Synziphosurina

Synziphosurina is a paraphyletic group of chelicerate arthropods previously thought to be basal horseshoe crabs (Xiphosura).[4] It was later identified as a grade compose of various basal euchelicerates,[4][5][6][3][2] eventually excluded form the monophyletic Xiphosura sensu stricto[4] and only regarded as horseshoe crabs under a broader sense ('Xiphosura' sensu lato).[3][5] Synziphosurines survived at least since early Ordovician to early Carboniferous in ages, with most species are known from the in-between Silurian strata.[2]

Fossil of Weinbergina opitzi with preserved ventral appendages.

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  Dorsal morphology and variation of opisthosomal segmentation across synziphosurines.

Size comparison of various synziphosurines.

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
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Chelicerata
Clade:	Prosomapoda
Suborder:	†Synziphosurina Packard 1886
Genus[1][2][3]
Anderella Bembicosoma Bunaia Bunodes Camanchia Cyamocephalus Legrandella Limuloides Pasternakevia Pseudoniscus Venustulus Weinbergina Willwerathia

While Weinbergina and Willwerathia being exceptionally large,[7] most synziphosurines are small arthropods with body length ranging only about a few centimeters long.[8]

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.[2] With the exception of Pseudoniscus[8] and Pasternakevia,[9] the last 3 opisthosomal segments precede the spine-like telson are specialized into a narrow postabdomen (pretelson),[2] while the remaining wider segments referred to as preabdomen.[8] A reduced anteriormost tergite (microtergite) originated from the first opisthosomal segment is observable at least in some genera.[8][4] 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.[10][2] Evidences of appendages are scarce, fragmentary known from Anderella parva,[11] Camanchia grovensis[12] and Venustulus waukeshaensis[13] while exceptionally well-documented in Weinbergina opitzi.[14][2] 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.[4][15]

As of 2020, at least 13 genera and 20 species were considered to be synziphosurines.[2] 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.[16][5][6][17][2]