By Heiloi Yip
There is no doubt that the evolution of rigid bones and endoskeleton were critical to the evolution of humans and other animals, tracing all the way back to the first time the tetrapods emerged onto land. It has been traditionally believed that calcified bones were a unique trait to the Osteichthyes, a clade of vertebrates including the ray-finned fishes and the tetrapods. However, recent evidence from an Imperial-led paleontology team provides a different theory on the evolutionary history of bones, showing that it might have arisen earlier than initially thought.
The first clue to the evolutionary history of bones lied in how it was formed in modern organisms. Endochondral ossification is the process in which a cartilaginous structure in the body is converted into calcified bone, and this process is exhibited in all vertebrates with bony skeletons, including humans. The skeletal elements initially begin as cartilaginous structures, where the cartilage acts as a template for the future formation of bones. As the skeleton matures, a gradual calcification of the cartilage occurs over time. At a certain stage, there is dying back of cartilage tissues, forming an endochondrial cavity within the cartilaginous structure. The final step in bone formation is represented by the replacement of the endochondrial space with blood vessels and bone-related cells, leading to the eventual development of a bone matrix. This process, which has been extensively studied in humans, illustrates the close relationship between cartilage and bone, and applies to almost every vertebrate (Paxton, Knibbs & Peckham, 2020).
For many years, the evolution of bones was thought to be a simple process of cartilaginous skeletons evolving the more complex process of endochondral ossification. Among all extant vertebrates, calcified bones are unique to the Osteichthyes. Evidence for this can be found by analysing more distantly related groups of vertebrates, notably the Chondricthyes (a clade including sharks and rays) and the jawless fishes. These two clades lack calcified bone, instead possessing only a cartilaginous skeleton. Thus, a hypothesis was formed that calcified bones were a unique trait to Osteichthyes (Wagner & Aspenberg, 2011). There is also fossil evidence to support this claim. Such fossils mainly belong to a group of basal jawed fish known as placoderms, which were believed to be distantly related ancestors of Chondricthyes and Osteichthyes. Although a defining trait of placoderms were the bony plates on their faces, it is believed they were formed through a different process from endochondral ossification. While the bone plates of placoderms tend to be well-preserved, most fossils have very little traces of internal skeleton being preserved. An interpretation of this phenomenon is that since cartilage is worse at being preserved than calcified bone, the placoderms were hypothesized to have had a skeleton composed mainly of cartilage (Giles, Rücklin & Donoghue, 2013). As such, it was widely believed that cartilage evolved first, followed by the evolution of calcified bones during the divergence of Osteichthyes from the basal vertebrates (Wagner & Aspenberg, 2011).
However, this long-standing theory has since been challenged, due to a recent discovery of a placoderm fossil by leading researcher Dr. Martin Brazeau from Imperial College London. The fossil was that of a placoderm-like fish, Minjinia turgenensis, identified to be a basal ancestor to both Chondrichthyes and Osteichthyes. The fossil’s morphology contained the remarkably well-preserved skull-roof and braincase. Analysis of the braincase via X-ray computed microtomography (micro-CT) revealed bone-like features within the endochondral space of the braincase. Dr Brazeau remarked on this finding, stating that it shows “clear evidence of bony inner skeleton in a cousin of both sharks and, ultimately, us”. This finding implied that Minjinia had evolved some bone-like traits, despite being distantly related to the Osteichthyes. Two interpretations could be drawn from this: either Minjinia evolved calcified bones independently of the Osteichthyes, or the Chondrichthyes have secondarily lost calcified bones (Brazeau et al., 2020; Dunning, 2020). An earlier discovery of another ancient fossil supports the latter theory, this time of a basal shark (Gogoselachus lynbeazleyae) in western Australia. The well-preserved cartilage skeleton was analysed via micro-CT and electron microscopy, where paleontologists found evidence of bone cells within the cartilage. This could be a hint that the ancestors of sharks had a skeleton composed of structures that more closely resembled calcified bones than modern cartilaginous skeletal elements, indicating that Chondricthyes members gradually lost the ability to form calcified bones (Long et al., 2015). These two pieces of evidence suggest that endochondral ossification has emerged much earlier over the evolutionary history of vertebrates than previously thought.
These recent discoveries have shed more light onto the history of calcified bones and its role in the evolution of the vertebrates. It is very often in paleontology for pre-existing beliefs to become outdated upon the excavation of a novel fossil. The whole picture can only be uncovered gradually by the discovery of more fossils. Countless fossils still remain underground and yet to be excavated, perhaps with some being preserved with potentially groundbreaking qualities.
References:
Brazeau, M., Giles, S., Dearden, R., Jerve, A., Ariunchimeg, Y., Zorig, E., Sansom, R., Guillerme, T. & Castiello, M. (2020). Endochondral bone in an Early Devonian ‘placoderm’ from Mongolia. Nature Ecology & Evolution. 4(11), 1477-1484. Available from: doi: 10.1038/s41559-020-01290-2
Dunning, H. (2020). Ancient Bony Fish Forces Rethink Of How Sharks Evolved | Imperial News | Imperial College London. Available at: https://www.imperial.ac.uk/news/203413/ancient-bony-fish-forces-rethink-sharks/ [Accessed 7 December 2020]
Giles, S., Rücklin, M. & Donoghue, P. C. (2013). Histology of “placoderm” dermal skeletons: Implications for the nature of the ancestral gnathostome. Journal of morphology. 274(6), 627–644. Available from: doi:10.1002/jmor.20119
Long, J., Burrow, C., Ginter, M., Maisey, J., Trinajstic, K., Coates, M., Young, G. & Senden, T. (2015). First Shark from the Late Devonian (Frasnian) Gogo Formation, Western Australia Sheds New Light on the Development of Tessellated Calcified Cartilage. PLOS ONE. 10(5), 0126066. Available from: doi:10.1371/journal.pone.0126066
Paxton, S., Knibbs, A. & Peckham, M. (2020). The Leeds Histology Guide. Available at: https://www.histology.leeds.ac.uk/bone/bone_ossify.php [Accessed 7 December 2020]
Wagner, D. & Aspenberg, P. (2011). Where did bone come from? Acta Orthopaedica. 82(4), 393-398. Available from: doi: 10.3109/17453674.2011.588861.
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