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Extinct genus of sea spiders From Wikipedia, the free encyclopedia
Palaeoisopus is a monotypic genus of fossil pycnogonid (sea spider), known only by one species, Palaeoisopus problematicus, discovered from the Lower Devonian Hunsrück Slate of Germany.[1] 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.[1]
| Palaeoisopus Temporal range: | |
|---|---|
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| Fossil | |
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| Reconstruction | |
| Scientific classification | |
| Kingdom: | |
| Phylum: | |
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| Class: | |
| Order: | †Palaeoisopoda |
| Family: | †Palaeoisopodidae Dubinin, 1957 |
| Genus: | †Palaeoisopus Broili, 1928 |
| Species: | †P. problematicus |
| Binomial name | |
| †Palaeoisopus problematicus Broili, 1928 | |
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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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]
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