repeat after me – pterosaurs were not dinosaurs!

From yesterday’s RSNZ news headlines:

Size mattered for flying dinosaurs: New research into pterosaurs and pelycosaurs shows their grand headcrests and sails were developed to attract a mate, not to regulate body temperature as first thought.

Now, you may think me a palaeontological pedant (& this is not a cue for cracks about my age from the cheap seats!), but I find that loose use of the word ‘dinosaur’ really irritating. Why? – because neither pterosaurs nor pelycosaurs were dinosaurs. I know the headline’s more eye-catching than saying ‘size mattered for prehistoric flying reptiles’, but still, it bugs me.

So what’s the difference?

Pelycosaurs – animals like Dimetrodon & Edaphosaurus – pre-dated the dinosaurs by a considerable length of time. They first appeared in the Carboniferous period (around 300 million years ago – here’s a link to a geological time scale) & were the dominant animal group during the Permian (280-260 mya). So what, you say. They could have been ancestral to dinosaurs. Well, no, they weren’t – & the evidence for this lies in their skulls. These splay-legged reptiles, some of which sported great ‘sails’ on their backs, were synapsids – a term reflecting the presence of a single opening (behind the eye) in the dermal bone of the skull. (Have a look at the picture at that link.) Mammals are also synapsids, & this means that the mammalian lineage has its roots much further back in time than that of the dinosaurs, who first appeared on the scence in the Triassic. What’s more, dinosaurs are diapsids – a feature that they share with birds & other living reptiles. (Which is why modern phylogenetic trees see birds branching off the reptiles rather than having a branch of their own, which is the way I was taught it way back when.)

The name ‘pterosaur’ means ‘winged lizard’, but these prehistoric flying reptiles are only distantly related to modern lizards. These elegant fossils first turn up in rocks from the Triassic, but their complexity suggests that the group is probably older than that; it’s just that the fossils haven’t been found. Scientists used to think that pterosaurs were gliders, but the consensus has shifted to an acceptance that most (with the exception of the really big ones such as Quetzocoatlus) were capable of powered flight. (The evidence for this includes the fact that the bones of pterosaurs were hollow – like those of birds – and the forelimb bones had prominent muscle-attachment crests, suggesting the presence of flight muscles.) There’s also a suggestion, based on what looks to be fur on the bodies & flight membranes of some particularly well-preserved fossils, that at least some pterosaurs were endotherms ie capable of generating & maintaining their own body heat.

As for the dinosaurs, which evolved during the Triassic (around 225 mya)… I remember that Stephen Jay Gould once commented – as a throwaway line in one of his books – that people have a fascination with dinosaurs made all the more delicious by the fact that they were very fierce, very big – & very dead. (I wish I could remember the actual book I read that in; no time right now to go through my bookcase…) Of course, he would also have noted that this fascination is based on a misunderstanding of dinosaurs: they ranged in size from the giant sauropods like Diplodocus & the even larger titanosaurs, through the smaller but still large-by-human-standards tyrranosaurs, down to animals roughly the size of a turkey. I strongly suspect they’d have varied in ferocity as well 🙂

If asked how to distinguish between dinosaurs & other diapsids, a palaeontologist would look at things like the anatomy of the legs. The earliest dinosaurs were bipeds, walking erect on their hind legs. In other words, the early reconstructions of some dinosaurs that showed them as quadrupeds with legs akimbo were a long way from reality. They walked on their toes, with the soles of their feet raised off the ground, and the hind legs in particular were pulled in under the body – essentially the same as in modern birds. In addition, the hip socket had a small opening in it where the 3 bones that make up each side of the pelvis come together. (Features of the hip also allow us to divide dinosaurs into 2 main groups – the ‘lizard-hipped’ saurischians & the ‘bird-hipped’ ornithischians. The latter is something of a misnomer as birds are actually most closely related to a group of saurischians.)

So – dinosaurs, pterosaurs & pelycosaurs are 3 related but distinct groups; conflating them into one ignores all their fascinating differences & oversimplifies our view of the past.

(I’d like to be able to comment on the actual paper on which that headline was based. Unfortunately, while we supposedly have an institutional subscription, at present the website keeps asking me for $US, so until that’s sorted out a review will have to wait.)


10 thoughts on “repeat after me – pterosaurs were not dinosaurs!”

  • Jim Thomerson says:

    We used an introductory biology text which used a Dimetrodon as its example of a dinosaur. Will, I think introductory texts should have a few obvious mistakes for the instructor to call to the attention of the class.

  • herr doktor bimler says:

    cracks about my age from the cheap seats!
    Whom did you have in mind?
    If it’s any consolation, the wording in that Sydney Morning Herald is not as bad as the UK Daily Mail version, which explicitly describes the pelycosaurs as sail-backed dinosaurs.
    I’d like to be able to comment on the actual paper on which that headline was based.
    I’ll see if I can get a copy through the Massey library. The Smithsonian blog has a summary that’s based on the actual paper rather than on the press release that all the popular-press reports seem to be using:

  • Alison Campbell says:

    If you could get hold of a copy, that would be cool. I’m still waiting on our librarians to sort out just what our subscription lets us get…

  • herr doktor bimler says:

    Hmmm. Essentially the paper is appealing to an argument from ignorance. For the authors’ pteranodon data-set, the size of the crest is allometric for the 9 skulls for which they have measurements, to an extent that is not explained by the other explanations they can think of, so it must be sexual selection.
    The trouble is that the data points consist of three “putatively female” skulls and six “putatively male” skulls, where the smallest ‘male’ skull is the same size as the three ‘female’ skulls, and it has the same sized crest as them, contrary to what sexual selection would predict.
    I’m waiting for Tetrapod Zoology to weigh in on the question.

  • Alison Campbell says:

    I had wondered about that. I mean, just how do you sex a pelycosaur/pteranodon? ‘Putative females’ could surely equally well simply be smaller/younger individuals of either sex. (And there’s that reclassification of several separate dinosaur species as an age series in a single species as an example – I blogged on it a while ago.) A priori assumptions are never a good thing.

  • Stuart Humphries says:

    As one of the authors, I can say that we were very careful not to use the word dinosaur in the paper. Press offices and journalists, on the other hand are unlikely to be aware, or care, about the difference.
    I’m not quite sure how ruling out competing theories to leave a strong allometric pattern characteristic of another theory is “arguing from ignorance”.

  • Alison Campbell says:

    I think the press go for the catchy headline every time; the only trouble is that it perpetuates misconceptions that you & I have to deal with in our students 🙂
    I’ll leave herr doktor to justify his allometry comment…

  • Stuart Humphries says:

    I agree in principle, but in reality the vast majority of (potential) readers presumably won’t care -to them dinosaurs are a generic descriptor. Our students are probably a tiny minority of the readership (for good or bad 😉 )
    It’s nice to do things right, but when the press get hold of things perhaps the best we can hope for is a story that somehow reflects the research we published!

  • Alison Campbell says:

    I know exactly what you mean 🙂 A while ago I blogged on a piece the press had got very very wrong: the researchers had looked at the link between red flowers in a particular genus of Australian plants, levels of ‘grazing’ by birds, & cyanide content of said flowers. The press take on this was that scientists now knew why roses were red!

  • herr doktor bimler says:

    I’m not quite sure how ruling out competing theories to leave a strong allometric pattern characteristic of another theory is “arguing from ignorance”.
    My reasoning, such as`it is, is that it would be nice if there were more direct positive evidence for sexual selection of these features of pteranodons. “Positive evidence” might include a sufficiency of pteranodon skulls such that when crest size is plotted against orbit size, there are recognisably two superimposed populations with different slopes (i.e. levels of allometry) that might be identified as males and females. Or perhaps a single curve but with a kink in the middle, such that “all skulls less than some size” (with low allometry) might be females while “all skulls above that size” (with high allometry) might be male.
    In other words, I’m saying that it would be nice if the processes of fossilisation had given us all the pteranodon skeletons we want which now that it’s in other words is not as profound as I’d hoped.
    I agree that pointing out the inadequacies of alternative theories is a perfectly good way of arguing in favour of a theory. The trouble is that it’s my impression that we do not know a great deal about the actual life-style and ecological niche of Pteranodon; there’s a lot of speculation involved (if this impression is wrong then I’ll try to take any correction in good grace). And this must restrict the possibilities for generating alternative theories. That’s my rationale for “argument from ignorance”.
    Finally, in the absence of positive evidence for sexual dimorphism, there’s always the null hypothesis that allometry didn’t have any evolutionary advantage but was simply wired into the growth pattern of pteranodons, so that bigger ones developed disproportionally larger crests (Zombie Steven Jay Gould would be able to argue for that hypothesis better than I can).

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