Palaeoisopus

Palaeoisopus is the most common pycnogonid of Hunsrück Slate, with over 80 fossil specimens had been discovered as of 2024. When this arthropod was first described by Broili 1928, it was thought to be an isopod crustacean, hence the name Palaeoisopus. A few years later, it was reidentified as a pycnogonid by the same author. Even so, the anterior and posterior axis was reversed, with the overlapped chelifores and long abdomen being misinterpreted as a round abdomen and an elongated cephalon (head), respectively. This was corrected by Lehmann 1959, which using X-ray to identify cephalic structures such as ovigers and ocular tubercles between the "abdomen" (chelifores) and "fourth legs" (first legs). Bergström et al. 1980 using the same method to study more specimens, providing a detailed reconstruction of Palaeoisopus.