Stidham (1998) described the above fossil, a fragment of fused dentaries, from a find in the Hell Creek Formation of Wyoming, USA, as a potential morphologically modern psittaciform mandible. This jaw is remarkably parrot-like:
Stidham was relatively careful in his description, positively comparing the material to lorries, which unlike the parrot above, have a much lower, longer jaw. But Gareth Dyke and Gerald Mayr (1999) disagreed, arguing that morphologically primitive psittaciforms from the Eocene and Miocene were far more primitive-looking than the UCMP mandible, presuming that the morphology was convergent. Instead, they suggested it might be similar to caenagnathid oviraptorosaur theropods.
If Stidham (1998) is right, and he merely suggested similarities (including a series of vascular channels similar to lories), the jaw represents a possible psittaciform convergent on modern forms predating the appearance of modern-jaws parrots and lories by over 50ma. Convergence is likely even when Dyke and Mayr’s (1999) comments are considered, and as such, finding other morphological features to affirm the identity of this tiny jaw would be very useful.
A strong comparison to caenagnathids is inevitable, as to any oviraptorosaur (I am surprised the authors did not directly compare to the shorter, broader jaws of oviraptorids, but this would be problematic anyway, as no oviraptorid jaw is fused at the dentary symphysis; this feature only occurs in caenagnathid mandibles and in the jaw of the Uzbek Caenagnathasia martinsoni (Currie =, Godfrey and Nessov, 1994)).
I have my general doubts about a caenagnathid identity, largely based on the degree of gracility in the UCMP specimen, and that nearly all caenagnathid mandibles show detailed ridges and nodules ornamenting the surface. However, avian affinities cannot easily be ruled out, especially if the jaw is from a derived avialan such as a confuciusornithid- or Apsaravis-like bird, both of which are edentulous. If the jaw is nonavian, it is also unlikely to be a nondianosaurian reptile, as the morphology of crocodilians with edentulous regions of their jaws are far different, especially in the absence of fusion between both dentaries, and the extension of the distinct splenial into the intermandibular suture. As these are not apparent, the jaw is probably from a theropod (avian or not).
The entertaining premise, though, and one that may be far likelier than Stidham’s in some respects, is that this does, in fact, represent a caenagnathid or at least an oviraptorosaur or ornithomimosaur. The jaw fragment is around 15mm long, and is thus particularly tiny: if it were a caenagnathid mandible, it was from a very, very small animal — the complete jaw would be close to around 3.5 cm in length, and imply an animal around .4m, barely 14in, long. Intriguing jaw, nonetheless.
Currie, P. J., Godfrey, S. J. & Nessov, L. A. 1994. New caenagnathid (Dinosauria, Theropoda) specimens from the Upper Cretaceous of North America and Asia. Canadian Journal of Earth Sciences — Revue canadienne de Sciences de la terre 30(10-11):2255-2272.
Dyke, G.J. & Mayr, G. 1999. Did parrots exist in the Cretaceous period?. Nature 399:317-318.
Stidham T. 1998. A lower jaw from a Cretaceous parrot. Nature 396:29-30.