Palaeoisopus

Palaeoisopus is a large sea spider, with a body length (excluding proboscis and chelifores) of at least 12.5 cm and leg spans of about 32 cm, comparable to those of a modern Colossendeis (giant sea spider).[1] The margins of each of its body parts were covered by tubercles, the cephalon (head section that bore eyes, proboscis, chelifores, palps, ovigers and 1st leg pair) and 3 trunk somites (section that bore the remaining leg pairs) are well-defined by ring-like segmentation. The abdomen apparently compose of 4 abdominal somites and a styliform telson,[2] but based on the medial position of anus (which, in telson-bearing chelicerates, always located at the ventral boundary of abdomen and telson), the latter was also suggest to be a fusion of 5th abdominal somite and the original telson.[1]

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  Fossil of Palaeoisopus, showing the long, fully-extended 1st leg pair.
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  Ventral fossil, showing full set of appendages.
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  Fossil compose of 3 individuals, showing dorsal morphology.

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.[1] The anteriormost appendages were a pair of robust, pincer-like chelifores, which compose of 5 podomeres (3 for scape and 2 for pincer) instead of 3 or 4 (1 or 2 for scape and 2 for pincer) like those of the other pycnogonids.[3] Below the chelifores was a cylinderal proboscis that always tucked underneath the cephalon, making it almost invisible in dorsal view.[1] 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[1] such as Pycnogonidae and Phoxichilidiidae (female lacking ovigers).[4][5] Each of the leg base was surrounded by flexible, ring-like structure.[1] Among the 4 leg pairs the first one is significantly elongated, and its detail morphology slightly differ from the posterior counterparts as well (e.g. shorter basal segments, different setae arrangement, 4 flatten distal podomeres instead of 5).[1]

The large eyes, robust chelifores and oar-like legs suggest that Paleoisopus was a nektonic (swimming) visual predators, with associated stalked crinoid (sea lily) as a possible prey item.[1]

While some analysis placing Palaeoisopus within a derived position,[6] most studies suggest that Paleosiopus is a basal sea spider,[1][7][8] as the well-developed abdomen most likely represent a plesiomorphic condition of total-group Pycnogonida.[1][9]