Ossification sequence of the common tern (Sterna hirundo) and its implications for the interrelationships of the Lari (Aves, Charadriiformes)

The hypotheses of relationships within Lari (gulls) are highly unstable, and depend on whether morphological or molecular data are examined. Developmental sequence data are thought to contain phylogenetic information, but have never been applied to the problem of avian systematics. In this article, we describe the ossification sequence of the Common Tern (Sterna hirundo), and compare the pattern observed to published descriptions of other Charadriiformes, specifically the Great Skua (Stercorariidae) and various species of gulls (Laridae). We use changes in ossification sequence to elucidate the relationship between these three taxa, using both qualitative and systematic approaches. The first analysis of the ossification sequence data does not support a close relationship between Stercorariidae and Laridae, as has been proposed in some morphological analyses; however it was unable to differentiate between a Laridae-Sternidae sister-group relationship or a Sternidae-Stercorariidae sister-group relationship. The second analysis was unable to differentiate between any topology, including a polytomy, for these taxa. These results highlight the potential for use of ossification sequence data in an evolutionary context but caution that analyses are highly dependent on sequence resolution and the taxonomic level of the data set.