The sharks are a successful superorder of widespread cartilaginous fish from the Pridoli-Holocene of the entire world. While they did not get a proper name until the 16th century, sharks have captivated humanity for most of its existence, as many civilizations were fearful of sharks even before the 1975 horror movie Jaws released[1]. As well as this, sharks owe part of their success to their sleek body, which is torpedo-shaped in most species; this is a body plan that many other open-water predators - such as tuna and dolphins - have evolved. In fact, many other cartilaginous fish genera, such as Cladoselache, Stethacanthus, and Helicoprion have a very similar body plan, and are often mistaken for sharks because of this.
Physiology[]
The main trait that all sharks share in common is a skeleton made of cartilage rather than one made of bone; due to this, sharks almost never leave complete body fossils upon death, and teeth are the most common shark fossils one can find. Along with this, unlike bony fish, sharks have pectoral fins separate from their head, with 5-7 gill slits sitting between them on each side of the body. Their bodies are covered in small, pointed, tooth-like scales called dermal denticles.
Evolutionary history[]
Sharks likely split off from bony fish around the end of the Silurian, with their last common ancestor having been a placoderm fish; due to the discovery of endochondral bone in a placoderm fossil, it is believed that sharks were ancestrally bony, but lost their bony skeletal structure in favor of one made of cartilage.[2]
Although sharks have existed for millions of years beforehand, modern sharks did not evolve until the Early Jurassic. As a way to adapt to new environmental conditions, sharks evolved new modes of reproduction, such as oviparity and viviparity. While the latter is more common in modern sharks, the former is more reminiscent of Paleozoic sharks, and helps increase fecundity and evolutionary fitness as well as decreasing risk of extinction; evidence for this comes in the form of high rates of extinction for lamniform (mackerel shark) and squaliform sharks, both of which are viviparous and had evolved the most recently. Due to this as well as short time ranges for each species, it can be assumed that modern sharks in the Early Jurassic were smaller in size as well as oviparous, meaning they had higher substitution and mutation rates and thus underwent modifications faster; the rising sea levels and cool, humid climate allowed them to undergo an adaptive radiation before going into a diversity stasis during the Middle-Late Jurassic due to increased extinction rate.[3]
References[]
- Marx, Robert F. (1990). The History of Underwater Exploration. Courier Dover Publications. p. 3. ISBN 978-0-486-26487-5.
- Brazeau, M. D., Giles, S., Dearden, R. P., Jerve, A., Ariunchimeg, Y., Zorig, E., Sansom, R., Guillerme, T., & Castiello, M. (2020). Endochondral bone in an Early Devonian ‘placoderm’ from Mongolia. Nature Ecology and Evolution, 4(11), 1477–1484. https://doi.org/10.1038/s41559-020-01290-2
- Kriwet, J., Kiessling, W., & Klug, S. (2012). Diversification trajectories and evolutionary life-history traits in early sharks and batoids. Proceedings of the Royal Society B: Biological Sciences, 276(1658), 945–951. https://doi.org/10.1098/rspb.2008.1441
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