesiosaurs to make any sensible proposals about the existence of such behaviour, but it is at the very least possible that these aspects of behaviour were there.

Shingleback skink Tiliqua rugosa, one of several viviparous skinks known to engage in all sorts of complex social behaviour (pair-bonding, formation of juvenile groups, kin recognition etc.). Image from Lydekker's 1896 Royal Natural History.

One thing I have to note as a dinosaur specialist is that – in this high parental investment in a low number of babies – plesiosaurs now look rather different from many Mesozoic dinosaurs, most of which produced large egg clutches and practised little to no post-hatching parental care*. It would be somewhat ironic if lowly plesiosaurs – so long imagined as ‘mere reptiles’, and not as neat or interesting in behavioural terms as the terrestrial dinosaurs – were the ones practising the complex parental care so many people associate with modern birds and mammals. The idea of K-selected plesiosaurs also raises questions about how good plesiosaurs were at stocking environments, how quickly they could replenish their numbers after disasters, and so on. While it would be entertaining to speculate at length on these matters, I need to stop here. Congrats to the authors, it’s great to see this specimen described and analysed at last.

* Another caveat: as always, we don’t know as much as we might like to, and there is at least some evidence for post-hatching parental care in some dinosaurs.

For previous Tet Zoo articles on plesiosaurs, see…

Refs – -

Cheng, Y.-n., Wu, X.-c. & Ji, Q. 2004. Triassic marine reptiles gave birth to live young. Nature 432, 383-386.

O’Keefe, F. R. & Chiappe, L. M. 2011. Viviparity and K-selected life history in a Mesozoic marine plesiosaur (Reptilia, Sauropterygia). Science 333, 870-873.

Renesto, S., Lombardo, C., Tintori, A. & Danini, G. 2003. Nothosaurid embryos from the Middle Triassic of northern Italy: an insight into the viviparity of nothosaurs? Journal of Vertebrate Paleontology 23, 957-960.

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com!

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The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Yodelling Cyclist 9:46 am 08/12/2011

    Huzzah! Data at last! Though it’s a shame that egg-laying has gone out of the window. I liked the mental image (implausible though it was) of a mother plesiosaur hauling out to give birth.

    Link to this
  2. 2. Jurassosaurus 10:22 am 08/12/2011

    This has been a long time coming. I remember hearing about a plesiosaur with fossil embryos inside the body cavity (somewhere in the UK I think) some four years ago. That specimen still has yet to be described. I’m glad we found another one, and that the authors didn’t wait as long to publish (to prep, yes, but not to publish). Now we just need to see if metriorhynchids and protostegids were capable of similar feats, or if oviparity was the ultimate reason behind their smaller size (not that a 2 tonne _Archelon ischyros_ was something to sneeze at).

    Link to this
  3. 3. Jerzy New 12:36 pm 08/12/2011

    It opens the possibility that plesiosaurs were homothermic and could colonize cold seas and cold freshwater lakes.

    (Conciously trolling hoping that Daily Mail writes something like “New evidence that plesiosaur could live in Loch Ness”. ;)

    Link to this
  4. 4. Andreas Johansson 12:40 pm 08/12/2011

    “Homothermic” sounds decidely risqué.

    Link to this
  5. 5. Jerzy New 12:42 pm 08/12/2011

    Seriously, plesiosaurs are understudied. Did anybody bother on researching their body temperature, like for dinosaurs?

    Do we have any direct evidence of social behaviour like groups fossilized together?

    Are there any other groups of fossil reptiles where eggs and small juveniles are missing, suggesting they were born alive?

    Link to this
  6. 6. Halbred 2:56 pm 08/12/2011

    Weren’t ichthyosaurs K-selected, too? If so, could that explain the decline of icthyosaurs and plesiosaurs during the early days of the mosasaur takeover?

    Link to this
  7. 7. Heteromeles 5:57 pm 08/12/2011

    It’s so great to see something come out of LACNHM. I’ve seen lists of insect collections from there with specimens identified to “species nov.” and occasionally “gen. nov.” on them, from areas that are now built out. Probably most museums are like that, but I’m still happy that something neat saw daylight.

    Theoretical physiological question: Obviously that fetus required a lot of oxygen. Is it more likely to get that oxygen from a homeothermic mother, or a ectothermic one? Or is oxygen supply across the placenta not a limiting issue in this case?

    Link to this
  8. 8. Ranjit Suresh 6:51 pm 08/12/2011

    Bernard et al. 2010 provides evidence that plesiosaurs maintained relatively constant temperatures in the range of the high 30′s Celsius. But, it’s not clear to me if they were gigantotherms, had higher internal temperatures through vascular heat exchangers like some sharks, tuna, and swordfish, or were tachymetabolic like mammals and birds.

    The scientists used oxygen isotopes in their teeth to determine these temperature ranges. However, swordfish keep their eyes and brain at higher temperatures while allowing the rest of their bodies temperature to be controlled by the ambient environment. Although they don’t have teeth as adults, if they did, would they show up with similar results as plesiosaurs?

    Link to abstract:

    Link to this
  9. 9. falcon121 7:01 pm 08/12/2011

    Social skinks? …

    Link to this
  10. 10. Bytor 9:38 pm 08/12/2011

    Viviparity or ovoviviparity? Would we be able to tell from a fossil the different between a placenta with a trophic connection or an extremely thin- and pliable-shelled egg that ripped open to hatch internally before expulsion?

    Link to this
  11. 11. LeeB 1 10:28 pm 08/12/2011

    Sharks have a whole range of reproductive modes from oviparity to viviparity.

    But all the large offspring (over 1m long at birth)require more nourishment from the parent than can be provided from the yolk contained in a single egg alone.

    They either have young that eat other eggs while still in the womb; practice intra-uterine cannibalism with only one survivor in each half of the uterus; or have the equivalent of a placenta to feed the young.

    The size of the embryonic plesiosaur suggests that it would have been born live; probably after being fed by a placenta.

    And given the ratio of the embryo’s size to that of it’s mother it makes you wonder how big neonatal pliosaurs were.

    LeeB.

    Link to this
  12. 12. falcon121 11:37 pm 08/12/2011

    Placental nourishment does seem the most likely explanation for such a large reptilian neomate (are there any reptiles that practice intra-uterine cannibalism?) An interesting thought: were pliosaurs born precocial or altricial? Were they capable of finding food or did they need the help of a parent? Were unrelated adults likely to cannbalize young pliosaurs?

    Link to this
  13. 13. David Marjanović 10:53 am 08/13/2011

    I liked the mental image (implausible though it was) of a mother plesiosaur hauling out to give birth.

    It wasn’t that implausible before Keichousaurus reproduction was discovered. Given that their bellies were plated by the shoulder and hip girdles and the gastralia, I’m surely they were better able to support their weight when lying on land than any whale.

    Link to this
  14. 14. David Marjanović 10:54 am 08/13/2011

    Oops. Forgot that the <blockquote> tag doesn’t work.

    (…which is… majorly stupid on a site that contains science blogs. But I digress.)

    Link to this
  15. 15. LeeB 1 5:39 pm 08/13/2011

    I don’t know of any reptiles that practice intra-uterine cannibalism; it has been reported in salamanders, fish and sharks.

    Given their size I expect neonate plesiosaurs could feed themselves, certainly newborn sharks are capable of feeding themselves.

    Of course it is possible the plesiosaurs could have accompanied their parents, they would have been large enough to keep up.

    Crocodiles appear to react to the alarm calls of even unrelated young, attempting to attack whatever is threatening them; but are also reported to cannibalise smaller crocodiles.

    So whether or not pliosaurs would protect or predate unrelated young is difficult to predict.

    It may be size related; at some stage they might begin to see them as competitors.

    LeeB.

    Link to this
  16. 16. falcon121 8:55 pm 08/13/2011

    Now for another implausible mental image … small pliosaur family groups …

    Read up on social skinks btw its fascinating.

    Link to this
  17. 17. Jerzy New 9:05 am 08/14/2011

    Actually, at least smaller plesiosaurus might haul themselves on land AND give birth there. Live birth does not prevent going on land and does not strictly imply parental care.

    There is lots of talk that parental care in tetrapods is assocuated with “cuteness” or shortening facial part of the skull in young. Much of it is untrue, but still: were small plesiosaurs “cute”?

    Link to this
  18. 18. DMA12 9:37 am 08/14/2011

    I know plesiosaurs and icthyosaurs were believed to be endothermic, but is there any evidence for the metabolism of mosasaurs.

    Link to this
  19. 19. BilBy 9:41 am 08/14/2011

    What’s up with the scale bar on the ‘pregnant pachypleurosaur’ fossil image? 5cm? Were there plesiosaurs that small or are those actually the babies?

    Link to this
  20. 20. David Marjanović 10:52 am 08/14/2011

    Pachypleurosaurs (and Keichousaurus, which may or may not be a pachypleurosaur) actually were that small. The plesiosaurs are more closely related to the considerably larger nothosaurs.

    Link to this
  21. 21. David Marjanović 10:53 am 08/14/2011

    …probably.

    Link to this
  22. 22. BilBy 11:58 am 08/14/2011

    Wow – then I want to know more about these tiny pachypleurosaurs! Presumably they were not pelagic swimmers? In fact, excuse my complete ignorance but were plesiosaurs pelagic, or coastal, a combination, or what?

    Link to this
  23. 23. LeeB 1 4:23 pm 08/14/2011

    @DMA12

    The answer is yes.

    See the abstract of a paper in Science here:http://www.sciencemag.org/content/328/5984/1379

    LeeB.

    Link to this
  24. 24. David Marjanović 5:35 pm 08/15/2011

    Plesiosaurs were generally pelagic, though they’ve been found even in freshwater deposits. Pachypleurosaurs were coastal, and as their name (thick-rib lizards) says, they were pachyosteosclerotic — they used their massive bones as ballast for diving.

    Link to this

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