Coming in all different shapes, sizes and colours, living in different habitats, in every single ocean in the world, with different reproductive methods, different diets and different behaviours, sharks and their relatives are incredibly diverse. Yet, despite all these variations, the "Chondrichthyans" (including all sharks, skates and rays) are actually a "monophyletic group". This means that they all share a common ancestor that lived some 440 million years ago... This means that the ancestors of modern sharks were around before the dinosaurs! Today, it is thought there are over 1200 species of living Chondrichthyans, of which over 530 species are classified as sharks... and new species are still being discovered all the time! So how did sharks evolve? And what did some of their ancestors look like?
Sharks and Skates and Rays, Oh My!
In taxonomy, sharks skates and rays are referred to as "Elasmobranchs" within the class of cartilaginous fishes ("chondrichthyans"). That was a lot of really complicated scientific words that, at best are going to struggle to remember and at worst, you couldn't care less about. But don’t worry… all this means, is that all sharks, skates and rays are related to each other more so than they are to other fish species, like herrings or tuna (Abel et al, 2020, Ebert et al, 2021).
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 (Abel et al, 2020, Ebert et al, 2021).
However, it becomes clear when 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), that Elasmobranchs are remarkably diverse in terms of morphology (Abel et al, 2020, Ebert et al, 2021). So how did they diverge?
Sharks are Different to Bony Fishes
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 (Abel et al, 2020, Ebert et al, 2021). To learn more you can head over to Funny Bones.
Around the same time the Chimaeriformes (chimaera or 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 (Abel et al, 2020, Ebert et al, 2021).
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 (Abel et al, 2020, Ebert et al, 2021). To learn more, you can check out The Good, the Bad and the Ugly.
The Original Sharks
Sharks' cartilaginous skeletons degrade so quickly after death, that they rarely fossilise. So we have only have very patchy examples of ancient sharks in the fossil record. This means we cannot be absolutely certain when the first true shark arose (Abel et al, 2020, Ebert et al, 2021).
But we do know that 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 (Abel et al, 2020, Ebert et al, 2021).
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 (Abel et al, 2020, Ebert et al, 2021).
To learn more about the five mass extinction evens that sharks lived through, check out I Will Survive!
The Rise of Modern Sharks
Approximately 195 million years ago, the order Hexanchiformes evolved. These are the ancestors of todays six- and seven-gill sharks. In fact, some of the species in this group - such as the frilled shark (Chlamydoselachus anguineus) - have been around for so long that they are known as "living fossils" (Abel et al, 2020, Ebert et al, 2021).
During this period, 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 (Abel et al, 2020, Ebert et al, 2021).
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. 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 (Abel et al, 2020, Ebert et al, 2021).
It is thought that Megalodon and the modern day white shark (Carcharodon carcharias) actually coexisted as some point in deep history and that the great white actually 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 (Abel et al, 2020, Ebert et al, 2021).
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 eight other orders of modern sharks Echinirhiniformes, Heterodontiformes, Hexanchiformes, Lamniformes, Orectolobiformes, Pristiophoriformes, Squaliformes and Squatiniformes, together make up the remaining 45% of all the species alive today. To learn more you can check out The Magnificent 8* (Abel et al, 2020, Ebert et al, 2021).
Both today and in the past, sharks and their relatives have boasted remarkable diversity. Who knows what sharks might look like over the next 100 million years.
To learn more about extinct sharks, you can check out The Good, the Bad and the Ugly.
References
Abel CD, Grubbs RD, Pullen E & Dando M (2020). Shark Biology and Conservation: Essentials for Educators, Students and Enthusiasts. Johns Hopkins University Press, ISBN: 9781421438368.
Cole NJ & Currie PD (2007). Insights from sharks: Evolutionary and developmental models of fin development. Developmental Dynamics 236, 2421–2431. Access online.
Ebert DA, Dando M & Fowler S (2021). Sharks of the World: A Complete Guide. Wild Nature Press, ISBN: 9780957394605
Wourms JP & Demski LS (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
