Prehistoric fish: 11 of the largest, weirdest and most significant

The fossil fish collection we care for contains more than 100,000 specimens. Many of them were scientific firsts.

Emma Bernard, our Fossil Fish Curator, says, “marine reptiles and dinosaurs get all the attention, but fish aren’t these irrelevant animals that are just kind of there. There’s such a wide variety – they dominated the oceans and they’ve survived multiple mass extinction events. Without them, the oceans today would look very, very different!”

Meet 11 key species from the 500-million-year story of fishes.

1. Haikouichthys ercaicunensis – the oldest known fish

When it lived: 518 million years ago

Haikouichthys is the oldest fish ever found. It’s also likely the oldest vertebrate – meaning an animal with a backbone. We’re vertebrates – as are all birds, reptiles, mammals, amphibians and, of course, fish. Its ancient age means Haikouichthys is likely closely related to the last common ancestor of all vertebrates, including us.

It was discovered in 1999 alongside Myllokunmingia fengjiaoa, another prehistoric fish, from the extraordinarily rich Chengjiang Fossil Site in China. They date to the Cambrian Period, when there was a great explosion in life forms. This is when most animal body plans first evolved.

These early fish lacked jaws. In fact, jaws took more than 100 million years to evolve! But we know these jawless species are fish due to their gills and a unique arrangement of muscles called myotomes.

2. Cephalaspis lyelli

When it lived: 400 million years ago

All early fish were jawless, belonging to a group called Agnatha. These animals used their muscular mouths to suck, squeeze and pulsate their food.

The prehistoric Cephalaspis lyelli had a mouth on the underside of its head, like stingrays and catfish, suggesting it sucked up prey from the muddy seafloor. It also appears to have had some kind of sensory organ along the front and sides of its head, which may have been used to detect burrowing organisms in the mud.

Its name means ‘head shield’, acknowledging its flat, armoured head with spines arcing backwards. It’s unlikely individuals were able to fend off predators with these spines. Instead, they may have been used to churn up seafloor sediments and the tasty prey within.

3. Conodonts

When they lived: 541 million to 252 million years ago

Early fish may have been jawless, but they weren’t without bite. Conodonts are an enigmatic group of animals. For around 120 years we only knew them from numerous, microscopic tooth-like structures. These were sharp and around a millimetre long. They were first dissolved out of limestone rocks in 1856, but weren’t matched with any animal until the 1980s.

Remarkable fossils in Scotland and South Africa revealed conodont teeth associated with the soft bodies of eel-like animals. These early vertebrates had distinct heads, a feature that evolved around 450 million years ago. Relatives of the conodonts and other fossil jawless fish include the lampreys and hagfishes, which still exist today.

Research shows conodont teeth were small but mighty. Without jaws, they couldn’t apply much force, but scratches on the surface of some teeth show they were used to grasp, handle and slice prey.

4. Cheiracanthus murchisoni

When it lived: 385 million years ago

The earliest fish are known from their teeth or, on the rare occasions they were preserved, their soft bodies. But this all changed during the Devonian Period, a span of time that’s sometimes referred to as the age of fishes. Most of the main fish groups became much more diverse at this time, including sharks, ray-finned fishes and lobe-finned fishes.

Cheiracanthus murchisoni was one of the first of a group known as acanthodians or ‘spiny sharks’. The acanthodians are distant relatives of true sharks and share some features, such as a streamlined body for fast swimming, a skeleton made of cartilage, a strong, blunt head and an upturned tail.

New discoveries in China are revealing that jaws evolved slightly earlier, in the Silurian Period. But C. murchisoni was among the early species to make good use of them to catch, handle and devour prey.

Fish evolved jaws from arches used to hold open the mouth so water could pass easily through the gills. There’s also evidence gill arches evolved into shoulders, eventually leading to arms in vertebrates.

5. Materpiscis attenboroughi and the first live births of any animal

When it lived: 380 million years ago

Another change in the Devonian Period was live birth, as opposed to egg laying. An incredible specimen from the Gogo Formation in Australia – a famous Devonian fossil site – shows an embryo still attached to its mother by an umbilical cord. Before this discovery, scientists thought it was reptiles that first evolved live birth around 200 million years later.

The fish belongs to a group called Placodermi, which were early jawed fishes that had cartilaginous skeletons and armoured heads. The species was named Materpiscis attenboroughi in honour of Sir David Attenborough, who highlighted the Gogo site in the 1970s.

In 2020, another discovery – this time in the Orkney Islands in Scotland – pushed back the date of the earliest known live birth even further. An unborn embryo of Watsonosteus fletti, another placoderm, was revealed when scientists studied slices of an adult fish fossil. This fish lived 385 million years ago.

The discovery of placoderms with embryos and specialised ‘clasping’ bones used for copulation also pushed back the evolution of sex and internal fertilisation. Before this, fish would have fertilised eggs outside the body, like spawning fish do today.

6. Dunkleosteus – a fierce armoured predator

When it lived: 382 million to 358 million years ago

Placoderms went extinct at the end of the Devonian Period, during a mass extinction that killed off around 75% of all species. But before they did, one of their group became what may have been the first vertebrate apex predator.

Dunkleosteus was a Devonian ocean giant, growing up to four metres long, and would have used its large jaws to chomp prey.

Only the armoured front sections of its head and body are commonly fossilised. This means reconstructions of whole Dunkleosteus species are often informed guesses based on similar groups of fish. This has led to uncertainties in their size, but the skulls alone show they would have been among the biggest fish of their time. Some researchers have estimated them to be up to 10 metres long. However, analysis from 2022 suggests Dunkleosteus probably didn’t exceed five metres in length.

7. Tiktaalik – how animals moved from water onto the land

When it lived: 375 million years ago

Fish dominated the prehistoric seas, but some didn’t stop there, emerging onto land.

Tiktaalik is known as a transition fossil – an animal showing characteristics of two distinct groups of organisms. While likely not a direct ancestor of all four-limbed animals – also known as tetrapods – that now roam the land, Tiktaalik shows how the transition from ocean to land may have occurred.

It belongs to the lobe-finned fish group Sarcopterygii, meaning its fins were supported by robust muscles. This is in contrast to ray-finned fish in the Actinopterygii group, which have less muscular fins. It’s thought Tiktaalik may have used its sturdy limbs to help pull itself along in shallow waters or onto shoreline mudflats, like mudskippers do today.

Its anatomy shows a curious mix of fish and tetrapod characteristics. On the fish side, Tiktaalik has gills, scales and fins. On the tetrapod side, it has rib bones, a mobile neck and lungs. As for the in between parts, its limb bones and joints show a mix of both groups, with flexible wrists that support fins, instead of toes.

Emma says, “Tiktaalik and related fish have one bone in their upper limbs that attaches to their shoulder, and that’s the bone that we have as the humerus in our own bodies. Some of the bones in our skeleton you can trace back to our fishy ancestors.”

8. Helicoprion – a mouth like a circular saw

When it lived: 290 million to 270 million years ago

At the close of the Devonian, the age of fishes came to an end too. Although, fishes’ dominance didn’t really end then – the Carboniferous Period that followed came to be known as the golden age of sharks!

However, the weirdest fishes at this time were not sharks, but a closely related group called chimaeras. From this group, fossils of Helicoprion baffled scientists for more than 120 years, with their spiral arrangement of up to 150 teeth. It wasn’t until modern 3D scans that researchers revealed this whorl sat in the creature’s lower jaw and produced a slicing action like a circular saw.

Other Carboniferous chimaeras included the Stethacanthus, which had an anvil-shaped crest and a spiny whip attached to the dorsal fin. Chimaeras are still unusual today, with the group now consisting of deep-dwelling species given evocative common names like ghost sharks, rat fish and spookfish.

9. Leedsichthys – the largest known bony fish

When it lived: 165 million to 155 million years ago

No complete skeleton of Leedsichthys has ever been found, but we still know it was one of the largest fishes that has ever lived. A Leedsichthys tail, found in Jurassic rocks near Peterborough, is nearly three metres in height from tip to tip. This suggests that the whole animal would have measured 16 metres in length.

Leedsichthys was a ray-finned fish, and, like the modern blue whale, it was a filter feeder. This giant fish had gill rakers – comb-like projections – which it used to sift tiny creatures, such as shrimp and jellyfish, from the water as it swam.

Size seems to have been its only defence, leaving it vulnerable to attack from fearsome predators that prowled the Jurassic oceans. This would have included large marine reptiles, such as mosasaurs and ichthyosaurs.

10. The coelacanth Macropoma lewesiensis

When it lived: 85 million years ago

Coelacanths are lobe-finned fishes that were once believed to have gone extinct around 66 million years ago. However, as two living species of coelacanths have since been discovered, some now call this group ‘living fossils’.

Emma’s favourite coelacanth is an extinct species in the fossil fish collection we care for. Macropoma lewesiensis was the first coelacanth described in scientific literature. The original specimen was collected from Lewes in Sussex and is from the Gideon Mantell Collection, named after geologist Gideon Mantell who named the dinosaur Iguanodon.

Macropoma lewesiensis looks very similar to modern coelacanths.

“Most fossil fish can look like roadkill – a flat squish in the rock. But because this one is preserved in 3D, you can see a lot of the internal morphology,” Emma explains. “It’s an ideal specimen – it’s visually appealing, scientifically really important, historically important and it looks like what it’s meant to look like.”

“If all goes wrong, I’m running out the building with that specimen!” she adds, jokingly.

11. Megalodon – the largest fish to ever exist

When it lived: 23 million to 3.6 million years ago

Each era has its giants! The Mesozoic, which includes the Triassic, Jurassic and Cretaceous, was rich with huge dinosaurs, marine reptiles and fish, such as Leedsichthys. However, when a mass extinction at the end of the Cretaceous wiped the marine reptiles out, some sharks were able to exploit the openings left by these giants going extinct.

Otodus megalodon was the largest shark to ever live, with calculations suggesting its length may have exceeded 24 metres. This outclasses the Jurassic Leedsichthys and completely eclipses modern great white sharks, which only reach a maximum of six metres.

However, it’s hard to be certain without more body parts. Emma says, “I’d love to find a near-complete megalodon because it would resolve various theories about their size, colour and exact body shape. It would answer a lot of questions I get asked!”

Megalodon was undoubtedly large though and this size, along with some other factors, was its downfall, as larger animals are more vulnerable to environmental changes. A period of global cooling during the Pliocene Epoch, 5.3 million to 2.6 million years ago, saw up to a third of all large marine animals go extinct, including megalodon.