Megacheira ("great hands", also historically great appendage arthropods) is an extinct class of predatory

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 defined by their possession of spined "great appendages". 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 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, tic ...

. The homology of the great appendages to the cephalic appendages of other arthropods is also controversial. Uncontested members of the group were present in marine environments worldwide from the lower

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

to the upper

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

Morphology

Megacheirans are defined by their possession of uniramous "great appendages", which are their first pair of head appendages. The first one or two proximalmost segments/ podomeres are spineless (it has been argued that the supposed first of the two proximal podomeres is actually an arthrodial membrane), while the remaining 3–4 more distal podomeres each typically bear a single upward pointing spine attached towards the

distal Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...

end of the segment, with the spineless proximal segment/s typically being connected to the spined distal segments by an elbow-like joint, which curled upwards. The great appendages have been interpreted as

raptorial In biology (specifically the anatomy of arthropods), the term ''raptorial'' implies much the same as ''predatory'' but most often refers to modifications of an arthropod leg, arthropod's foreleg that make it function for the grasping of prey whi ...

limbs involved in predation, with those of some genera such as '' Yohoia'' being structurally comparable to the raptorial maxillipeds of

mantis shrimp Mantis shrimp are carnivorous marine crustaceans of the order Stomatopoda (). Stomatopods branched off from other members of the class Malacostraca around 400 million years ago, with more than 520 extant species of mantis shrimp known. All li ...

. The spines on the great appendages of leanchoilid megacheirans such as ''

Leanchoilia ''Leanchoilia'' is a megacheiran marine arthropod known from Cambrian deposits of the Burgess Shale in Canada and the Chengjiang biota of China. Description ''L. superlata'' was about long and had long, whip-like flagellae extending from i ...

'' and '' Yawunik'' are elongated into flagella-like structures, suggesting a sensory role alongside predatory function. The body is divided into the head and the trunk. The

biramous The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip, ...

limbs of megacheirans are homonomous (i.e. having little differentiation from each other), with endopods typically divided into seven segments/podomeres, and paddle-shaped exopods, which are fringed with thin lamellae. The morphology of the terminal

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 ...

segment is variable. The biramous limbs of at least some megacheirans have been suggested bear exites.

Taxonomy

Several subdivisions within the group are recognised including Jianfengiidae (including '' Fortiforceps'', '' Jianfengia'', '' Sklerolibyon'' and possibly '' Parapeytoia'') which are known from the Early Cambrian of China, as well as the Cheiromorpha (containing at least '' Yohoia'', '' Haikoucaris'', and Leanchoiliidae), known with certainty from the Early-Mid Cambrian of North America, China and Australia, which is distinguished from Jianfengiidae by having a fewer number of body segments (20+ in Jianfengiidae, as compared to typically only 11 to 13 in Cheiromorpha). The

monophyly 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 comm ...

of Megacheira is uncertain, with some studies recovering the group as

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 ...

. The latest unambiguous megacheiran is the leanchoiliid '' Lomankus'' from the Upper Ordovician of North America. '' Parapeytoia'' from the Cambrian of China which was formerly misinterpreted as a radiodont was later suggested to be a member of this group. Possible megacheirans include '' Enalikter'' described from the

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 ...

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 Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...

, and '' Bundenbachiellus'' from the Early

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

; due to their possession of great appendage-like cephalic appendages. However, their relationship to megacheirans has been questioned, due to the uncertain homology of their appendages. '' Kootenichela'' has been suggested to be a chimera of various arthropod taxa. Previous inclusion of some "bivalved" genera such as '' Forfexicaris'', '' Ovalicephalus'', and '' Occacaris'' to Megacheira was questioned by later investigations. The Late Cambrian Orsten taxon '' Oelandocaris'' typically considered to be a crustacean relative, has also been suggested in some studies to be a megacheiran.

List of genera

* †'' Tanglangia'' * †'' Oelandocaris''? * †Jianfengiidae ** '' Sklerolibyon'' ** '' Jianfengia'' ** '' Parapeytoia''? ** '' Fortiforceps'' * †Kootenichelidae? ** '' Kootenichela''? ** '' Worthenella''? * †Cheiromorpha ** '' Yohoia'' ** '' Haikoucaris'' ** Leanchoiliida *** Enaliktidae? **** '' Enalikter''? **** '' Bundenbachiellus''? *** Leanchoiliidae **** '' Actaeus'' **** '' Alalcomenaeus'' **** ''

'' **** '' Oestokerkus'' **** '' Yawunik'' **** '' Kanoshoia'' **** '' Lomankus''

Relationship to other arthropods

Megacheirans are either suggested to be stem-group chelicerates or stem-group arthropods, with the former hypothesis based on the

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 ...

-like morphology of the great appendages alongside neuroanatomy and the presence of a reduced labrum resembling those of modern chelicerates, it being argued that chelicerae and the great appendages are homologous structures. Other studies suggest that the megacheirans are stem-group arthropods based on the argument that the great appendages are homologous to the frontal appendages of stem-group arthropods like ''

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 radiodonts. This identity is disputed, with other authors suggesting that the frontal appendages of radiodonts are homologous to the labrum of modern arthropods.

References

{{Authority control Arthropod classes Prehistoric protostome classes