Henri Rönkkö wrote-
All right. So what is the earliest known bird having a kinetic skull called?
Well, Protopteryx probably had a kinetic skull, as it lost the postorbital-orbital contact and probably the quadratojugal-squamosal contact. However, the presence of a craniofacial hinge cannot be ascertained from the literature and an ectopterygoid was probably still present, limiting movement compared to modern birds. Enantiornithines certainly lost postorbital-orbital and quadratojugal-squamosal contact and I think had craniofacial hinges, although they still had ectopterygoids, so their kinesis was probably not as well developed as neornithines either. Hesperornithids are the most basal birds to be described with a prokinetic skull. Perhaps earlier euornithines like Patagopteryx or Chaoyangia had prokinesis too, but this is undescribed at present.
Further discoveries may complicate the picture however. The alvarezsaurid Shuvuuia has a prokinetic skull and Archaeopteryx was recently reported (Elzanowski, 2001) to have a form of cranial kinesis different from any living bird. I'm unsure of the condition in more basal theropods, although I'm fairly certain no other non-avian theropods have been found with prokinetic skulls.