Palaeoisopus

Palaeoisopus is a genus of fossil pycnogonid (sea spider). The only known species is Palaeoisopus problematicus from the Lower Devonian Hunsrück Slate of Germany. It is characterized by several features unusual for a pycnogonid, such as swimming legs of different sizes and a long, segmented abdomen.^[2][3]

Palaeoisopus Temporal range: Lower Devonian PreꞒ Ꞓ O S D C P T J K Pg N
Fossil
Reconstruction
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Chelicerata
Class:	Pycnogonida
Order:	†Palaeoisopoda
Family:	†Palaeoisopodidae Dubinin, 1957
Genus:	†Palaeoisopus Broili, 1928[1]
Species:	†P. problematicus
Binomial name
†Palaeoisopus problematicus Broili, 1928

Discovery

Palaeoisopus is the most common pycnogonid of Hunsrück Slate, with over 80 fossil specimens had been discovered as of 2024.^[3]

When this arthropod was first described by Broili 1928,^[1] it was thought to be an isopod crustacean, hence the name Palaeoisopus.^[3] A few years later, it was re-identified as a pycnogonid by the same author.^[4] Even so, the anterior and posterior axis was reversed, with the overlapped chelifores and long abdomen being misinterpreted as a round abdomen and an elongated cephalon (head), respectively. This was corrected by Lehmann 1959, which using X-ray to identify cephalic structures such as ovigers and ocular tubercles between the "abdomen" (chelifores) and "fourth legs" (first legs).^[5] Bergström et al. 1980 using the same method to study more specimens, providing a detailed reconstruction of Palaeoisopus.^[2] Sabroux et al. 2024 redescribed this genus alongside other Hunsrück pycnogonid fossils, by using both X-ray and RTI method they discover some new details, mostly those of the cephalic structures.^[3]

Morphology

Fossil showing the long, fully-extended front legs

Details of second to fourth legs

• 
  Main body
• 
  Chelifores, palps and ovigers

Palaeoisopus is a large pycnogonid, with a body length (excluding proboscis and chelifores) of at least 12.5 cm and leg spans of up to 40 cm, comparable to a modern Colossendeis (giant sea spider).^[2][3]

The margin of its exoskeleton are ornamented with tubercles. The body is widest at the box-shaped cephalon, with a cylinderal proboscis folded underneath it. The succeeding 3 trunk segments have pairs of dorsal tubercle and narrow towards the long abdomen. The abdomen apparently compose of 4 flexible segments and a styliform (sword-like) telson,^[6][3] 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 fifth abdominal segment and the original telson.^[2]

There is a dorsal ocular tubercle at the front of its cephalon. In the classical reconstruction by Lehmann 1959 and Bergström et al. 1980, it compose of a pair of large eyes and 2 smaller eyes arranged in a midline, unlike the 2-paired ocelli of other pycnogonids.^[2] In the redescription by Sabroux et al. 2024, it was re-interpreted as having no eyes, with the purported eye-like structures representing either just tubercles or lateral sense organs.^[3]

The first appendages are a pair of robust, pincer-like chelifores in front of the cephalon. Based on Bergström et al. 1980, each chelifore compose of 5 segments (podomere): 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.^[7] This was questioned by Sabroux et al. 2024, as the putative "second and third scape segment" might be just a second scape segment with an additional midway ridge.^[3] The palps and ovigers located laterally and lateroventrally to the anterior cephalic region, respectively. Each apparently have more segments (11 and 12) than those of other pycnogonids (up to 9 and 10), but their 3 basal "segments" might represent annulations of a subdivided first segment (coxa 1) as seen in the legs. The palps have a spur on its fourth distalmost segments and the ovigers terminted by a claw. The ovigers were apparently absent in some specimens, which may represent sexual dimorphism as seen in some modern pycnogonid taxa^[2] such as Pycnogonidae and Phoxichilidiidae (female lacking ovigers).^[8][9]

The remaining appendages are 4 pairs of enormous legs. Unlike other pycnogonids with subequal legs, each pair of them are different in sizes (smaller towards the posterior). The base was surrounded by multiple ring-like annulations (4 for leg 1, 3 for leg 2, 2 for leg 3-4) that represent coxa 1, connected to the short lateral processes of cephalon and trunk. The distal section (beyond the fourth segment/femur, which is unusually short for a pycnogonid) was flatten, lined with marginal setae and terminated by a robust, hook-like claw. The first legs are distinctive: they have significantly widen coxae, their flatten distal section compose of 4 segments with subequal width and bore a few ventral setae at each of their distal corner. In comparison, the second to fourth legs have 5 flatten distal segments that narrow towards the end and possess double rows of long ventral setae.^[2][3]

Paleoecology

specimen associated with a crinoid

Palaeoisopus may have been a nektonic pycnogonid that swam around the sea floor by moving its oar-like legs.^[2][3] The robust chelifores and hooked claws suggest it was a predator. Based on the purported large eyes, Bergström et al. 1980 suggest it rely on visual cues to find preys, with associated stalked crinoid (sea lily) as a possible target.^[2] This was questioned by Sabroux et al. 2024, as they re-interpreted it as blind and the association with crinoid was limited only to juvenile specimens.^[3]

Taxonomy

Haliestes

Paleosiopus may had been a basal, stem-group pycnogonid,^[2][10][11] as its long, segmented abdomen and telson likely represent an ancestral traits, suggesting it branched off before the pycnogonid stem lineage started to reduce the abdomen into a short, unsegmented tubercle.^[2][12] There are some other Paleozoic pycnogonids with similar leg morphology to this genus (possess annulated coxa 1, flatten distal segments, marginal setae, hooked claws and first leg pair with 1 less segment) such as the older Haliestes and coexisting Pentapantopus, but it is unknown if this leg type represent a clade or evolutionary grade of pycnogonid stem-group.^[3] While some analysis placing them within Pantopoda (crown-group pycnogonids),^[13] this result is questionable as they have low support value and based on outdated reconstruction of the fossil taxa.^[14][15][3]