Evolution, conservatism and overlooked homologies of the mammalian skull

Abstract

In the last decade, studies integrating palaeontology, embryology and experimental developmental biology have markedly altered our homological understanding of the mammalian skull. Indeed, new evidence suggests that we should revisit and restructure the conventional anatomical terminology applied to the components of the mammalian skull. Notably, these are classical problems that have remained unresolved since the ninteenth century. In this review, I offer perspectives on the overlooked problems associated with the homology, development, and conservatism of the mammalian skull, aiming to encourage future studies in these areas. I emphasise that ossification patterns, bone fusion, cranial sutures and taxon-specific neomorphic bones in the skull are virtually unexplored, and further studies would improve our homological understanding of the mammalian skull. Lastly, I highlight that overlooked bones may exist in the skull that are not yet known to science and suggest that further search is needed. This article is part of the theme issue ‘The mammalian skull: development, structure and function’.

Anatomical and Morphological Structure of the Skull of a Juvenile Specimen of Myotis myotis (Chiroptera: Vespertilionidae)

“…The bony elements of the skull are markedly reduced compared to other amniotes [26]. The skull of a contemporary adult mammal contains about 30 free elements [15,27].…”

“…For example, Koyabu [27] highlights that patterns of ossification, cranial sutures, and taxon-specific neomorphic bones of the skull have almost never been investigated, and that further research would provide insight on the structure of mammalian skulls. The authors of this paper share Koyabu's [27] opinion that the skull may contain bones that have not yet been confirmed or discovered by science, which suggests a need for further research, especially concerning the skulls of bats. This results from the high interspecies variation in the heterochrony of ossification in mammals [69][70][71].…”

“…To date, comparative anatomy has defined thousands of terms describing anatomical parts of vertebrates. However, handbooks often do not clearly define all the elements of the skull [27]. Consequently, it is worth demonstrating the perspectives for typically ignored problems related to the homology, development, and conservatism of the mammalian skull in order to encourage research in these areas.…”

The mammalian skull: development, structure and function

“…The broad overview by Koyabu [ 14 ] explores the osteological variability of the mammalian skull, highlighting ongoing challenges to understanding the homology, ossification patterns, bone fusion, morphology of the sutures and the phenomenon of the neomorphic bones, all from the evo-devo perspective. The contribution by Kaucka [ 15 ] is a survey of the craniofacial development regulation and plasticity, exploring genetic patterning during the development of the cranial diversity across mammalian species.…”