By Heiloi Yip
Out of all the anatomical features unique to vertebrate animals, the skull is arguably one of the most significant traits. As a skeletal structure that not only protects the brain, it also comprises the structure of the jaw and contains the sensory organs, making the skull a very valuable trait for any vertebrates to possess. Among modern tetrapods, we see a diverse range of skull shapes adapted to the various lifestyles of each species. While natural selection can explain the evolution of skulls to adapt, it answers only one half of the question of how such diversity could arise in the first place. The other half may be answered by looking at the composition of the skull and understanding the mechanisms that allow the skull to transform itself.
The evolution of the skull can be traced back to the Ordovician period by analysing the fossils of jawless fishes. These fossils were the ancestors of jawed vertebrates that featured tough dermal plates on the dorsal and ventral side of the head, which were for protection against predators. Fossils from later periods would feature dermal plates that were subdivided into smaller fragments, including a subset of segments that would become parts of the hinged jaw. It is hypothesized that at some point in the evolution of vertebrates, the dermal plates became internalised and incorporated into the skeletal system, giving rise to the skull as seen in modern vertebrates (Kaucka & Adameyko, 2019). The spaces bordering the smaller fragments of bone were retained in modern skulls, better known as sutures. These sutures act as sites of bone growth, where stem cells differentiate and cell signalling pathways coordinate the growth of the skull.
Understanding how the skull forms during embryogenesis will help explain how its shape can be modified drastically. Skull development begins when cranial neural crest cells (CNCC) migrate to the anterior end of the embryo. Over time, different migrating clusters of CNCCs differentiate into a variety of tissues, including skeletal tissues that would give rise to the skull. The different skeletal elements grow and expand until they interact with each other, where the sutures are established to border the bones (White, Goswami & Tucker, 2021).
The sutures, by separating the various bones that comprise the skull, grants a modular flexibility to the skull shape, allowing changes to be made (almost) independently to each individual bone. This makes it vastly easier to modify the shape of the skull to a greater range of shapes, thus enabling vertebrate skulls to diversify to such a great degree. Mainly, the modularity of the skull affects its own evolution via a phenomenon called mosaic evolution. Due to the increased independence between the bones of the skull, different parts can evolve in divergent ways. For example, one bone may evolve much quicker than the rest of the skull, or that one bone is subjected to different selection pressures than another bone. One example of how mosaic evolution acts can be explained via heterochrony, the phenomenon where different parts of the body develop at different rates during embryogenesis. In this case, changing the timings at which one or multiple bones develop relative to the rest of the skull can have drastic effects on the resultant skull shape in the adult organism (Morris & Abzhanov, 2021).
It should be clear by now that the sutures are one of the reasons why the skull has evolved into so many shapes and sizes over evolutionary time. With a wide array of possible forms that the skull could evolve into, multiple vertebrate species exploited this feature in different ways to occupy a variety of ecological niches. Many examples of adaptive radiation can be exhibited to changes in the skull, with one of the most famous examples being the Galapagos-exclusive Darwin’s finches. This group of birds contained species with varied beak shapes that were specialised for different diets, yet they were all descended from a common ancestor with a single beak shape. By modifying the timings at which the beak bones developed via heterochrony, this led to an adaptive radiation of beak shapes from the finches’ ancestors (Navalón et al., 2020). This diversifying phenomenon is not exclusive to birds, as another example can be seen in the skulls of Phyllostomid bats. Different species within this group use different sensory modes to navigate their surroundings, which in turn demand different skull morphologies in order to house the suitable sensory organs. Mosaic evolution acts on the skull of the bats by imposing different selection pressure on each bone that corresponds with the various sensory organs, once again resulting in divergent skull shapes (Arbour, Curtis & Santana, 2021). Many more instances of adaptive radiation of the skull can be found in other groups of birds and mammals, as well as among reptiles, amphibians, and fishes. By studying and comparing how the sutures develop between different species, more can be inferred of the natural history of the vertebrate skull, demonstrating the importance of the relationships between evolution and development.
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Kaucka, M. & Adameyko, I. (2019). Evolution and development of the cartilaginous skull: From a lancelet towards a human face. Seminars in Cell & Developmental Biology. 91, 2–12. Available from: doi:10.1016/j.semcdb.2017.12.007
Morris, Z. S. & Abzhanov, A. (2021). Chapter Eight – Heading for higher ground: Developmental origins and evolutionary diversification of the amniote face, in Gilbert, S. F. (ed.) Current Topics in Developmental Biology. Academic Press (Evolutionary Developmental Biology). 241–277. Available from: doi:10.1016/bs.ctdb.2020.12.003
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White, H. E., Goswami, A. & Tucker, A. S. (2021). The Intertwined Evolution and Development of Sutures and Cranial Morphology. Frontiers in Cell and Developmental Biology. 9. Available from: doi:10.3389/fcell.2021.653579