A survey of the uncinate bone and other poorly known ossicles associated with the lacrimal/ectethmoid complex of the avian skull.

In several taxa of Neornithes (crown group birds), the lacrimal/ectethmoid complex exhibits small bones, the comparative osteology of which is poorly studied. Some of these ossicles-which are commonly known as uncinate bones (ossa uncinata or ossa lacrimopalatina)-were already described two centuries ago, but knowledge of their distribution and morphological variability in higher level clades is incomplete. In the present study, a detailed survey of the occurrence of uncinate bones is given, and these ossicles are for the first time reported in the gruiform Psophiidae, some Rallidae, and in the Otidiformes. Their presence in the latter taxon is of particular interest, because in current molecular analyses, the Otidiformes result as close relatives of the Musophagiformes, in which the uncinate bone is particularly large. The uncinate bones of most other neornithine clades, however, appear to have evolved multiple times independently through parallel evolution from the same ligamentous structures. A few earlier authors assumed that the uncinate bone is homologous to the ectopterygoid of non-avian theropods. Although this remains a viable hypothesis, more data on the occurrence of the ectopterygoid in Mesozoic birds are needed for well-supported conclusions. Here, it is noted that the ontogenetic development of the uncinate bone appears to be correlated with that of the ectethmoid, which is another bone in the skull of neornithine birds that is of unknown origin.