:en:Pseudoniscus

{{Short description|Extinct genus of chelicerate}}

{{Automatic taxobox

| name = Pseudoniscus

| fossil_range = {{fossil range|Silurian}}

| image = 20200921 Pseudoniscus roosevelti.png

| image_caption = Restoration of Pseudoniscus roosevelti

| display_parents = 3

| taxon = Pseudoniscus

| authority = Nieszkowski, 1859

| type_species = †Pseudoniscus aculeatus

| type_species_authority = Nieszkowski, 1859

| subdivision_ranks = Species

| subdivision =

• †P. aculeatus Nieszkowski, 1859
• †P. clarkei Ruedemann, 1916
• †P. falcatus Woodward, 1868
• †P. roosevelti Clarke, 1902

| synonyms =

• Neolimulus Woodward, 1868

}}

Pseudoniscus is a genus of synziphosurine, a paraphyletic group of fossil chelicerate arthropods.{{Cite journal|last1=Bicknell|first1=Russell D. C.|last2=Pates|first2=Stephen|date=2020|title=Pictorial Atlas of Fossil and Extant Horseshoe Crabs, With Focus on Xiphosurida|journal=Frontiers in Earth Science|language=en|volume=8|page=98|doi=10.3389/feart.2020.00098|bibcode=2020FrEaS...8...98B|issn=2296-6463|doi-access=free}} Pseudoniscus was regarded as part of the clade Planaterga.{{Cite journal|last1=Selden|first1=Paul A.|last2=Lamsdell|first2=James C.|last3=Qi|first3=Liu|title=An unusual euchelicerate linking horseshoe crabs and eurypterids, from the Lower Devonian (Lochkovian) of Yunnan, China|url=https://www.academia.edu/14633144|journal=Zoologica Scripta|year=2015|volume=44|issue=6|pages=645–652|doi=10.1111/zsc.12124|s2cid=55264483|issn=0300-3256|doi-access=free}}{{Cite journal|last1=Lamsdell|first1=James C.|last2=Briggs|first2=Derek E. G.|last3=Liu|first3=Huaibao P.|last4=Witzke|first4=Brian J.|last5=McKay|first5=Robert M.|date=2015|title=A new Ordovician arthropod from the Winneshiek Lagerstätte of Iowa (USA) reveals the ground plan of eurypterids and chasmataspidids|url=https://www.researchgate.net/publication/282127302|journal=The Science of Nature|language=en|volume=102|issue=9–10|pages=63|doi=10.1007/s00114-015-1312-5|pmid=26391849|bibcode=2015SciNa.102...63L|s2cid=8153035|issn=0028-1042}}{{Cite journal|last1=Bicknell|first1=Russell D. C.|last2=Lustri|first2=Lorenzo|last3=Brougham|first3=Tom|date=2019-12-01|title=Revision of "Bellinurus" carteri (Chelicerata: Xiphosura) from the Late Devonian of Pennsylvania, USA|journal=Comptes Rendus Palevol|language=en|volume=18|issue=8|pages=967–976|doi=10.1016/j.crpv.2019.08.002|bibcode=2019CRPal..18..967B|issn=1631-0683|doi-access=free}} Fossils of the genus have been discovered in deposits of the Silurian period in the United Kingdom (Patrick Burn Formation), the United States and Estonia. Pseudoniscus is one of the two members of the family Pseudoniscidae, the other being Cyamocephalus.{{Cite journal|last=Anderson|first=Lyall I.|date=1999-01-01|title=A new specimen of the Silurian synziphosurine arthropod Cyamocephalus|url=http://www.sciencedirect.com/science/article/pii/S0016787899800716|journal=Proceedings of the Geologists' Association|language=en|volume=110|issue=3|pages=211–216|doi=10.1016/S0016-7878(99)80071-6|bibcode=1999PrGA..110..211A|issn=0016-7878}}{{cite book|chapter-url=http://www.wsc.nmbe.ch/resources/fossils/Fossils19.0.pdf|title=World Spider Catalog|last1=Dunlop|first1=J. A.|last2=Penney|first2=D.|last3=Jekel|first3=D.|publisher=Natural History Museum Bern|year=2018|chapter=A summary list of fossil spiders and their relatives|pages=1–296}}{{cite journal|last=Lamsdell|first=James C.|year=2013|title=Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura|journal=Zoological Journal of the Linnean Society|volume=167|issue=1|pages=1–27|doi=10.1111/j.1096-3642.2012.00874.x|issn=0024-4082|doi-access=free}}

The prosoma of Pseudoniscus covered by a carapace with recurved posterior margin and pointed genal spines. Most of the dorsal feature on the carapace (e.g. ophthalmic ridges, cardiac lobe) are not significantly expressed. At least some species have a median spine in front of the carapace like the close-related Cyamocephalus. Unlike most other synziphosurines with opisthosoma subdivided into a wide preabdomen and narrow postabdomen, the 10-segmented opisthosoma of Pseudoniscus possess undivided, metameric segmentation{{Cite journal|last1=Eldredge|first1=Niles|last2=Smith|first2=LeGrande|date=1974|title=Revision of the suborder Synziphosurina (Chelicerata, Merostomata) : with remarks on merostome phylogeny. American Museum novitates ; no. 2543|hdl=2246/2745|url=http://digitallibrary.amnh.org/handle/2246/2745|language=en-US}} similar to Pasternakevia.{{Cite web|title=Silurian synziphosurine horseshoe crab Pasternakevia revisited - Acta Palaeontologica Polonica|url=https://www.app.pan.pl/article/item/app20080074.html|access-date=2020-10-02|website=www.app.pan.pl|language=en}} Within Pseudoniscus, P. roosevelti is one of the few synziphosurines that confirmed to have lateral compound eyes, while the remain species lacking unambiguous evidence of it.{{Cite journal|last1=Bicknell|first1=Russell D. C.|last2=Amati|first2=Lisa|last3=Ortega-Hernández|first3=Javier|date=2019-11-14|title=New insights into the evolution of lateral compound eyes in Palaeozoic horseshoe crabs|url=https://www.researchgate.net/publication/335016132|journal=Zoological Journal of the Linnean Society|language=en|volume=187|issue=4|pages=1061–1077|doi=10.1093/zoolinnean/zlz065|issn=0024-4082}}