A peculiar arthropod, evidently related to the Pycnogonida and represented by a number of well-preserved fossils from the Devonian Hunsruck shales. It was formerly considered by a number of paleontologists to have an anterior jointed proboscis and a bulbous terminal abdomen, but studies of material under ultraviolet light have demonstrated that the bulbous abdomen is in fact a pair of robust, well-developed chelae (see illus.). Furthermore, additional appendages (palps and ovigers) not apparent in previously studied material have been discerned, bringing its complement of anterior appendages into agreement with that of the Pycnogonida. As reoriented, Palaeoisopus has anterior flattened appendages that separate it from extant families of Pycnogonida.