Development of the ethmoid in a wallaby and implications for the homology of turbinal elements in marsupials

The homologies of the turbinals (scroll bones) of the ethmoid are not well understood, including the potential implication for understanding mammalian phylogeny. Here we examine the postnatal development of this anatomical system in a marsupial mammal because previous work has shown that the adult pattern of five endoturbinals (ethmoturbinals) and two ectoturbinals (frontoturbinals) is conserved. Furthermore, marsupial phylogeny is fairly well resolved and provides a solid evolutionary framework for examining turbinal homologies. In this study, we documented the development of the ethmoid of the tammar wallaby, Notamacropus eugenii, using histology and computed tomography imagery of a growth series of pouch young. The pattern of development of the turbinal elements in the wallaby was compared to that in didelphids, as described in previous work. We found that four ethmoturbinals initially develop, followed later in development by an interturbinal; these five elements then develop into the bony endoturbinals found in adults. These data support the idea that endoturbinal III, derived from an interturbinal, has a distinctive development pattern from the other endoturbinals. This is consistent with what is seen in the didelphid marsupials, Caluromys philander and Monodelphis domestica, suggesting this is a common developmental pattern for marsupials. This article is part of the theme issue 'The mammalian skull: development, structure and function'.