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Chelicerata split from Mandibulata by the mid-Cambrian, as evidenced by stem-group chelicerates like Habeliida and Mollisonia present by this time. [2] The surviving marine species include the four species of xiphosurans (horseshoe crabs), and possibly the 1,300 species of pycnogonids (sea spiders), if the latter are indeed chelicerates.
Merostomata is a class of chelicerate arthropods that contains the extinct Eurypterida (sea scorpions) and the extant Xiphosura (horseshoe crabs). The term was originally used by James Dwight Dana to refer to Xiphosura only, but was emended by Henry Woodward to cover both groups.
The jumping spider Phidippus audax.The basal parts of the chelicerae are the two iridescent green mouthparts. The chelicerae (/ k ə ˈ l ɪ s ər iː /) are the mouthparts of the subphylum Chelicerata, an arthropod group that includes arachnids, horseshoe crabs, and sea spiders.
The name "Mandibulata" was originally used for a subgroup of insects by Joseph Philippe de Clairville in 1798. [7] In the 1930s, Robert Evans Snodgrass used the name to encompass myriapods, hexapods and crustaceans, which he considered to be united by a number of morphological simlarities, including but not limited to the presence of mandibles. [8]
The clade is united by the presence of various characteristics. These include an elongated dorsal exoskeleton, seven articulating tergites and similarly sized cephalic and pygidial shields, [ 1 ] in addition to only having three pairs of walking legs and the rest of the limbs being used as gills (which likely places them nearest to ...
The Crustacea were generally considered the closest relatives of the Uniramia, and sometimes these were united as Mandibulata.However, the competing hypothesis — that Crustacea and Hexapoda form a monophyletic group, the Pancrustacea, to which the Myriapoda are the closest relatives — has support from molecular and fossil evidence.
Tetrapulmonata is a non-ranked supra-ordinal clade of arachnids.It is composed of the extant orders Uropygi (whip scorpions), Schizomida (short-tailed whip scorpions), Amblypygi (tail-less whip scorpions) and Araneae (spiders).
Limulus is a genus of horseshoe crab, with one extant species, the Atlantic horseshoe crab (Limulus polyphemus).One fossil species is currently assigned to the genus though several other species have been named, which have since been assigned to other genera.