Stylonurus is a stylonurid eurypterid merostomate arthropod from the Pridoli-Middle Devonian of the United Kingdom and possibly the United States of America. It was named in 1856 by David Page. It was one of the most successful eurypterids, as well as one of the most unique.
Stylonurus was a unique arthropod, as it greatly differed from the generic eurypterid build. It had a rounded cephalothorax, mid-sized eyes, a pair of mandibles, 4 pairs of long walking legs (the 1st two bearing comb-like spines), and a long tail ending in a spear-like telson. Its body would have been covered in a hard, chitinous exoskeleton.
Stylonurus was a predator, preying on fish and small invertebrates. Its 1st (and 2nd) pair of walking limbs had spines on them, and were used to capture prey and pass them into its mouth.
Unlike other eurypterids of its time, Stylonurus did not have a pair of paddles used for swimming; however, its long legs had many uses. Most importantly, it would use them for striding across the seafloor, and would only swim occasionally, using its legs and telson to propel itself through the water. As well as this, the very first pair of its legs were marginally smaller than the rest, and had comb-like spines on them, so it would have used them to catch small animals. Finally, along with its aforementioned ability to walk on the bottom of the ocean, it would have been able to step out of the sea to mate and lay eggs, similar to horseshoe crabs.