Megacheira ("great hands", also historically great appendage arthropods) is an extinct class of predatory defined by their possession of spined "great appendages".
Morphology
Megacheirans are defined by their possession of
uniramous (unbranched) "great appendages", which are their first pair of head appendages. The first one or two proximalmost segments/
closest towards the body 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 towards the ends of the limbs each typically bear a single upward pointing spine attached towards the
distal 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 limbs involved in predation, with those of some genera such as
Yohoia being structurally comparable to the raptorial maxillipeds of
mantis shrimp.
The spines on the great appendages of leanchoilid megacheirans such as
Leanchoilia 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 (divided into two branches) limbs of megacheirans are homonomous (i.e. having little differentiation from each other), with
(the lower, leg-like branches) typically divided into seven segments/podomeres, and paddle-shaped
(the upper limb branches), which are fringed with thin lamellae. The morphology of the terminal
telson segment is variable.
The biramous limbs of at least some megacheirans have been suggested bear
.
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 of Megacheira is uncertain, with some studies recovering the group as
paraphyletic.
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.
List of genera
Relationship to other arthropods
Megacheirans are either suggested to be stem-group
or stem-group arthropods,
with the former hypothesis based on the
chelicerae-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 and
radiodonts. This identity is disputed, with other authors suggesting that the frontal appendages of radiodonts are homologous to the labrum of modern arthropods.
