Palaeoisopus is a monotypic genus of fossilpycnogonid, known only by one species, Palaeoisopus problematicus, discovered from the Lower DevonianHunsrück Slate of Germany. It have several characters unusual for a pycnogonid, such as swimming legs with alternating size, medially-arranged eyes, and most significantly, a long, segmented abdomen, which were highly reduced in modern counterparts.
Morphology
Palaeoisopus is a large sea spider, with a body length of at least 12.5 cm and leg spans of about 32 cm, comparable to those of a modern Colossendeis. The margins of each of its body parts were covered by tubercles, the cephalon and 3 trunk somites are well-defined by ring-like segmentation. The abdomen apparently compose of 4 abdominal somites and a styliform telson, but based on the medial position of anus, the latter was also suggest to be a fusion of 5th abdominal somite and the original telson. Medial to the anterodorsal margin of cephalon was an eye-bearing ocular tubercle. Unlike the paired 4-eyed arrangement of most pycnogonids, it compose of a pair of large eyes and 2 smaller eyes that arranged anteroposteriorly in a midline. The anteriormost appendages were a pair of robust, pincer-likechelifores, which compose of 5 podomeres instead of 3 or 4 like those of the other pycnogonids. Below the chelifores was a cylinderal proboscis that always tucked underneath the cephalon, making it almost invisible in dorsal view. The palps and ovigers have similar morphology, the former terminated with a subchelate structures and the latter was apparently absent in some specimens, which may represent sexual dimorphism as seen in some modern pycnogonid taxa such as Pycnogonidae and Phoxichilidiidae. Each of the leg base was surrounded by flexible, ring-like structure. Among the 4 leg pairs the first one is significantly elongated, and its detail morphology slightly differ from the posterior counterparts as well.
Paleoecology
The large eyes, robust chelifores and oar-like legs suggest that Paleoisopus was a nektonicvisualpredators, with associated stalked crinoid as a possible prey item.
Phylogeny
While some analysis placingPalaeoisopus within a derived position, most studies suggest that Paleosiopus is a basal sea spider, as the well-developed abdomen most likely represent a plesiomorphic condition of total-group Pycnogonida.
