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Fishing around in the fossil collections

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Some of the most attractive, spectacular, or just strange specimens in the Ulster Museum’s collections are those of fossil fish.

They range from the tiny scales of the ancient jawless Astraspis to the enormous toothsome Xiphactinus. Intact fossil fish are rare. Scavengers and rapid decay usually see the bones and scales scattered within a matter of days. However, unusual conditions may bury the fish before this has happened, entombing it for perhaps hundreds of millions of years until revealed by the tap of a geologist’s hammer.

Here are just a few exquisite specimens. Some are on display while others remain in storage.

Astraspis desiderata

Astraspis desiderata

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Cheiracanthus murchisoni

Cheiracanthus murchisoni

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Palaeoniscus catopterus

Palaeoniscus catopterus

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Ptychodus polygyrus

Ptychodus polygyrus

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Shark centra

Shark centra

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Sygnathus acus

Sygnathus acus

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Paleobalistum goedeli

Paleobalistum goedeli

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Aspidorhynchus comptoni

Aspidorhynchus comptoni

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Xiphactinus audax

Xiphactinus audax

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Priscacara liops

Priscacara liops

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Scomberodon speciosum

Scomberodon speciosum

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Atractosteus strausi

Atractosteus strausi

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Fish from the depths of time

Image: Astraspis desiderata, Harding Sandstone, Ordovician Period (450 million years ago) - Colorado, USA. K20696
Astraspis desiderata, Harding Sandstone, Ordovician Period (450 million years ago) - Colorado, USA. K20696

This small piece of sandstone, about 450 million years old, is packed with small boney plates and scales of some truly ancient fish. It is a piece of the Harding Sandstone, a type of rock found in Colorado, USA, which for many years held the honour of being the source of the earliest fossil fish known.

These 'fish', just a few cm long, were not like any found today. They had no jaws or bony skeleton, and their bodies were covered in small bony plates and scales. They may perhaps have been close relatives of some of our distant ancestors. The most common type in the Harding Sandstone is called Astraspis desiderata.We now know that the first fish appeared more than 70 million years earlier than these but the Harding Sandstone is still, to this day, an important source of some of the oldest known fish.

Spiney Shark

Image: Cheiracanthus murchisoni, Devonian Period (about 390 million years old) - Tynet Burn, Fochabers, north-east Scotland. K17100
Cheiracanthus murchisoni, Devonian Period (about 390 million years old) - Tynet Burn, Fochabers, north-east Scotland. K17100

Acanthodians, or 'spiny sharks', are an extinct group of fishes that were common in lakes and rivers during the Devonian Period around 400 million years ago. The front of each of their fins is supported by a long slender spine, hence their name, but they are not closely related to modern sharks.

This is just one of about 15 different species of fossil fish that have been found in Devonian rocks exposed in the banks of Tynet Burn, near Elgin in northeast Scotland. The fish, found in flattened limestone nodules, are often perfectly preserved. Some look a little like fish alive today while others, like the spiny sharks, are quite alien-looking. All are long extinct. The Ulster Museum holds a large collection of fossil fish from Tynet Burn.

Fish in a desert?

Image: Palaeoniscus catopterus, Sherwood Sandstone Group, Lower Triassic (about 230 million years ago) - Rhone Hill, Dungannon, County Tyrone. K1620
Palaeoniscus catopterus, Sherwood Sandstone Group, Lower Triassic (about 230 million years ago) - Rhone Hill, Dungannon, County Tyrone. K1620

This small slab of red sandstone contains three complete fossil fish. Intact fish are rare as fossils because they decay rapidly and fall apart soon after death. Very few have been found in Northern Ireland but the most remarkable were found more than 170 years ago. Quarrymen working red sandstone at Rhone Hill, near Dungannon, in County Tyrone, came across dozens of small fish crowded into just a few square metres of one particular sandstone bed. News of the discovery rapidly spread and several of the most famous geologists of the day visited the site soon after its discovery. This unique deposit was soon quarried away but a few of the fish found their way into the collections of the Ulster Museum.

What is particularly remarkable about these fish is that the red sandstones in which they occur were deposited in a semi-desert environment around 230 million years ago. It seems likely that these fish became trapped in a rapidly shrinking pool and were then buried by mud brought down by a flood.

Shell crusher

Image: Ptychodus polygyrus tooth, Hibernian Greensands Formation, Upper Cretaceous Period (about 90 million years ago) - County Antrim, Northern Ireland. K13904
Ptychodus polygyrus tooth, Hibernian Greensands Formation, Upper Cretaceous Period (about 90 million years ago) - County Antrim, Northern Ireland. K13904

This rather peculiar looking object is actually just one tooth from an extinct type of shark called Ptychodus. The shape of the teeth is like that of modern skates and rays, rather than typical sharks. It suggests that Ptychodus used these teeth to break open the shells of molluscs or crustaceans. The two smooth patches on the top of the tooth are wear facets, created by this tooth rubbing against another in its top jaw every time its mouth closed. It shows that this tooth had seen a lot of use by the time its owner died.

This tooth was found in a group of rocks called the Hibernian Greensands, which were deposited around 90 million years ago across what is now the eastern part of Co. Antrim.

A giant fossil fish from Cave Hill

Image: Shark centra, Ulster White Limestone Formation, Upper Cretaceous (about 80 million years ago) - Cave Hill, County Antrim, Northern Ireland. K14013
Shark centra, Ulster White Limestone Formation, Upper Cretaceous (about 80 million years ago) - Cave Hill, County Antrim, Northern Ireland. K14013

The large disc-like objects in this block of white limestone are sections from the backbone of a giant fossil shark. From their size we can estimate that the entire animal must have been at least five metres long! The skeleton of a shark is made of cartilage rather than bone, so fossils like this are rarely preserved. This specimen was found in the limestone quarry on Cave Hill, on the outskirts of Belfast, in the 19th Century. The Ulster White Limestone, or Chalk, was originally deposited on the bed of the sea that covered what is now Northern Ireland about 80 million years ago.

Pipefish

Image: Sygnathus acus, Marecchia River Formation, Lower Pliocene (about 5 million years ago) - Poggio Berni, Rimimini Province, Italy. K5211
Sygnathus acus, Marecchia River Formation, Lower Pliocene (about 5 million years ago) - Poggio Berni, Rimimini Province, Italy. K5211

The first pipefish appeared around 50 million years ago and they have changed little since then. These slow-moving fish use their long slender shape to blend in with beds of eel grass or other seaweeds. Because of this association with particular marine plants, the presence of a fossil pipefish in a particular layer of rock can tell us about aspects of their environment that have not been preserved as fossils.

The peculiar pycnodonts

Image: Paleobalistum goedeli, Sublithographic Limestone Formation, Middle Cretaceous Period (about 95 million years ago) - Haqel, Lebanon. K25133
Paleobalistum goedeli, Sublithographic Limestone Formation, Middle Cretaceous Period (about 95 million years ago) - Haqel, Lebanon. K25133

This bizarre-looking fish is called a pycnodont. With its deep flattened body and rather small fins, it was not a fast swimmer. Whatever it ate must have been pretty slow-moving too and its blunt rounded teeth suggest that it may have preyed on hard-shelled creatures, such as bivalves or crustaceans.

This fish inhabited a warm shallow sea around 95 million years ago in what is now Lebanon. The thinly layered limestones that accumulated on the sea bed here are rich in well-preserved fossil fish, with more than 70 genera known.

Built for speed

Image: Aspidorhynchus comptoni, Santana Formation, Lower Cretaceous (about 95 million years ago) - Araripe, Brazil. K1113
Aspidorhynchus comptoni, Santana Formation, Lower Cretaceous (about 95 million years ago) - Araripe, Brazil. K1113

The sleek elongate body of this fish and its long beak-like mouth, suggest that it was a fast-swimmer, perhaps rather like a modern garpike. It was just one of many different fish that inhabited a shallow inland sea around 95 million years ago in what is now the Araripe region of northeastern Brazil.

These fossil fish are enclosed by limestone nodules that must have grown around them as they lay in the mud of the sea bed. Many of the fish are almost perfectly preserved and not even squashed flat, so the processes of fossilisation - converting the soft flesh and mud to hard stone - must have begun almost as soon as the fish had died.

Bulldog Fish

Image: Xiphactinus audax, Niobrara Chalk Formation, Upper Cretaceous Period (About 90 million years ago) - Scott County, Kansas, USA. K29810
Xiphactinus audax, Niobrara Chalk Formation, Upper Cretaceous Period (About 90 million years ago) - Scott County, Kansas, USA. K29810

This extraordinary fossil fish, almost four metres long, was found in the Chalk of Kansas. It swam the oceans which covered what is now the central USA almost 90 million years ago. Its bulldog-like jaws and fearsome teeth show that it preyed on fish and seabirds near the surface. With its long body and large tail it could catch them with sudden bursts of speed. In many ways the bulldog fish is rather like a modern tarpon, only far bigger.

This prize specimen was discovered in 1992 by Robert and Alan Detrich. They noticed part of the tail projecting from the side of a gulley and excavated back to uncover the virtually complete fossil.

Acquired by the Ulster Museum in 2004, it is one of the largest fossil fish on display anywhere in Europe.

Perfect Perch

Image: Priscacara liops, Green River Formation, Eocene (about 50 million years ago) - Fossil Lake, Kemmerer, Wyoming, USA
Priscacara liops, Green River Formation, Eocene (about 50 million years ago) - Fossil Lake, Kemmerer, Wyoming, USA

Perch are common predatory fish in lakes and rivers around the world today, and clearly have been for many millions of years. This fossil Perch is around 50 million years and thrived in one of several large lakes in the Green River region of what is now the western USA. After death most of the fish in these lakes would have decayed quickly or been eaten by scavengers. Occasionally low levels of oxygen on the lake bed prevented this from happening, allowing time for the corpse to be buried with barely a bone out of place.

Fish within a fish

Image: Scomberodon speciosum, Eocene Period (50 million years ago) - Monte Bolca, Verona, Italy. K8300
Scomberodon speciosum, Eocene Period (50 million years ago) - Monte Bolca, Verona, Italy. K8300

Small animals often fall prey to bigger ones. The small fish in the stomach of this larger one must have been swallowed only hours before the larger fish died, and before it had time to digest its last meal. A terminal case of indigestion, perhaps?

Image: Scomberodon speciosum close up showing small fish inside stomach.
Scomberodon speciosum close up showing small fish inside stomach.

This specimen is from the famous fossil locality of Monte Bolca, where fossil fish have been mined for centuries. The fossil-bearing limestone slabs always contain numerous natural breaks, and so the fossils have to be pieced back together.

Replica or real?

Image: Atractosteus strausi, Eocene Period (50 million years ago) - Messel, near Darmstadt, Germany. K12239
Atractosteus strausi, Eocene Period (50 million years ago) - Messel, near Darmstadt, Germany. K12239

This fossil fish, a relative of the modern Garpike, is real but the surrounding ‘rock’ is not. It is no longer embedded in the shale in which it was found, but in a slab of synthetic resin. Why? This is just one of many remarkable fossils - from fish, to snakes, bats and horses - that are found in the oil shales that were once quarried from a huge pit at Messel, in Germany. Unfortunately, the shales - and their contained fossils - disintegrate as they dry out so palaeontologists have devised a method of preserving them by transfer from the shale into resin.