Synziphosurine
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Synziphosurina is a
paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
group of
chelicerate The subphylum Chelicerata (from Neo-Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. Chelicerates include the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks ...
arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...
s previously thought to be basal horseshoe crabs (
Xiphosura Xiphosura (; , in reference to its sword-like telson) is an order of arthropods related to arachnids. They are more commonly known as horseshoe crabs (a name applied more specifically to the only extant family, Horseshoe crab, Limulidae). They fir ...
). It was later identified as a grade composed of various basal euchelicerates, eventually excluded from the
monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...
Xiphosura ''sensu stricto'' and only regarded as horseshoe crabs under a broader sense ('Xiphosura' ''sensu lato''). Synziphosurines survived at least since
early Ordovician The Early Ordovician is the first epoch (geology), epoch of the Ordovician period, corresponding to the Lower Ordovician series (stratigraphy), series of the Ordovician system. It began after the Cambrian Stage 10, Age 10 of the Furongian epoch o ...
to
early Carboniferous Early may refer to: Places in the United States * Early, Iowa, a city * Early, Texas, a city * Early Branch, a stream in Missouri * Early County, Georgia * Fort Early, Georgia, an early 19th century fort Music * Early B, stage name of Jamaican d ...
in ages, with most species are known from the in-between
Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...
strata. File:20200920 Synziphosurina dorsal segmentation.png, 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 The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. (The terms ''prosoma'' and ''opisthosoma'' are equivalent to ''cepha ...
covered by a dome-like
carapace A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...
(prosomal dorsal shield) and an
opisthosoma The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma ( cephalothorax). It is a distinctive feature of the subphylum Chelicerata (arachnids, horseshoe crabs and others). Although it is similar in most respects ...
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 The telson () is the hindmost division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment (biology), segment of the arthropod body, or an additional division that is not a true segm ...
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 ''Weinbergina'' is a genus of synziphosurine, a paraphyletic group of fossil chelicerate arthropods. Fossils of the single and type species, ''W. opitzi'', have been discovered in deposits of the Devonian period in the Hunsrück Slate, Germany ...
opitzi'') or clear (e.g. '' Legrandella lombardii'') evidences of lateral
compound eye A compound eye is a Eye, visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidium, ommatidia, which are tiny independent photoreception units that consist of a cornea, lens (anatomy), lens, and p ...
s on their carapaces. Evidences of
appendages An appendage (or outgrowth) is an external body part or natural prolongation that protrudes from an organism's body such as an arm or a leg. Protrusions from single-celled bacteria and archaea are known as cell-surface appendages or surface app ...
are scarce, fragmentary known from ''
Anderella ''Anderella'' is a genus of synziphosurine, a paraphyletic group of fossil chelicerate arthropods. ''Anderella'' was regarded as part of the clade Prosomapoda. Fossils of the single and type species, ''A. parva'', have been discovered in deposit ...
parva'', '' Camanchia grovensis'' and '' Venustulus waukeshaensis'' while exceptionally well-documented in ''
Weinbergina ''Weinbergina'' is a genus of synziphosurine, a paraphyletic group of fossil chelicerate arthropods. Fossils of the single and type species, ''W. opitzi'', have been discovered in deposits of the Devonian period in the Hunsrück Slate, Germany ...
opitzi''. The prosoma possess a pair of
chelicerae The chelicerae () are the arthropod mouthparts, mouthparts of the subphylum Chelicerata, an arthropod group that includes arachnids, horseshoe crabs, and sea spiders. Commonly referred to as "jaws", chelicerae may be shaped as either articulated ...
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 ''Offacolus'' is an extinct genus of euchelicerate, a group of chelicerate arthropods. Its only species, ''O. kingi'', has been found in deposits from the Silurian period ( Homerian epoch) in the Wenlock Series Lagerstätte of Herefordshire, ...
'' and ''
Dibasterium ''Dibasterium'' is an extinct genus of euchelicerate, a group of chelicerate arthropods. Fossils of the single and type species, ''D. durgae'', have been discovered in the Coalbrookdale Formation of the Middle Silurian period (Homerian age) in H ...
'', the questionable genus ''
Borchgrevinkium ''Borchgrevinkium'' is an extinct genus of chelicerate arthropod. A fossil of the single and type species, ''B. taimyrensis'', has been discovered in Deposition (geology), deposits of the Early Devonian, Devonian period (Lochkovian, Lochkovian e ...
'', as well as the
Dekatriata Dekatriata is a clade of planatergan chelicerates including the groups Arachnida, Chasmataspidida, Eurypterid Eurypterids, often informally called sea scorpions, are a group of extinct marine arthropods that form the Order (biology), order E ...
-related '' Houia'' and '' Winneshiekia'', may also regarded as members of synziphosurines in some literatures.


References

{{Taxonbar, from=Q7662951 Xiphosura Paraphyletic groups Early Ordovician first appearances Carboniferous extinctions