Megacheira ("great hands") is an extinct class of predatory arthropods that possessed a pair of great appendages,[1] hence the class name as well as the common name "great appendage arthropods". Their taxonomic position is controversial, with studies either considering them stem-group euarthropods, or stem-group chelicerates.[2] Most of them were found in marine environments throughout the world from the lower to middle Cambrian. Megacheirans were important components of several faunas, including the Burgess, Wheeler and Maotianshan Shales Lagerstatten.
The homology between megacheiran great appendages and cephalic appendages of other arthropods had been discussed for decades. There is controversy over whether they are homologous to both dinocaridid (radiodonts and gilled lobopodians) frontal appendages, the frontalmost appendages of Isoxys and chelicerates' chelicerae.[3][4] Based on neuroanatomical evidences, many studies support their homology to chelicerae and first antennae of other arthropods (which are deutocerebral),[5] but not dinocaridid frontal appendages (protocerebral),[6][7][8] A 2020 study suggests that the great appendages of megacheirans and radiodonts are homologous and casts doubt on the validity of suspected neuroanatomical evidences.[9]
The monophyly of Megacheira is uncertain, and megacheirans are either suggested to be stem-group chelicerates or stem-group arthropods,[7] with the former hypothesis based on the chelicerae-like morphology of great appendages[10][3][4] alongside neuroanatomy[5] and reduced labrum[11] resembled to those of a modern chelicerate. As an example of the latter hypothesis, A study in 2020 found that Megacheria was paraphyletic, forming a grade of arthropods more derived than Dinocaridids and Isoxys, but less than crown arthropods (chelicerates + mandibulates), with Jianfengiids being more basal and the Haikoucaris + Yohoia + Leancholiids clade (Cheiromorpha) closer to crown arthropods.[9]
Genera referred to the class include Yohoia, Haikoucaris, Leanchoilia, Alalcomenaeus, Yawunik, Oestokerkus, Jiangfengia, Fortiforceps, Tanglangia, Actaeus, Kootenichela, Worthenella and Sklerolibyon.[12] Parapeytoia, which formerly misinterpreted as a radiodont was later revealed to be a member of this group.[13][14][15] The class might also contain Enalikter aphson described from the Silurian Herefordshire Lagerstätte of the United Kingdom, and Bundenbachiellus giganteus known from the Early Devonian of Germany;[16][17] although the interpretation of these taxa as megacheirans was challenged by Struck et al. (2015).[18] The claim that Enalikter was an annelid was subsequently rebutted by some of the original authors in 2017.[19] Kootenichela has been suggested to be a chimera of various arthropod taxa.[9] Previous inclusion of some "bivalved" genus such as Forfexicaris, Ovalicephalus, and Occacaris to Megacheira was questioned by later investigations.[7]