• SharkieSophie

The Evolution of Sharks

Updated: Apr 23

The "Chondrichthyans" are a "monophyletic group" with a common ancestor approximately 420 million years ago. This means that the ancestors of modern sharks were around before the dinosaurs! Today, it is thought there are over 700 species of “extant” Chondrichthyans, of which approximately 400 - 500 species are classified as sharks, and new species are still being discovered.



In taxonomy, sharks are termed the "Selachimorpha", which is a superorder within the subclass "Elasmobranchii", which is a branch of the class "Chondrichthyes". That is a lot of words you are going to struggle to remember, but don’t worry… all this means, is that all sharks, skates, rays and chimeras are related to each other more so than they are to other fish species.


The plough nosed chimaera (Image source: www.practicalfishkeeping.co.uk)

If you closely examine sharks and rays, looking past what may seem like wildly contrasting morphologies, you will notice some details which hint that they are cousins. All have 5 - 7 gills slits, rigid dorsal fins, a jaw not fused to the cranium and "dermal denticles" on the skin. The swim bladder is absent and buoyancy in the water is maintained via a liver rich in oils. However, as is clear if you compare a whale shark (Rhincodon typus) to an eagle ray (Myliobatis aquila), or a saw shark (Pristiophorus cirratus) to a manta ray (Mobula birostris), Elasmobranchs are remarkably diverse in terms of morphology.


So how did they diverge?


The Chondrichthyans first diverged from the “Osteichthyes” (bony fishes) approximately 420 million years ago. These lineages differ in the structure of their skeletons. At a glance bony fish and cartilaginous fish may seem similar in shape, but the skeletons of the Chondrichthyans are made up entirely of hardened cartilage, which never ossifies into bone, as is found in the bony fishes. A primitive shark, called “Elegestolepis” lived around 420 million years ago, but (as sharks’ cartilaginous skeletons break down and only the teeth fossilise) we cannot be sure how they might have looked.


Artist's impression of Cladoselache (Image source: www.wikipedia.org)

Around the same time the “Chimaeriformes” (chimaera aka ghost sharks) also diverged into a separate group. The chimaera have a cartilaginous skeleton, like sharks, but differ in jaw morphology and share some features with the bony fish. Shark-like animals, called Cladoselache, arose around 380 million years ago, but we cannot be certain whether these were truly ancestors of modern sharks or if they were part of the chimaera lineage.


Artist’s impression of Helicoprion (Image source: www.reddit.com)

However, by 360 million years ago, shark ancestors were definitely prevalent. Diversification of sharks exploded, after a mass extinction created gaps in the ecosystem which could be exploited by adapting organisms. This is sometimes referred to as ‘The Golden Age of Sharks’. It is during this period that we see spectacular diversity of form, such as the weird “Helicoprion” and the wonderful “Stethacanthus”.


Artist’s impression of Stethacanthus (Image source: www.bbc.co.uk)

Yet, whilst this extinction event benefited shark evolution, the next extinction event, around 250 million years ago, wiped out many shark species and down-scaled their diversity. Only a handful of lineages survived and it is from these groups that our modern sharks evolved. One of the lucky few, gave rise to “Hybodus”; which lived somewhere between 65 and 300 million years ago.



Artist's impression of Hybodus (Image source: www.dinopedia.fandom.com)

Approximately 195 million years ago, the order “Hexanchiformes” evolved. These are the ancestors of todays six- and seven-gill sharks, such as Hexanchus griseus. Sharks developed flexible, protruding jaws, which meant they could handle larger prey, even animals bigger than themselves! The speed with which they could swim also increased. By 100 million years ago, the large-bodied, speedy-form modern sharks had appeared.


In the last 65 million years, the ancestors which became the “Lamniformes” order arose. This included “Otodus obliquus”, which gave rise to the iconic “Megalodon”, which eventually evolved into our modern-day great white shark (Carcharodon carcharias). Megalodon evolved to become larger than Otodus, as a size-based arms race between predator and prey, pumped body forms to enormous sizes. Over millions of years, as prey evolved to be larger (and thus more difficult for predators to hunt), predators evolved to become larger, and vice versa. However, it is thought that Meagalodon and the modern day white shark actually coexisted as some point in deep history and that the great white had the advantage with its relatively small size. Changing climatic conditions and shifting prey abundances were unfavourable for such a gargantuan size, so white sharks prevailed where Megalodon became extinct.



However, some Lamniformes evolved along a different track. Also during the period of the last 65 million years, some lineages evolved away from predation and became filter-feeders, which gave rise to ancestors of the modern day basking shark (Cetorhinus maximus) and the megamouth sharks (Megachasma pelagios). Entirely separate lineages also adapted to become filter feeders (“convergent evolution”), which gave rise to modern “Orectolobidae”, such as the whale shark (Rhinocodon typus).


The most recently evolved group of sharks is the “Sphyrnidae” family, aka the hammerhead sharks, appeared as recently as 35 million years ago. These sharks are placed within the order “Carcharhiniformes”, which is the most diverse of modern shark orders; including over 270 different species of shark. The seven other orders of modern sharks Heterodontiformes, Hexanchiformes, Lamniformes, Orectolobiformes, Pristiophoriformes, Squaliformes and Squatiniformes, together make up the remaining 45% of all the species alive today.


If you enjoyed learning about the weird and wonderful forms of extinct sharks, you can check out The Good, the Bad and the Ugly.


References

Cole NJ & Currie PD (2007). Insights From Sharks: Evolutionary and Developmental Models of Fin Development. Developmental Dynamics 236, 2421–2431. Access online.


Martin AP & Naylor GJP (2010). Independent Origins of Filter-Feeding in Megamouth and Basking Sharks (Order Lamniformes) Inferred from Phylogenetic Analysis of Cytochrome b Gene Sequence, In: Yano K, Morrissey JF, Yabumoto Y & Nakaya K (Eds). Biology of the Megamouth Shark. Tokai University Press, Tokyo, Japan. Access online.


Wourms JP. & Demski L.S. (1993). The reproduction and development of sharks, skates, rays and ratfishes: introduction, history, overview, and future prospects. Environmental Biology of Fishes, 38, 7-21. Access online.


By Sophie A. Maycock for SharkSpeak.

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