Modularity and Complete Natural Homeoses in Cervical Vertebrae of Extant and Extinct Penguins (Aves: Sphenisciformes)

The cervical system of extant penguins (Aves: Sphenisciformes) is organised into morphological modules, each with its biomechanical function. Indeed, for these marine pelagic birds to acquire hydrodynamic morphology, the folding of the neck is essential. Despite a common general structure, the cervical vertebrae exhibit morphological differences depending on their positioning. These characteristics are identified as apparent cases of complete natural homeotic transformations—therefore, the composition of some modules varies. Two types of complete cervical homeoses are identified between species, but the second type can also occur within some species when the post hatching development is considered. The fossil material analysed here makes it apparent that the two modular configurations characterising the anterior part of the neck—a consequence of the first homeosis—existed 36 My and 25 My ago, for one, and circa 10 My ago, for the other. These comparisons also reveal a clear differentiation in vertebral features between the fossil species of the Oligocene-Miocene ages and the more recent and extant penguins. Ultimately, these observations make the proposal of a hypothesis in relation to the ontogenetic influence of Hox genes, and their regulators, based on the changes observed in the cervical segment of Sphenisciformes.