Fishing Through Geological Time

By Matthew Sutton

This article was originally published in The Oxford Scientist Michaelmas Term 2021 edition, Change.

Earth’s oceans are an immense and foreboding place. They occupy 71% of the surface area of the planet and have a total volume exceeding 1.3 billion cubic kilometres. Occupying every corner of every part of this gargantuan biome are the most ubiquitous backboned animals on the planet: the fishes. Biologists often divide vertebrates up into 5 groups based on their evolutionary history and shared features – mammals, birds, reptiles, amphibians and fishes. Each of the first four groups evolved from piscine ancestors. Not only that, but in the present day the fishes have both a greater total mass and higher number of species than all other vertebrates combined. In other words, if you’re a backboned animal living on Earth today, odds are that you’re a fish.

Fishes are by far the most ancient vertebrate group. They have swum through our planet’s seas for well over 500 million years, being almost as old as complex multicellular life itself.

Well over 90% of fish species swimming through oceans, rivers and lakes today belong to a group called the teleosts. These fishes first emerged around 300 million years ago and include everything from eels to piranhas to seahorses. A much less diverse but more ancient group are the cartilaginous fishes, so called because their skeletons are made up of cartilage (similar to that found in human ears and noses) rather than bone. Sharks and rays are the two most prominent and charismatic members of this group.

Sharks and rays can seem like aliens even in the modern ocean, but by looking into the fossil record we can see that some ancient cartillagenous fishes had truly bizarre and remarkable adaptations. Take the 350-million-year-old Akmonistion, with its inexplicable anvil-like dorsal fin, the function of which remains totally unknown. Or how about Helicoprion, a long-extinct shark with a curving set of teeth in its lower jaw that resemble a vicious buzzsaw. The true geological diversity of sharks and rays has been beyond the reach of palaeontologists thanks to the fact that their skeletons are very unlikely to be preserved over millions of years when compared to bony fishes like teleosts. Fossil remains of sharks and rays do exist but are only preserved in exceptional circumstances. Reconstructing the history of these animals from such scant remains is like trying to understand the Roman Empire using nothing but a handful of bricks.

A novel means of illuminating the history of not just sharks, but all fishes, has recently been pioneered by Elizabeth Sibert, a researcher at Yale University. She investigates the hardy microscopic remains of fish teeth and scales – ichthyoliths (meaning ‘fish rock’) – that are preserved in sediments at the bottom of the sea. Teeth and scales are much hardier than most other fish body parts and might be shed during life or settle to the seafloor after a fish has died. By developing methods to isolate these microscopic but widespread ichthyoliths and applying those methods to sediment drilled from the seafloor, she has unearthed long-hidden chapters in the story of the fishes. For example, in a June 2021 paper, she and a fellow researcher presented evidence of a previously unknown major extinction event that wiped out 70% of shark diversity around 19 million years ago.

Unfortunately, the cartilaginous fishes have also been at the sharp end of humanity’s impact on our environment. An unholy trinity of overfishing, habitat degradation and climate change threaten their future survival, and a 2021 study estimated that their numbers have reduced by 70% over the last 50 years. A separate study this year also found that almost 40% of shark and ray species face the imminentthreat of extinction due to human activity.

At the same time that their future survival is threatened, the fate of ancient sharks and rays is finally being brought into the spotlight. By looking back at how these organisms reacted to periods of rapid environmental change during Earth’s deep history, we might begin to illuminate how they will respond in the future to changes brought about by humans. The full story of the fishes, across half a billion years of evolution and five mass extinctions, is only now becoming clear.