Palaeoisopus Temporal range:
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Reconstruction | |
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Order: | †Palaeoisopoda
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Family: | †Palaeoisopodidae Dubinin, 1957
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Genus: | †Palaeoisopus
Broili, 1928
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Species: | †P. problematicus
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Binomial name | |
†Palaeoisopus problematicus Broili, 1928
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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 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]
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]