Megacheira ("great hands") is an extinct class of predatory

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...

s that possessed a pair of

great appendage Great appendages are claw-like appendages which attach to the heads of the "great appendage arthropods", a name usually refers to Megacheira, a class of extinct arthropod characterized by a pair of "short-great appendages" bearing in front of the ...

s, 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 In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

euarthropods, or stem-group

chelicerates The subphylum Chelicerata (from New Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks, and mite ...

. 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

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and

Maotianshan Shales The Maotianshan Shales are a series of Early Cambrian deposits in the Chiungchussu Formation, famous for their '' Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized organisms or traces. The Maotianshan Shales ...

Lagerstatten.

Homology of great appendages

The

homology Homology may refer to: Sciences Biology *Homology (biology), any characteristic of biological organisms that is derived from a common ancestor * Sequence homology, biological homology between DNA, RNA, or protein sequences *Homologous chrom ...

between megacheiran

s and cephalic appendages of other arthropods had been discussed for decades. There is controversy over whether they are homologous to both dinocaridid (

radiodont Radiodonta is an extinct order of stem-group arthropods that was successful worldwide during the Cambrian period. They may be referred to as radiodonts, radiodontans, radiodontids, anomalocarids, or anomalocaridids, although the last two original ...

s and gilled lobopodians) frontal appendages, the frontalmost appendages of ''

Isoxys ''Isoxys'' (meaning "equal surfaces") is a genus of extinct bivalved Cambrian arthropod; the various species of which are thought to have been freely swimming predators. It had a pair of large spherical eyes (which are the most commonly preserve ...

'' and chelicerates'

chelicerae The chelicerae () are the 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 fangs, or similarl ...

. Based on neuroanatomical evidences, many studies support their homology to chelicerae and first antennae of other arthropods (which are deutocerebral), but not dinocaridid frontal appendages (protocerebral), A 2020 study suggests that the great appendages of megacheirans and radiodonts are homologous and casts doubt on the validity of suspected neuroanatomical evidences.

Taxonomy

The

monophyly In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

of Megacheira is uncertain, and megacheirans are either suggested to be stem-group

chelicerate The subphylum Chelicerata (from New Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks, and mite ...

s or stem-group arthropods, with the former hypothesis based on the chelicerae-like morphology of great appendages alongside neuroanatomy and reduced labrum 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 ''

'', but less than

crown A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, partic ...

arthropods (chelicerates + mandibulates), with Jianfengiids being more basal and the '' Haikoucaris'' + ''

Yohoia ''Yohoia'' is an extinct genus of megacheiran arthropod from the Cambrian period that has been found as fossils in the Burgess Shale formation of British Columbia, Canada. It has been placed among the arachnomorpha, a group of arthropods that in ...

'' + Leancholiids clade (Cheiromorpha) closer to crown arthropods. Genera referred to the class include ''

'', '' Haikoucaris'', ''

Leanchoilia ''Leanchoilia'' is an megacheiran arthropod known from Cambrian deposits of the Burgess Shale in Canada and the Chengjiang biota of China. It was about long and had long, whip-like feelers mounted on frontal arm-like appendages. Its internal o ...

'', ''

Alalcomenaeus ''Alalcomenaeus'' is one of the most widespread and longest-surviving arthropod genera of the Early and Middle Cambrian. Known from over 300 specimens in the Burgess Shale and the Chengjiang biota. it is a member of the group Megacheira. Morph ...

'', ''

Yawunik ''Yawunik'' is an extinct genus of Cambrian megacheiran ("Great appendage" arthropod) known from the Burgess Shale in Canada (Marble Canyon locality). The type species has been named ''Yawunik kootenayi'' after the Kootenay, both a geographic a ...

'', '' Oestokerkus'', '' Jiangfengia'', '' Fortiforceps'', '' Tanglangia'', ''

Actaeus In Greek mythology, Actaeus (; Ancient Greek: Ἀκταῖος ''Ἀktaῖos'' means "coast-man"), also called Actaeon, was the first king of Attica, according to Pausanias. Family Actaeus was the father of Aglaurus, and father-in-law to Cecr ...

'', ''

Kootenichela ''Kootenichela deppi'' is an extinct arthropod described from the Middle Cambrian of the Kootenay National Park, Canada. It is originally considered to be a member of "great appendage arthropods", although subsequent studies questioned its affini ...

'', ''

Worthenella ''Worthenella'' is a genus of stem-group arthropod from the Burgess Shale. Charles Doolittle Walcott named the species in 1911. In 2013, David Legg placed it in the family Kootenichelidae as a sister to ''Kootenichela ''Kootenichela deppi'' i ...

'' and '' Sklerolibyon''. '' Parapeytoia'', which formerly misinterpreted as a

was later revealed to be a member of this group. The class might also contain '' Enalikter aphson'' described from the Silurian

Herefordshire Lagerstätte Coalbrookdale Formation, earlier known as Wenlock Shale or Wenlock Shale Formation and also referred to as Herefordshire Lagerstätte in palaeontology, is a fossil-rich deposit ('' Konservat-Lagerstätte'') in Powys and Herefordshire at the Engla ...

of the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the European mainland, continental mainland. It comprises England, Scotlan ...

, and '' Bundenbachiellus giganteus'' known from the Early Devonian of

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...

; although the interpretation of these taxa as megacheirans was challenged by Struck ''et al.'' (2015). The claim that ''Enalikter'' was an annelid was subsequently rebutted by some of the original authors in 2017. ''

'' has been suggested to be a

chimera Chimera, Chimaera, or Chimaira (Greek for " she-goat") originally referred to: * Chimera (mythology), a fire-breathing monster of Ancient Lycia said to combine parts from multiple animals * Mount Chimaera, a fire-spewing region of Lycia or Cilici ...

of various arthropod taxa. Previous inclusion of some "bivalved" genus such as '' Forfexicaris'', '' Ovalicephalus'', and ''

Occacaris ''Occacaris oviformis'' is an extinct nektonic predatory arthropod from the Lower Cambrian Maotianshan shale Lagerstätte. It bears a superficial resemblance to the Cambrian arthropod, '' Canadaspis'', though, was much smaller, and had a pair of ...

'' to Megacheira was questioned by later investigations.

External links

{{Authority control Arthropod classes Prehistoric protostome classes