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Tetrapod Zoology

Tetrapod Zoology

Amphibians, reptiles, birds and mammals - living and extinct

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.

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

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