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Synziphosurina Temporal range: Early Ordovician–Mississippian PreꞒ Ꞓ O S D C P T J K Pg N
Venustulus waukeshaensis (top left), Cyamocephalus loganensis (top right), Weinbergina opitzi (center), Limuloides limuloides (bottom left), and Legrandella lombardii (bottom right).
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Chelicerata
Clade:	Euchelicerata
Suborder:	†Synziphosurina Packard 1886
Genera
Anderella Bembicosoma Bunaia Bunodes Camanchia Cyamocephalus Legrandella Limuloides Pasternakevia Pseudoniscus Venustulus Weinbergina Willwerathia

Synziphosurina is a paraphyletic group of chelicerate arthropods previously thought to be basal horseshoe crabs (Xiphosura). It was later identified as a grade composed of various basal euchelicerates, eventually excluded form the monophyletic Xiphosura sensu stricto and only regarded as horseshoe crabs under a broader sense ('Xiphosura' sensu lato). Synziphosurines survived at least since early Ordovician to early Carboniferous in ages, with most species are known from the in-between Silurian strata.

• Dorsal morphology and variation of opisthosomal segmentation across synziphosurines.

While Weinbergina and Willwerathia being exceptionally large, most synziphosurines are small arthropods with body length ranging only about a few centimeters long.

The body of synziphosurine composed of a prosoma covered by a dome-like carapace (prosomal dorsal shield) and an opisthosoma with usually unfused 9-11 segments expressed by tergites. With the exception of Pseudoniscus and Pasternakevia, the last 3 opisthosomal segments precede the spine-like telson are specialized into a narrow postabdomen (pretelson), while the remaining wider segments referred to as preabdomen. A reduced anteriormost tergite (microtergite) originated from the first opisthosomal segment is observable at least in some genera. Most synziphosurines are possibly blind, with only a few species showing possible (e.g. Weinbergina opitzi) or clear (e.g. Legrandella lombardii) evidences of lateral compound eyes on their carapaces. Evidences of appendages are scarce, fragmentary known from Anderella parva, Camanchia grovensis and Venustulus waukeshaensis while exceptionally well-documented in Weinbergina opitzi. The prosoma possess a pair of chelicerae and at least 5 pairs of walking legs while the opisthosoma probably has 6 pairs of plate-like opercula.

As of 2020, at least 13 genera and 20 species were considered to be synziphosurines. The even basal euchelicerates Offacolus and Dibasterium, the questionable genus Borchgrevinkium, as well as the Dekatriata-related Houia and Winneshiekia, may also regarded as members of synziphosurines in some literatures.

References

1. W. Krzemiński, E. Krzemińska, and D. Wojciechowski. 2010. Silurian synziphosurine horseshoe crab Pasternakevia revisited. Acta Palaeontologica Polonica 55(1):133-139
2. ^ Bicknell, Russell D. C.; Pates, Stephen (2020). "Pictorial Atlas of Fossil and Extant Horseshoe Crabs, With Focus on Xiphosurida". Frontiers in Earth Science. 8: 98. Bibcode:2020FrEaS...8...98B. doi:10.3389/feart.2020.00098. ISSN 2296-6463.
3. ^ Dunlop, J. A.; Penney, D.; Jekel, D. (2020). "A summary list of fossil spiders and their relatives" (PDF). World Spider Catalog. Natural History Museum Bern. pp. 1–296.
4. ^ Lamsdell, James C. (2013-01-01). "Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura". Zoological Journal of the Linnean Society. 167 (1): 1–27. doi:10.1111/j.1096-3642.2012.00874.x. ISSN 0024-4082.
5. ^ Selden, Paul A.; Lamsdell, James C.; Qi, Liu (2015). "An unusual euchelicerate linking horseshoe crabs and eurypterids, from the Lower Devonian (Lochkovian) of Yunnan, China". Zoologica Scripta. 44 (6): 645–652. doi:10.1111/zsc.12124. ISSN 0300-3256. S2CID 55264483.
6. ^ Lamsdell, James C.; Briggs, Derek E. G.; Liu, Huaibao P.; Witzke, Brian J.; McKay, Robert M. (2015). "A new Ordovician arthropod from the Winneshiek Lagerstätte of Iowa (USA) reveals the ground plan of eurypterids and chasmataspidids". The Science of Nature. 102 (9–10): 63. Bibcode:2015SciNa.102...63L. doi:10.1007/s00114-015-1312-5. ISSN 0028-1042. PMID 26391849. S2CID 8153035.
7. Anderson, Lyall I.; Poschmann, Markus; Brauckmann, Carsten (1998). "On the Emsian (Lower Devonian) arthropods of the Rhenish Slate Mountains: 2. The synziphosurine Willwerathia". Paläontologische Zeitschrift. 72 (3–4): 325–336. doi:10.1007/BF02988363. ISSN 0031-0220. S2CID 128464147.
8. ^ Eldredge, Niles; Smith, LeGrande (1974). "Revision of the suborder Synziphosurina (Chelicerata, Merostomata) : with remarks on merostome phylogeny. American Museum novitates ; no. 2543". hdl:2246/2745. {{cite journal}}: Cite journal requires |journal= (help)
9. Krzemiński, Wiesław; Krzemińska, Ewa; Wojciechowski, Dariusz (2010). "Silurian Synziphosurine Horseshoe Crab Pasternakevia Revisited". Acta Palaeontologica Polonica. 55 (1): 133–139. doi:10.4202/app.2008.0074. ISSN 0567-7920.
10. Bicknell, Russell D. C.; Amati, Lisa; Ortega-Hernández, Javier (2019-11-14). "New insights into the evolution of lateral compound eyes in Palaeozoic horseshoe crabs". Zoological Journal of the Linnean Society. 187 (4): 1061–1077. doi:10.1093/zoolinnean/zlz065. ISSN 0024-4082.
11. Moore, Rachel A.; McKENZIE, Scott C.; Lieberman, Bruce S. (2007). "A Carboniferous Synziphosurine (xiphosura) from the Bear Gulch Limestone, Montana, Usa". Palaeontology. 50 (4): 1013–1019. doi:10.1111/j.1475-4983.2007.00685.x. ISSN 1475-4983. S2CID 59449700.
12. MOORE, RACHEL A.; BRIGGS, DEREK E. G.; BRADDY, SIMON J.; SHULTZ, JEFFREY W. (2011). "Synziphosurines (Xiphosura: Chelicerata) from the Silurian of Iowa". Journal of Paleontology. 85 (1): 83–91. doi:10.1666/10-057.1. ISSN 0022-3360. JSTOR 23019501. S2CID 115132351.
13. Moore, Rachel A.; Briggs, Derek E. G.; Braddy, Simon J.; Anderson, Lyall I.; Mikulic, Donald G.; Kluessendorf, Joanne (2005). "A new synziphosurine (Chelicerata : Xiphosura) from the Late Llandovery (Silurian) Waukesha Lagerstatte, Wisconsin, USA". Journal of Paleontology. 79 (2): 242–250. doi:10.1666/0022-3360(2005)079<0242:ANSCXF>2.0.CO;2. ISSN 0022-3360. S2CID 56570105.
14. Moore, Rachel A.; Briggs, Derek E. G.; Bartels, Christoph (2005). "A new specimen ofWeinbergina opitzi (Chelicerata: Xiphosura) from the Lower Devonian Hunsriick Slate, Germany". Paläontologische Zeitschrift. 79 (3): 399–408. doi:10.1007/BF02991931. ISSN 0031-0220. S2CID 84994966.
15. Dunlop, Jason A.; Lamsdell, James C. (2017). "Segmentation and tagmosis in Chelicerata". Arthropod Structure & Development. 46 (3): 395–418. doi:10.1016/j.asd.2016.05.002. ISSN 1467-8039. PMID 27240897.
16. Briggs, Derek E. G.; Siveter, Derek J.; Siveter, David J.; Sutton, Mark D.; Garwood, Russell J.; Legg, David (2012-09-25). "Silurian horseshoe crab illuminates the evolution of arthropod limbs". Proceedings of the National Academy of Sciences. 109 (39): 15702–15705. Bibcode:2012PNAS..10915702B. doi:10.1073/pnas.1205875109. ISSN 0027-8424. PMC 3465403. PMID 22967511.
17. Bicknell, Russell D.C.; Lustri, Lorenzo; Brougham, Tom (2019). "Revision of "Bellinurus" carteri (Chelicerata: Xiphosura) from the Late Devonian of Pennsylvania, USA". Comptes Rendus Palevol. 18 (8): 967–976. doi:10.1016/j.crpv.2019.08.002.

• Synziphosurina
• Xiphosura
• Paraphyletic groups
• Early Ordovician first appearances
• Carboniferous extinctions