At first glance, this handsome fellow looks surprisingly modern—rather like a herring decided to grow teeth, become a serious predator and pose magnificently for the fossil record.
Thrissops formosus was an early teleost, belonging to the enormously successful branch of ray-finned fishes that includes most living fish species. The name “ray-finned” refers to fins supported by slender bony rays rather than the muscular, fleshy lobes seen in coelacanths, lungfish and the distant ancestors of land vertebrates.
With its elongated body, pointed head and deeply forked tail, Thrissops was built for swimming through open water. Its jaws carried rows of small, sharp teeth suited to seizing other fish.
Some individuals may have approached 80 centimetres in length, making T. formosus one of the larger predatory teleosts of its ecosystem—not a sea monster, certainly, but probably an unwelcome sight if you happened to be a small Jurassic fish going about your day.
Its narrow tail base and strongly forked caudal fin suggest an active swimmer capable of bursts of speed. The dorsal and anal fins were positioned far back on the body, where they would have helped stabilize the fish during pursuit. This was not an animal designed to shuffle politely along the seabed. Thrissops belonged in the water column, where lunch was mobile and catching it required some effort.
Fossils of Thrissops formosus are best known from the famous lithographic limestones of the Solnhofen Archipelago in Bavaria, southern Germany.
The species was originally named by Swiss naturalist Louis Agassiz in 1833, with its type material coming from Late Kimmeridgian rocks near Kelheim. Additional specimens are known from Late Kimmeridgian and Early Tithonian deposits at localities including Solnhofen, Eichstätt and Ettling.
Fossils assigned to the species have also been reported from the Late Jurassic limestones of Cerin in eastern France. Together, these finds place the species near the close of the Jurassic Period. A recent taxonomic review documents the species and its principal localities.
At the time, this part of Europe was not the continuous landscape we know today. It was an archipelago of small islands surrounded by warm, shallow seas along the northern margin of the Tethys Ocean.
Quiet, restricted lagoons accumulated extremely fine carbonate mud. When animals sank into these low-oxygen environments, scavenging and decay could be slowed long enough for remarkably detailed fossils to form.
These same delicious deposits preserved Archaeopteryx, pterosaurs, marine reptiles, crustaceans, ammonites, insects and an extraordinary variety of fishes.
The fine-grained limestone captured delicate structures that would normally disappear, including fin membranes, soft tissues and, in exceptional Thrissops specimens, traces of the fish’s original colour pattern.
Dark markings have been detected within or associated with the scales of some specimens. These are linked to preserved melanin—the pigment responsible for many dark colours in living animals.
We cannot yet restore the fish’s entire wardrobe with confidence, but the fossils indicate that Thrissops formosus was not necessarily the plain silver torpedo we might otherwise imagine. After 150 million years, even a hint of its original patterning feels astonishingly intimate.
One beautifully preserved specimen from Ettling also carries evidence of an injured tail. The damage appears to have healed while the fish was alive, meaning this particular Thrissops escaped an attack and continued swimming. The likely attacker may have been another predatory fish. Apparently, the Jurassic seas offered both excellent fossilization and absolutely dreadful customer service.
As for its family connections, Thrissops formosus belonged to the extinct order Ichthyodectiformes. These fishes first appeared during the Jurassic and survived into the Late Cretaceous. They were generally streamlined marine predators, although the group eventually produced a remarkable variety of sizes and body forms.
The closest relatives of T. formosus were other species of Thrissops and closely allied Jurassic ichthyodectiforms such as Allothrissops.
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| Cretaceous Predator, Xiphactinus |
At around five metres long, Xiphactinus was the sort of relative whose arrival would cause everyone else at the family reunion to quietly leave the swimming pool.
That relationship does not mean Thrissops was a miniature Xiphactinus or its direct ancestor.
Both belonged to the ichthyodectiform branch, but they occupied different positions within a lineage extending across more than 100 million years. Thrissops represents one of the early experiments in the fast-swimming, fish-eating body plan that later ichthyodectiforms carried to much larger—and occasionally outrageous—proportions.
Its relationship to living fish requires a little more care. Thrissops formosus has no close living equivalent and the entire ichthyodectiform lineage is extinct. Some evolutionary studies place Ichthyodectiformes just outside Teleocephala, the great group containing the vast majority of living teleost fishes.
In that interpretation, Thrissops was close to the early evolutionary assembly of modern teleost anatomy, but it was not itself a member of any living family. Its precise position remains debated as researchers continue comparing skulls, vertebrae, fins and tail skeletons across early fossil teleosts. One broad analysis recovered T. formosus as the sister taxon to Teleocephala.
Recent research has also shown that the genus Thrissops was more diverse than previously recognized. Newly described species from the Kimmeridge Clay of Dorset, England, and the Ettling deposits of Bavaria demonstrate that several related forms occupied Late Jurassic European seas. This work reinforces the importance of Thrissops as one of the earliest groups of comparatively large predatory teleosts.
What makes Thrissops formosus so compelling is the combination of familiarity and deep time. Its streamlined shape, forked tail and predatory lifestyle would not look entirely out of place in a modern ocean, yet this fish lived alongside ammonites, marine crocodile relatives, pterosaurs and Archaeopteryx.
It emerged during an important chapter in teleost evolution, long before ray-finned fishes achieved their present extraordinary diversity.
Today, teleosts inhabit nearly every aquatic environment on Earth, from coral reefs and mountain streams to polar seas and the deepest ocean trenches.
Thrissops formosus belonged near the beginning of that astonishing story: a swift Jurassic hunter preserved in limestone, carrying within its skeleton clues to the evolutionary rise of the fishes that would eventually conquer the world’s waters.
Not bad for an animal that looks, at first glance, like someone left a particularly ambitious herring pressed between the pages of Earth’s oldest scrapbook.
Lead Image: Asset ID: 2711635879. Vernadsky State Geological Museum, Moscow, Russia