Eurypterus podolicus, UA

A well-preserved cephalothorax (fused head and midbody) on matrix.

Age: Ordovician, Wenlock;
Locality: Kamjanez-Podilskyj, Khmelnytskyj oblast, western Ukraine
Head size: 1 x 1,5 cm
Matrix: 6 x 7 cm

Eurypterids, also known as sea scorpions because of their pointed tails, evolved in the Ordovician.

As the dominant predators of the Upper Ordovician, Silurian and Lower Devonian seas, they had their greatest species diversity at that time. The largest representatives of this order could grow up to 3 meters long, while E. podolicus only reached a few modest centimeters.
The group of eurypterids, to which Eurypterus naturally belongs, had swimming paddles for their last pair of legs. Having two respiratory systems - book gills and gill plates - they were also able to walk on land, as trace fossils show. Unfortunately, they did not manage the leap to land completely: like their close relatives, the horseshoe crabs, they used the tips of their legs to chew, which is much more efficient underwater than on land.
But under water, they faced increasing competition from fish, so that they were driven further and further into the river systems.
During the Permian there were only a few species left, but they were probably the heaviest arthropods of all time.
The Great Dying led to the extinction of the last representatives of this group.

The colors on the pictures may differ slightly from the original due to technical reasons.