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Sauropterygians NEVER FORGET

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


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You’ve heard of plesiosaurs (and probably the short-necked plesiosaurs known vernacularly as pliosaurs). But unless you’re a palaeontologist or zoology uber-nerd, you might well not have heard of placodonts, pachypleurosaurs, nothosauroids and pistosaurids – the other lineages that, together with plesiosaurs, form Sauropterygia, a major clade of Mesozoic marine reptiles. Here’s a much-simplified cladogram showing hypothesised sauropterygian affinities. Cartoons depict the main lineages.

Non-plesiosaurian sauropterygians are mostly Tethyan in distribution: that is, they inhabited the warm, equatorial Triassic Tethys Sea. Back then it extended from western Europe all the way to eastern Asia. Only with the evolution of paraxially swimming plesiosaurs did sauropterygians become truly pelagic and hence able to disperse about the seas of the Mesozoic world.

Sauropterygians have highly distinctive skeletal elements. In the skull, they possess the unusual euryapsid skull condition (where the laterotemporal opening is secondarily closed, only the supratemporal opening remaining*), have retracted external nostrils, a closed, extensive bony palate that extends all the way to the braincase, a large retroarticular process (a prong that projects from the posterior end of the lower jaw), and lack several of the bones otherwise primitive for reptiles (like lacrimals, tabulars**, postparietals and supratemporals). Postcranially, the ilium is small and the scapula lies superficially to the clavicle – a very weird feature that became developed to an extreme in plesiosaurs. Talking of plesiosaurs, here’s a cake, made specially for the Arthur Cruickshank celebration of 2009…

* This assumes that sauropterygians are part of the major reptile clade Diapsida, as they seem to be based on other lines of morphological evidence.

** My mistake. Some authors do indeed say that sauropterygians are unusual in lacking tabulars, but Nick Gardner reminds me that tabulars have been lost across numerous diapsid lineages (as well as elsewhere in Reptilia, as in captorhinids). In fact, absence of the tabular may be a neodiapsid autapomorphy (though the ‘younginiform’ Claudiosaurus may perhaps have tabulars). Tabulars are paired bones at the back of the skull, located above the occipital surface and on either side of the also paired interparietals.

Sauropterygians of various sorts have been covered on Tet Zoo a few times, but I’ve never gotten round to covering any of them in detail. ARGH THE FRUSTRATION.

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! Follow on Twitter @TetZoo.

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





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  1. 1. Halbred 7:46 pm 10/24/2012

    Well, you wrote that very informative paper about placodonts, but I’d love an updated version on Tet Zoo. As for everyone between them and plesiosaurs, the names are recognizable but I know virtually nothing about them, especially pachypleurosaurs.

    Link to this
  2. 2. naishd 4:21 am 10/25/2012

    Duly noted. I have loads of material on these groups and am very keen to write about them. It’s just a lack of time that constrains me. The early evolution of viviparity, the evolution of durophagy and suction-feeding, underwater olfaction, the transition from axial undulatory locomotion to paraxial locomotion, the use of bones as ballast, and so much more.

    Darren

    Link to this
  3. 3. naishd 5:01 am 10/25/2012

    Oh yeah, my placodont article (now quite out of date due to all the new Chinese stuff, the debate over placodont lifestyle and so on) is…

    Naish, D. 2004. Fossils explained 48. Placodonts. Geology Today 20 (4), 153-158.

    Pdf here at my academia.edu site.

    Darren

    Link to this
  4. 4. John Harshman 10:13 am 10/25/2012

    I see you are continuing your clever tactic of leaving out the turtles. Shouldn’t we note that Olivier Rieppel, at least, would put them somewhere on this tree?

    Link to this
  5. 5. jeiman 11:34 am 10/25/2012

    http://www.sciencecodex.com/old_skull_bone_rediscovered-96507
    Here’s an interesting article concerning the embryonic retention of the tabulars in mammals. Perhaps they were retained in sauropterygians as well, if only in the embryonic stage.

    Link to this
  6. 6. Mark Evans 5:49 am 10/26/2012

    I thought this was going to be “Sauropterygians never forget” as in “Elephants never forget”. Never mind.

    I see you’ve gone for the “alternate stroke” locomotion model for your ?rhomaleosaurid plesiosaurian.

    Most pliosaurids either regain the lacrimal, or have a neomorphic element in that position, but other than that it’s comparatively easy to work on plesiosaurian skulls as they lose all those pesky bones.

    Link to this
  7. 7. naishd 6:27 am 10/26/2012

    Those pesky turtles (comment 4): well, at best, remember that the hypothesis is that turtles might be related to sauropterygians, not that they are nested within sauropterygians. So, even if that hypothesis does have merit, I should be ok in ignoring turtles again.

    The idea for the title (comment 6) is not that sauropterygians never forget things (so far as I know, this claim remains untested), but that we must never forget them, because they’re so awesome. Yes, that leptocleidid-type plesiosaur is indeed alternate stroking its way across the page. I was young and foolish back then. As for the loss and reappearance of certain skull bones.. yes, as you know there are definitely plesiosaurian lineages with things that look like nasals and so on.

    I assume everyone knows about the feathered ornithomimosaurs just out in Science. How neat.

    Darren

    Link to this
  8. 8. Andreas Johansson 12:06 pm 10/26/2012

    I don’t recall hearing of pistosaurids. From the citation marks in the diagram, I presume they’re likely not a clade? So a grade of not-quite-plesiosaurs?

    (A close relative of Plesiosauria ought be named Plesioplesiosaurus.)

    Link to this
  9. 9. John Harshman 12:10 pm 10/26/2012

    I was looking for good images of turtle-like placodonts (e.g. Henodus) and came across this really cool creationist web site, though in fact it’s down the Google list a little bit from David Peters, who appears at least twice.

    http://www.earthhistory.org.uk/transitional-fossils/origin-of-turtles

    Link to this
  10. 10. naishd 12:26 pm 10/26/2012

    Comment 8: yes, it now seems that ‘pistosaurids’ (Pistosaurus, Augustasaurus and Yunguisaurus) are a series of out-groups to Plesiosauria. They seem to be ‘proto-plesiosaurs’, with limbs approaching those of plesiosaurs in form and proportion. See…

    Cheng, Y.-n., Sato, T., Wu, X.-C. & Li, C. 2006. First complete pistosauroid from the Triassic of China. Journal of Vertebrate Paleontology 26, 501-504.

    Rieppel, O., Sander, P. M. & Storrs, G. W. 2002. The skull of the pistosaur Augustasaurus from the Middle Triassic of northwestern Nevada. Journal of Vertebrate Paleontology 22, 577-592.

    Sander, P. M. Rieppel, O. C. & Bucher, H. 1997. A new pistosaurid (Reptilia: Sauropterygia) from the Middle Triassic of Nevada and its implications for the origin of the plesiosaurs. Journal of Vertebrate Paleontology 17, 526-533.

    Sues, H.-D. 1987. Postcranial skeleton of Pistosaurus and interrelationships of the Sauropterygia (Diapsida). Zoological Journal of the Linnean Society 90, 109-131.

    Darren

    Link to this
  11. 11. David Marjanović 2:00 pm 10/26/2012

    the laterotemporal opening is secondarily closed

    Not at all. It’s open ventrally, as in most diapsids. The bar between it and the dorsal temporal opening is somewhat broader than usual, but that’s all; you can still see an embayment.

    the also paired interparietals

    In my experience, they’re only called “interparietal” when they fuse in mammals. Elsewhere they’re “postparietals” (even when fused).

    Olivier Rieppel, at least, would put them somewhere on this tree

    No; he has lepidosaurs + turtles as the sister-group of what he calls Sauropterygia and what was called Euryapsida for 150 years. The name Sauropterygia used to be applied to what he calls Eosauropterygia, a flat-out misleading name.

    Link to this
  12. 12. David Marjanović 2:01 pm 10/26/2012

    BTW, Darren, your nothosauroid has really tiny limbs. :-)

    Link to this
  13. 13. Jerzy v. 3.0. 3:33 pm 10/27/2012

    Re: 7. After this discovery of ornithomimid feathers in coarse sediment, and a strong proof that fossil dinosaur bones have inside recognizable blood vessels, cells etc.

    I wonder if somebody analyses microstructure of “rocky matrix” around some articulated dinosaur fossils. Maybe sometimes a soft tissue outside bones preserves too, but was macroscopically invisible and overlooked – and this will shed more light on dinosaur anatomy?

    Link to this

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