:en:Palaeoisopus

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

When this arthropod was first described by Broili 1928,{{Cite journal |last=Broili |first=F. |date=1928 |title=Crustaceenfunfe aus dem rheinischen Unterdevon |journal=Sitzungsberichte der mathematisch-naturwissenschaftlichen Abteilung der Bayerischen Akademie der Wissenschaften zu München 1928 |pages=197–201}} it was thought to be an isopod crustacean, hence the name Palaeoisopus. A few years later, it was re-identified as a pycnogonid by the same author.{{Cite journal |last=Broili |first=F. |date=1932 |title=Palaeoisopus ist ein Pantopode |journal=Sitzungsberichte der mathematisch-naturwissenschaftlichen Abteilung der Bayerischen Akademie der Wissenschaften zu München |volume=2 |pages=45–60}} 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).{{Cite journal |last=Lehmann |first=W. M. |date=1959 |title=Neue Entdeckungen an Palaeoisopus |url=https://link.springer.com/article/10.1007/BF02988981 |journal=Paläontologische Zeitschrift |language=de |volume=33 |issue=1 |pages=96–103 |doi=10.1007/BF02988981 |issn=1867-6812}} Bergström et al. 1980 using the same method to study more specimens, providing a detailed reconstruction of Palaeoisopus. 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.

File:Palaeoisopus (7992699932).jpg

File:2024 Sabroux et al. Fig 16.jpg

2024 Sabroux et al. Fig 02.jpg|Main body

2024 Sabroux et al. Fig 10.jpg|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).

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,{{Cite journal|last1=Dunlop|first1=Jason A.|last2=Lamsdell|first2=James C.|title=Segmentation and tagmosis in Chelicerata|url=https://www.academia.edu/28212892|journal=Arthropod Structure & Development|year=2017|language=en|volume=46|issue=3|pages=395–418|doi=10.1016/j.asd.2016.05.002|pmid=27240897 }} 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.

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. 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.

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.{{Cite journal|last1=Brenneis|first1=Georg|last2=Arango|first2=Claudia P.|date=December 2019|title=First description of epimorphic development in Antarctic Pallenopsidae (Arthropoda, Pycnogonida) with insights into the evolution of the four-articled sea spider cheliphore |journal=Zoological Letters|language=en|volume=5|issue=1|pages=4|doi=10.1186/s40851-018-0118-7 |pmc=6330760|pmid=30656062 |doi-access=free }} 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. 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 such as Pycnogonidae and Phoxichilidiidae (female lacking ovigers).{{Cite journal|last1=Bain|first1=Bonnie A.|last2=Govedich|first2=Fredric R.|date=December 2004|title=Courtship and mating behavior in the Pycnogonida (Chelicerata: Class Pycnogonida): a summary|url=https://www.researchgate.net/publication/233026662|journal=Invertebrate Reproduction & Development|language=en|volume=46|issue=1|pages=63–79|doi=10.1080/07924259.2004.9652607|s2cid=84993360 }}{{Cite journal|last1=Ballesteros|first1=Jesús A.|last2=Setton|first2=Emily V. W.|last3=López|first3=Carlos E. Santibáñez|last4=Arango|first4=Claudia P.|last5=Brenneis|first5=Georg|last6=Brix|first6=Saskia|last7=Cano-Sánchez|first7=Esperanza|last8=Dandouch|first8=Merai|last9=Dilly|first9=Geoffrey F.|last10=Eleaume|first10=Marc P.|last11=Gainett|first11=Guilherme|date=2020-02-02|title=Phylogenomic resolution of sea spider diversification through integration of multiple data classes |url=https://www.biorxiv.org/content/10.1101/2020.01.31.929612v1|journal=bioRxiv|volume=38 |issue=2 |language=en|pages=686–701|doi=10.1101/2020.01.31.929612|pmid=32915961 |s2cid=213417127|doi-access=free|pmc=7826184}}

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.

File:2024 Sabroux et al. Fig 01.jpg

Palaeoisopus may have been a nektonic pycnogonid that swam around the sea floor by moving its oar-like legs. 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. 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.

File:20200623 Haliestes dasos.png]]

Paleosiopus may had been a basal, stem-group pycnogonid,{{Cite journal|last1=Siveter|first1=Derek J.|last2=Sutton|first2=Mark D.|last3=Briggs|first3=Derek E. G.|last4=Siveter|first4=David J.|date=October 2004|title=A Silurian sea spider|url=https://www.nature.com/articles/nature02928|journal=Nature|language=en|volume=431|issue=7011|pages=978–980|doi=10.1038/nature02928|pmid=15496921|s2cid=4420863 }}{{Cite journal|last1=Poschmann|first1=Markus|last2=Dunlop|first2=Jason A.|date=2006|title=A new sea spider (Arthropoda: Pycnogonida) with a flagelliform telson from the Lower Devonian Hunsrück Slate, Germany |journal=Palaeontology|language=en|volume=49|issue=5|pages=983–989|doi=10.1111/j.1475-4983.2006.00583.x |doi-access=free}} 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.{{Cite journal|last1=Kühl|first1=Gabriele|last2=Poschmann|first2=Markus|last3=Rust|first3=Jes|date=May 2013|title=A ten-legged sea spider (Arthropoda: Pycnogonida) from the Lower Devonian Hunsrück Slate (Germany)|url=https://www.researchgate.net/publication/235635210|journal=Geological Magazine|language=en|volume=150|issue=3|pages=556–564|doi=10.1017/S0016756812001033|s2cid=129801458 }} 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. While some analysis placing them within Pantopoda (crown-group pycnogonids),{{Cite journal |last1=Arango |first1=Claudia P. |last2=Wheeler |first2=Ward C. |date=June 2007 |title=Phylogeny of the sea spiders (Arthropoda, Pycnogonida) based on direct optimization of six loci and morphology |journal=Cladistics |language=en |volume=23 |issue=3 |pages=255–293 |doi=10.1111/j.1096-0031.2007.00143.x |pmid=34905863 |s2cid=84031914}} this result is questionable as they have low support value and based on outdated reconstruction of the fossil taxa.{{Cite journal |last1=Brenneis |first1=Georg |last2=Scholtz |first2=Gerhard |date=2014-04-15 |title=The 'Ventral Organs' of Pycnogonida (Arthropoda) Are Neurogenic Niches of Late Embryonic and Post-Embryonic Nervous System Development |journal=PLOS ONE |language=en |volume=9 |issue=4 |pages=e95435 |doi=10.1371/journal.pone.0095435 |doi-access=free |issn=1932-6203 |pmc=3988247 |pmid=24736377}}{{Cite journal |last1=Siveter |first1=Derek J. |last2=Sabroux |first2=Romain |last3=Briggs |first3=Derek E. G. |last4=Siveter |first4=David J. |last5=Sutton |first5=Mark D. |date=2023 |title=Newly discovered morphology of the Silurian sea spider Haliestes and its implications |url=https://onlinelibrary.wiley.com/doi/10.1002/spp2.1528 |journal=Papers in Palaeontology |language=en |volume=9 |issue=5 |pages=e1528 |doi=10.1002/spp2.1528 |issn=2056-2802|hdl=1983/267d44cb-bd22-4a1d-9d00-b3916c453784 |hdl-access=free }}

{{Reflist|30em}}

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Category:Pycnogonid genera

Category:Devonian arthropods of Europe

Category:Fossils of Germany

Category:Hunsrück Slate fossils

Category:Fossil taxa described in 1928

Category:Monotypic prehistoric arthropod genera

Category:Monotypic chelicerate genera