VARIATION IN THE OS PALATINUM AND ITS STRUCTURAL RELATION TO THE PALATUM OSSEUM OF BIRDS (AVES)

Abstract

Morphological variation in the avian palatal skeleton (palatum osseum) was publicized by Huxley (1867) and expanded upon subsequently by many authors, but variation in the included nominal element, the palatinum (os palatinum), is less well known. Here we describe, illustrate, and name variable features of the avian palatinum that we judge to be potentially important for phylogenetic inference. To standardize anatomical nomenclature, we introduce formal names (per ICAAN) and revise others for palatal features provisionally considered to be homologous among modern birds (Neornithes). The palatinum can be divided topographically into four parts—pars maxillaris, pars choanalis, pars lateralis, and pars pterygoideus—each having variation in organization, function, and phylogenetic corollaries that facilitate comparisons within Neornithes and with Mesozoic allies. Emphasis on study of juvenile specimens of modern birds in which intact sutures are discernible revealed details in the structure of arcus pterygopalatinus and arcus pterygovomeralis that facilitated diagnosis of both palaeognathous and neognathous birds, and (within the latter) the Galloanserae and its sister-group, Neoaves. Articulatio vomeroparasphenoidalis may represent a synapomorphy of palaeognathous birds (ratites and tinamous). Although articulatio intrapterygoidea commonly is thought to provide the flexibility within arcus ptery-gopalatinus required for cranial kinesis in neognathous birds, we found that six patterns of articulations and flexible zones (zonae flexoriae) serve that function; these patterns are probably apomorphous, sometimes convergently, within Aves. Hesperornis apparently displays homoplasious flexibility within arcus pterygopalatinus. We found developmental evidence against paedomorphosis (“neoteny”) as an explanation of certain distinguishing features of palaeognathous palates, and comparison with palatal remnants of Archaeopteryx, Gobipteryx, Hesperornis, and several nonavian theropods suggest plesiomorphy of other palaeognathous features. Reconsideration of “palaeognathous” characters found in adults of some neognathous taxa, previously explained as evolutionary reversals, led us to a variety of alternative hypotheses. We believe that analysis of individual characters within the avian palatinum and palatal skeleton holds significantly greater promise for phylogenetic and functional inferences than the continued use of the classical, composite palatal types—palaeognathous, schizognathous, desmognathous, and aegithognathous—for phylogenetic reconstructions.