| Synziphosurina Temporal range: [1] | |
|---|---|
 | |
| 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: | Euchelicerata |
| Suborder: | † Synziphosurina Packard 1886 |
| Genera [2] [3] [4] | |
Synziphosurina is a paraphyletic group of chelicerate arthropods previously thought to be basal horseshoe crabs (Xiphosura). [5] It was later identified as a grade composed of various basal euchelicerates, [5] [6] [7] [4] [3] eventually excluded from the monophyletic Xiphosura sensu stricto [5] and only regarded as horseshoe crabs under a broader sense ('Xiphosura' sensu lato). [4] [6] Synziphosurines survived at least since early Ordovician to early Carboniferous in ages, with most species are known from the in-between Silurian strata. [3]
Dorsal morphology and variation of opisthosomal segmentation across synziphosurines.
While Weinbergina and Willwerathia being exceptionally large, [8] most synziphosurines are small arthropods with body length ranging only about a few centimeters long. [9]
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. [3] With the exception of Pseudoniscus [9] and Pasternakevia , [10] the last 3 opisthosomal segments precede the spine-like telson are specialized into a narrow postabdomen (pretelson), [3] while the remaining wider segments referred to as preabdomen. [9] A reduced anteriormost tergite (microtergite) originated from the first opisthosomal segment is observable at least in some genera. [9] [5] 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. [11] [3] Evidences of appendages are scarce, fragmentary known from Anderella parva, [12] Camanchia grovensis [13] and Venustulus waukeshaensis [14] while exceptionally well-documented in Weinbergina opitzi. [15] [3] 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. [5] [16]
As of 2020, at least 13 genera and 20 species were considered to be synziphosurines. [3] 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. [17] [6] [7] [18] [3]