Having grabbed a few minutes to go through the Tawa paper and then finding time to put pen to paper (or in these times, fingers to keyboard) there was one thing I wanted to raise about this new dinosaur. (For those who have missed out, this not-feathered theropod was named and described this week by Sterling Nesbitt and his team and is one of the earliest known theropod dinosaurs).
The thing that interested me was the evidence for pneumatisation of parts of the skeleton (hence the awful pun in the title of this post). Those who keep up with the excellent SV-POW (or even the occasional comments on here on the subject) will know that a variety of archosaurs have (or rather had in the case of the extinct ones) extensions of the lungs called air sacs that sat alongside or invaded various bones of the skeleton. Of obvious note are the birds, pterosaurs and sauropods, but actually most theropods show at least some pneumaticy (though some, like Areosteon, more than others).
The relevant point here is that although there is some evidence to suggest that all archosaurs had at least some minor pneumaticy early on in their evolution, there remains the strong possibility that the pterosaurs, sauropods and theropods all gained their pneumaticy independently. The fossil evidence is rather ambivalent. While the idea that theropods and saruopods are very closely related is hardly troubled by the possibility that they did not inherit pneumatic bones from their saurischian ancestors, it does seem a little odd and of course causes a few problems for the ‘pterosaurs-as-ornithodirans’ idea.
Happily, while not exactly solving this, Tawa does provide some new information here. In addition to being an especially basal theropod, Tawa does have pleurocoels on some of its vertebrae – they are pneumatic (and there are pneumatic invasions in parts of the skull too). The new information that Tawa brings to the table also appears to help resolve some phylogenetic questions about early theropods and other saurischians, and another early and pneumatic theropod, Chindesaurus, can now be positioned with more confidence.
Between the two this provides quite a high level of confidence that pneumaticy was around at least in the very earliest theropods and thus closes the gap between the later pneumatic theropods and the sauropods and pterosaurs. In other words while there is still no perfect continuum of pneumatic taxa between the groups, the gap has been shrunk with this new find.
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