Теңіз шаянкөздерінің тарихи және қазіргі кездегі жағдайы

Order of marine chelicerates
Automatic taxobox
| taxon = Xiphosura
| fossil range = Earliest Hirnantian–Present,
| image = 20200813 Lunataspis aurora. png
| image caption = Restoration of Lunataspis, the oldest known xiphosuran
| image2 = Limulus polyphemus (Atlantic Horseshoe Crab) adult underside (26923436154). jpg
| image2 caption = The extant Atlantic horseshoe crab (Limulus polyphemus)
| authority = Latreille, 1802
| subdivision ranks = Groups
| subdivision = * †Lunataspis , in reference to its sword like tail) 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, Limulidae). They first appeared in the Hirnantian (Late Ordovician). Currently, there are only four living species. Xiphosura contains one suborder, Xiphosurida, and several stem genera. The group has hardly changed in appearance in hundreds of millions of years; the modern horseshoe crabs look almost identical to prehistoric genera and are considered to be living fossils. The most notable difference between ancient and modern forms is that the abdominal segments in present species are fused into a single unit in adults. Xiphosura were historically placed in the class Merostomata, although this term was intended to encompass also the eurypterids, whence it denoted what is now known to be an unnatural (paraphyletic) group (although this is a grouping recovered in some recent cladistic analyses). Although the name Merostomata is still seen in textbooks, without reference to the Eurypterida, some have urged that this usage should be discouraged. The Merostomata label originally did not include Eurypterida, although they were added in as a better understanding of the extinct group evolved. Now Eurypterida is classified within Sclerophorata together with the arachnids, and therefore, Merostomata is now a synonym of Xiphosura. Several recent phylogenomic studies place Xiphosura within Arachnida, often as the sister group of Ricinulei; included among them are taxonomically comprehensive analyses of both morphology and genomes, which have recovered Merostomata as a derived clade of arachnids.

Modern xiphosurans reach up to in adult length, but the Paleozoic species were often far smaller, some as small as long. Their bodies are divided into an anterior prosoma and a posterior opisthosoma, or abdomen. The upper surface of the prosoma is covered by a semicircular carapace, while the underside bears five pairs of walking legs and a pair of pincer like chelicerae. The mouth is located on underside of the center of the prosoma, between the bases of the walking legs, and lies behind a lip like structure called the labrum. The exoskeleton consist of a tough cuticle, but do not contain any crystalline biominerals. Like scorpions, xiphosurans have an exocuticular layer of hyaline which exhibits UV fluorescence. Xiphosurans have up to four eyes, located in the carapace. Two compound eyes are on the side of the prosoma, with one or two median ocelli towards the front. The compound eyes are simpler in structure than those of other arthropods, with the individual ommatidia not being arranged in a compact pattern. They can probably detect movement, but are unlikely to be able to form a true image. In front of the ocelli is an additional organ that probably functions as a chemoreceptor. These are thought to be vestiges of the limbs of an absorbed first opisthosomal segment. The underside of the opisthosoma carries the genital openings and five pairs of flap like gills. Through a series of successive moults, the larva develops additional gills, increases the length of its caudal spine, and gradually assumes the adult form. Modern xiphosurans reach sexual maturity after about three years of growth. No xiphosurans are known from the following Silurian. Xiphosurida first appears during the late Devonian. A major radiation of freshwater xiphosurids, the Belinuridae is known from the Carboniferous, with the oldest representatives of the modern family Limulidae also possibly appearing during this time, though they only appear in abundance during the Triassic. Another major radiation of freshwater xiphosurans, the Austrolimulidae, is known from the Permian and Triassic. All extant species are estimated to have diverged from a common ancestor that lived about 135 million years ago in the early Cretaceous. As a group they have never showed much diversity in regard of species. Less than 50 fossil species are known from the Carboniferous period, when they were at their most diverse.

Cladogram after Lasmdell 2020.