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!