They were swimming in a sea like in the Caribbean; they reached over 2 m in length and were covered with sturdy scales that resembled a shell. They stunned their prey with their powerful heads. The remains of such fish from 150 million years ago have been discovered by palaeontologists in the quarry Owadów-Brzezinki (Łódź province).
The discovered remains come from two groups of ray-finned fishes, Halcomorphi and Pachycormiformes.
Pachycormiformes are extinct fishes with no evolutionary offspring. The largest specimens known from fossil record were 12 m long! "In the prehistoric seas, they occupied the place of today\'s whales, and they were very similar to them. Some of the fish from this group fed on plankton, but the fossils we discovered in Owadów belonged to predatory fish" - told PAP Daniel Tyborowski, doctoral student from the Museum and Institute of Zoology PAS, who described both specimens.
It looked very similar - and led a similar lifestyle - to the second fish (Halecomorphi, Caturus giganteus). The only known contemporary living relative comes from North America. "Unlike its ancestors from millions of years ago, it is a very small species" - noted Tyborowski.
"Today fish scales are generally very thin and flexible. The body of that fish was very smooth and shiny, and at the same time resistant to mechanical injuries. It was an armour" - the scientist explained. This was because of ganoin - a very hard substance similar to enamel, which covered large and massive scales. In addition to the protective function, the scales also had nutritional function, because - like the bones - they stored minerals that the body could use on in a less favourable periods.
The discovery was made in the quarry Owadów-Brzezinki in the northern part of the Świętokrzyskie Mountains. It is one of the richest fossil sites in Poland - many of the specimens found there represent new species for science. The most interesting fossils dating back 148 million years include the remains of the first flying reptile discovered in Poland - pterosaur, recently described new species of marine ichthyosaur, the first known in Poland Jurassic dragonfly - and the fish.
During that period, the area of today\'s Poland resembled today\'s Caribbean - it was a warm, shallow sea with thousands of islands and islets. Its bottom was the so-called carbonate platform (it consisted of calcite, aragonite and various mineral debris, made of calcium carbonate). "Fossils are well preserved in such environments. When the fish we study died and droppped to the bottom, they were surrounded by this specific, delicate sediment that, despite millions of years, preserved their tissues to this day" - said the doctoral student.
Daniel Tyborowski noted that fish remains always attract the attention of palaeontologists, because they are rarely found among fossils. Meanwhile, it is the second most common group of fossils in this quarry (after clams). A similar diversity of fish is known from the site in Solenhofen, Germany, one of the most famous and richest fossil sites.
The researcher examined the two best preserved specimens: jaws with some teeth. "At the beginning it was not clear whether we were dealing with the remains of fish, or rather large marine reptiles, ichthyosaurs" - he recalled.
In order to solve this problem, Tyborowski applied a novel method in the Polish palaeontology - microtomography, which allows to "view" the fossils without extracting them from limestone in which they are preserved (which could mean a high risk of damage).
The scan was performed in a laboratory at the University of Silesia. "The method worked, there was a noticeable difference in density between the specimen and the rock, which was mapped by X-rays" - explained Tyborowski.
The scan helped to identify the organisms and allowed to obtained data, based on which a 3D print was made of the jaw magnified several times. "The print was originally intended to be used as a prop for popular science lectures, but it turned out that it allowed us to see details that were not so clear when analysing the image on a computer screen" - the scientist said.
The use of tomography and dentine density analysis allowed to determine the details of the structure of the teeth. In one of the fish (Pachycormiformes), the upper part of the front teeth was covered with acrodentin - a variant of the heavily mineralized dentin. "This means that the front teeth were very resistant to mechanical damage. At the same time, the jaw of the fish was very massive. In the front, it had a single, large and sharp tooth" - the scientist described.
How did this fish from millions of years ago hunt? It would probably hit its prey at high speed like a ram and drive it on the front tooth, just like the barracuda does today" - Tyborowski said.
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