Over the last three and a half billion years, evolution has given rise to a myriad of different life forms on our planet... All originating from some simple, single-celled organism living in the oceans, evolution has created everything from corals to dinosaurs, from flowering plants to elephants... viruses, emus, whales, bacteria and giraffes. Whilst this process is beautiful, it is also incredibly violent, with "survival of the fittest" continuously wheedling out those too weak to survive and massive extinction events periodically wiping out nearly all life on our planet. Some species only exist for a very short period of time. Others are so successful that they survive for millions of years, and continue to adapt and change into new species... like the sharks... which have existed in some form for at least 450 million years! Believe it or not, that means sharks were around before trees!
The OG Sharks
During the earliest periods of evolution there was a lot of action in the oceans! It is thought that life first began on our planet in the deep ocean around 3.7 billion years ago. These single-celled organisms, subsequently adapted and evolved into different species of microbes. It is thought that the first plant-like organisms evolved in the sea around 3000 million years ago, when ancient cyanobacteria became able to photosynthesise (meaning they gain their energy from the sun). Eventually life advanced to be multi-cellular and became more complex. Then animals appeared: sponges arose, then worms, then jellyfish, and eventually early fish around 530 million years ago. It wasn't until about 470 million years ago that a type of green algae actually made it out of the oceans and the first simple plants began to adapt to life on land. This simple plant then advanced into mosses and then ferns.
Fossils have shown us that the ancient ancestors of sharks first arose around 450 million years ago, during the Late Ordovician period. Yet, it wasn't until around 350 million years ago that the first trees arose! That is a full 100 million years later than the earliest sharks! So sharks truly are older than trees! Mind-blowing! (Abel et al, 2020; Ebert et al, 2020)
Whatsmore, the early sharks survived multiple major extinction events, including that which killed the dinosaurs at the end of the Cretaceous period 65 million years ago (yes, your maths is correct - sharks were around before dinosaurs too!). They were pretty tough survivors! (Abel et al, 2020; Ebert et al, 2020)
Scientists have been able to learn so much about ancient sharks through fossils... We know of several wonderfully weird species, such as Helicoprion (with a whirling jaw that looks like an ammonite) and Stethacanthus (with an anvil-shaped dorsal fin). However, the fossil record is very patchy. This is because the skeletons of sharks are made from cartilage, which degrades much more rapidly than bone after death, and rarely leaves fossils. This means we have only a few informative specimens that show us how their the bodies looked. We usually only have the teeth to go by, as their hard, enamel coating, means they do fossilise well (Abel et al, 2020; Ebert et al, 2020). To learn more about these strange, ancient sharks you can check out The good, The bad and The Ugly.
Taking Flight
There is surely a huge amount we do not yet know about shark evolution... Which means there are still amazing discoveries to be made... One research team working in Mexico recently made one such discovery... A fossil, which had laid dormant in rock for some 93 million years, showed us that ancient sharks were even more diverse than we previously thought (Vullo et al, 2021).
The researchers described a shark which lived during the Late Cretaceous period, alongside the dinosaurs. It had a broad mouth, similar to many ray species, which was probably used for filter-feeding. It also had a large tail fin, much like a modern shark, which they suggested was used for forward propulsion. The scientists named the new species the eagle shark (Aquilolamna milarcae) for it's incredible elongated wing-like pectoral fins, which could span a whopping 1.9 metres across! Artistic reconstructions of how this shark might have looked, have shown us that millions of years ago, there were sharks that 'flew' through the water, using their fins for propulsion and manoeuvrability, much like modern-day manta rays do (Vullo et al, 2021).
This is remarkable find because it tells us that 'soaring sharks' evolved much earlier than we thought - as many as 30 million years before the ancestors of todays manta and devil rays (Family Mobulidae) evolved the same body plan. As this shark is thought to be more closely related to modern Lamniforms, like the great white (Carcharodon carcharias), as opposed to rays, this means that 'wings' for underwater flight did not just evolve once, but multiple times over the course of evolutionary history (Ebert et al, 2020; Vullo et al, 2021).
No Filter
Whilst there are over 540 different species of sharks alive today (and new ones are discovered all the time), they are all actually relatively similar; mostly with a long, streamlined body, with a large tail for swimming, paired pectoral fins and teeth at the front for catching prey. This discovery tells us that evolution did explore very different body plans amongst the sharks of old, but that these species went extinct (Ebert et al, 2020; Vullo et al, 2021).
The researchers believe that the eagle sharks were wiped-out during the great Cretaceous-Tertiary extinction event that killed the dinosaurs. At this point, their absence left an "ecological niche" open for another lineage to explore ... So the manta rays evolved to employ a very similar 'wing'-shape for swimming and a mouth adapted for filter-feeding, and other groups of surviving sharks arose into the lineages we know (and love) today (Ebert et al, 2020; Vullo et al, 2021).
To learn more about ancient sharks and their evolution you can check out my paleobiology articles.
References
Abel D & Grubbs D (2020). Shark Biology and Conservation Essentials for Educators, Students, and Enthusiasts. Johns Hopkins University Press.
Ebert DA, Dando M& Fowler S (2021). Sharks of the World: A Complete Guide, Second Edition. Princeton University Press: UK. IBAN: 978-0-691-20599-1.
Stein RW, Mull CG, Kuhn TS, Aschliman NC, Davidson LNK, Joy JB, Smith GJ, Dulvy NK & Mooers AO (2018). Global priorities for conserving the evolutionary history of sharks, rays and chimaeras, Nature Ecology & Evolution. Access online.
Vullo R, Frey E, Ifrim C, González MAG, Stinnesbeck ES & Stinnesbeck W (2021).
Manta-like planktivorous sharks in Late Cretaceous oceans. Science, 371:6535, 1253-1256. Access online.
By Sophie A. Maycock for SharkSpeak
