A Derived Morphology of the Quadrate May Support a Previously Unrecognized Major Higher-Level Clade of Neoavian Birds

The avian quadrate plays a critical role in cranial kinesis, but few comparative studies exist of its morphological variation across higher-level taxa. The present paper surveys the occurrence of a markedly concave articular facet of the condylus medialis. It is detailed that this feature, for which the term trochlea lateralis is introduced, may represent an apomorphy of a higher-level clade that includes the Aequornithes (gaviiforms, procellariiforms, suliforms, pelecaniforms, and allies), Phaethontimorphae (tropicbirds, sunbittern, and kagu), Mirandornithes (flamingos and grebes), and Gruiformes (cranes and allies). Like many other morphological characters, the occurrence of the trochlea lateralis shows homoplasy. However, at least one analysis of sequence data found a clade including the aforementioned four taxa, the interrelationships of which are not conclusively resolved in other studies. A trochlea lateralis is present in birds with different cranial morphologies and feeding adaptations, so that its occurrence often seems to have a phylogenetic (shared common ancestry) rather than a functional origin. The morphology of the condylus medialis of the quadrate may also bear on the affinities of some fossil taxa, such as the early Eocene Halcyornithidae and Messelasturidae, in which a trochlea lateralis is present.