There are few living things older than sharks. As a group, these underwater predators emerged in the Late Ordovician roughly 450 million years ago. This makes them older than trees, as well as the North Star, the Atlantic Ocean, and even Saturn’s Rings! However, shark's origins remain a mystery. The key to unlocking this mystery lies with Cladoselache.
What was Cladoselache?
Cladoselache is an extinct, shark-like chondrichthyan - the scientific name for a group of fishes characterised by having skeletons primarily made of cartilage, rather than bone. It lived in what is now North America during the Late Devonian (372-359 million years ago), around the same time that the first trees were beginning to establish themselves on land.
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While its name may mean ‘branch shark’, palaeontologists debate whether Cladoselache was actually a shark at all. It’s often called the ‘earliest shark’, though it is only distantly related to modern sharks and may actually be more closely related to chimaeras than true sharks.
When did sharks first appear?
The earliest confirmed modern sharks - the selachians - first appeared in the Early Jurassic, around 200 million years ago. That said, there’s evidence of shark-like fish living a lot earlier, perhaps as early as 450 million years ago in the Late Ordovician. This was a time when almost all life on Earth was confined to the oceans.
Known from just a few scant scales, these early, shark-like fishes are very mysterious and the jury’s out on whether or not they’re true sharks or distant relatives. Other common shark fossils, like teeth, are missing from these early time periods, which has led some researchers to suggest these mysterious maybe-sharks were toothless.
Although physical evidence may be lacking, in studying the genomes of modern sharks, skates, rays, and chimaeras, palaeontologists have found that the chondrichthyans diverged from one another roughly 420 million years ago. These so-called ‘molecular clock’ studies hint at a very deep origin for sharks.
The first shark-like animals known from more than just a handful of scales appeared roughly 380 million years ago and are represented by the likes of Antarctilamna and Cladoselache.
Both looked like living sharks, with torpedo-shaped bodies and characteristically pointy dorsal fines. They were also active predators and filled similar ecological niches to sharks today.
Is Cladoselache actually a shark?
Since Cladoselache was discovered in the 19th century, its identity as a shark has been fiercely debated.
Some of the earliest studies on Cladoselache argued that it lay close to the base of (or perhaps even at the base of) a group known as Elasmobranchii. This is a group of chondrichthyans that includes modern sharks, rays, skates, and sawfish.
However, following the discovery of more Cladoselache fossils and subsequent studies on its anatomy, an alternative hypothesis has caught traction.
Instead of belonging to Elasmobranchii, Cladoselache may belong to a much smaller group of bizarre chondrichthyans known as the symmoriiforms, which themselves are closely related to chimaeras - a group of strange-looking, deep-sea fish.
While it may be biologically incorrect to call Cladoselache a ‘shark’, it certainly fits the bill in a broad, ecological sense.
What did Cladoselache look like?
Like modern sharks, Cladoselache had a long, torpedo-shaped body and pointed fins. However, instead of having just one dorsal fin, Cladoselache sported two - the first of which was significantly taller than the second and preceded by a large spine, known as a fin spine.
A lot of prehistoric sharks possessed fin spines. The acanthodians, appropriately named ‘spiny sharks’, took this to the extreme, with some species exhibiting dozens of fin spines across their bodies.
The streamlined body of Cladoselache is very similar to that of mackerel sharks, such as great whites and makos. Its head, on the other hand, is flatter and more box-shaped, which is reminiscent of modern frilled sharks. These eel-like sharks might look like the stuff of nightmares, but at just 2m in length and living in the deep sea, they’re hardly a threat to humans.
Cladoselache’s jaws were also quite similar to those of modern frilled sharks. Its teeth were distinctly shark-like too, with a large and sharp central cusp sandwiched between a pair of smaller cusps.
Like modern sharks, Cladoselache could replace its teeth, with a series of widely spaced tooth rows constantly unfurling new teeth outward. They weren’t replaced quite as quickly as they are in living sharks, though. The presence of worn surfaces on some teeth suggests Cladoselache retained certain teeth for long periods.
What did Cladoselache eat? And how did it hunt?
The teeth of Cladoselache were quite slender and had smooth edges, unlike those of later superpredators such as Megalodon and great whites. Based on this shape, palaeontologists think Cladoselache’s teeth were designed for grasping rather than tearing or chewing.
Instead of dismembering large prey, Cladoselache would grasp small fish in its jaws before swallowing them whole. This is evidenced by fossilised gut contents, which largely include small, ray-finned bony fishes, as well as crustaceans, conodonts, and hagfish-like animals.
Some of these remains were found tail first within the stomach, suggesting Cladoselache was a fast and agile hunter that struck from behind, gobbling up its prey before it even knew it was being hunted.
Why did Cladoselache become extinct?
Despite being a pioneer, Cladoselache didn’t exist for very long and was actually wiped out shortly after it first appeared in the fossil record, approximately 370 million years ago.
Cladoselache was one of countless casualties of the Late Devonian mass extinction - the second of Earth’s five major mass extinctions and one of the most devastating events in the history of marine life. More than three-quarters of all marine species disappeared, forever changing life underwater.
The timing of this mass extinction, as well as the cause, is uncertain. Some suggest it was caused by widespread anoxia (a total lack of oxygen) and that it may have been split into several distinct events, spread over the course of nearly 25 million years. Others argue it happened much faster, over the course of just half-a-million years.
While Cladoselache didn’t make it through this mass extinction, sharks - or at least their ancestors - did. They went on to survive the next three mass extinctions too, surviving right up until the present day.
Although each extinction reduced their diversity, sharks repeatedly recovered and diversified again, eventually reaching their all-time peak during the Middle Eocene, around 45 million years ago - long after the last of the five major mass extinctions.
What came after Cladoselache?
In the periods following the Late Devonian - the Carboniferous and the Permian (359-252 million years ago) - sharks diversified rapidly. This was a golden evolutionary age for them and a time when many species experimented with some truly bizarre body plans.
Some strange species from this time include: Helicoprion, the ‘buzzsaw shark’ whose teeth were attached to a spiral-shaped structure that resembled a spiky octopus tentacle; Stethacanthus, a shark with an ironing board-shaped dorsal fin covered in tiny barbs; and Xenacanthus, an eel-like shark that, unlike most extinct and extant species, lived in freshwater.
By the Early Cretaceous (approximately 145 million years ago), lamniform sharks had evolved and had begun to take over the world’s oceans. The lamniformes are better known as the ‘mackerel sharks’ and include some of today’s most familiar species, such as great whites, as well as several lesser-known species, such as goblin sharks and the megamouth sharks.
The lamniformes also includes the largest shark that ever lived, Megalodon. This ‘big-toothed’ behemoth was nearly the size of a blue whale (24.3m in length) and terrorised marine ecosystems worldwide from 23 to 3.6 million years ago.
Although Megalodon was more than ten times larger than Cladoselache, traces of this early, shark-like pioneer can still be seen in its anatomy - a reminder that even the largest, most ferocious predators have humble beginnings.
