Saturday, November 10, 2007

Yes, I know, non-avian maniraptors had uncinates, too.

There's a new paper out at the Royal Society, finally detailing one of the more ignored aspects of maniraptoran anatomy: the uncinate processes. Codd, et al. for perhaps the first time as far as I can remember, actually discusses the role and phylogenetic importance of these structures, which must have been quite widespread among the Maniraptora. The role of these rib protrusions has never been entirely clear, but Codd, et al. suspect that they have something to do with respiration. I had always learned that the uncinates strengthened the boxy structure of a bird's torso. While this is almost certainly one part of the equasion, it would seem that uncinates serve several purposes. Of minor importance is that, at least in Sphenodon punctatus (tuatara), the uncinates connect to the gastralia via external oblique muscles. Non-avian maniraptoran dinosaurs (and even some avian ones) seem to have had well-developed gastralia, so that function may have been retained.
But more importantly, in modern birds, the uncinates act as levers that, together with muscle action and sternal ribs, actually raise and lower the sternum during respiration. The "pump" action of the sternum has long been known as a major factor in avian breathing, but it's surprising to see the uncinates actually facilitating this movement. But that's not even the focus of the paper. Rather, Codd et al. merely strive to understand the phylogenetic consequences of uncinates in the Theropoda, and what that might mean for theropod activity levels.
The paper is actually disappointingly short, especially when you realize that it could've actually been shorter. It turns out that uncinate processes have a fuzzy preservation record. This should not surprise us, because they are attached to the ribs with cartilage, and the processes themselves were thin, strut-like bones. At any rate, they are only known with certainty in Oviraptor, Citipati, Khaan, Velociraptor, Deinonychus, and Microraptor. Thanks to phylogenetic bracketing, we can pretty safely conclude that the common ancestor, at least, of Oviraptor and Deinonychus also had uncinates. So that means we can expect to find more complete remains of, say, Nothronychus with uncinates, and troodontids, too.
In modern birds, the sternum's keel provides a key muscle attachement site for respiration, but Codd et al. suggest that, because non-avian maniraptors did not have keels, they may have retained their gastralia for essentially the same purpose. Short paper, but these are things that needed to be said!

1 comment:

goognik said...

Yes - a good paper in terms of functionality, as was the earlier paper on UPs Codd did with other collaborators.

I'd always assumed their major function was for adding leverage to inspirational rib movements, and this is largely confirmed, though they have a number of different muscle attachments and different uses.

They are going to prove one of the most useful phylogenetic aids, since they are about 99.9% present in modern birds and really do seem to be completely absent from genuine enant birds. Finding the reliable phylogenetic markers (and indeed identifying the reliability of all characters) is essential for intelligent phylogenesis, and UPs offer a staggering degree of reliability.

I think we've now got enough good troodonts to say they didn't have them; I'd guess therizino's lacked them too. I don't know how many years/decades I'm going to have to keep repeating that confusiuornithids don't have them.

Alarmingly, Manning seems to have made a suggestion that just might be worth considering - the close-up of the UV pic of the head of Archaeopteryx (fig 2B) in:

"A Well-Preserved Archaeopteryx
Specimen with Theropod Features "
Gerald Mayr, Burkhard Pohl, Stefan Peters
SCIENCE VOL 310 2 DECEMBER 2005 1483

...does look like a possible UP, and although I'd tend to suspect it wasn't, it wouldn't be long before the descendants of late Archaeopteryx examples did evolve them.

UPs were useful in Sphenodon and crocs because they both have special reasons for strengthening inspirational movements.