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 animal's head.[1]

Reconstruction of Yawunik kootenayi, showing great appendages with elongated flagella.

Great appendages of various megacheirans, featured by Leanchoilia (A), Yawunik (B), Parapeytoia (C), Fortiforceps (D), Yohoia (E,F) and Haikoucaris (G).

In general, megacheiran's great appendage have 6 segments, with the promixal two segments forming a peduncle and the finger-like distal four segments forming a claw, both connect by an elbow joint.[2] Great appendages are interpret as raptorial limbs involved in predation, with those of some genus such as Yohoia are structurally comparable to the raptorial maxillipeds of mantis shrimp.[2] While the great appendages of leanchoilid megacheirans such as Leanchoilia and Yawunik have elongated flagella, suggest a sensory role alongside predatory function.[3]

Radiodonts such as Anomalocaris have multi-segmented frontal appendages, which are suggested to be either homologous or non-homologous with the megacheiran's great appendages.

Radiodont's frontal appendages have controversial relationships to those of the megacheirans. They have been suggested to be homologous with the antennule of the euarthropods ("true" arthropods that differ from the stem-groups like radiodonts),[4] or more specifically the chelicerae of chelicerates,[2][5] although the latter possibility had been discounted by some authors.[6][7] As of late 2010s, most studies reject the homology of this two type of frontalmost appendages, with neural structures clarifying the segmental affinities of radiodont frontal appendages as protocerebral (homologous to euarthropod's labrum),[8] and megacheiran's short-great appendages as deutocerebral (homologous to euarthropod's antennule and chelicerae).[1] Similar appendages also found in other Cambrian arthropods such as Isoxys and Occacaris, but their segmental affinities remain conjectural.[1] On the other hand, some 2020 studies suggested that the frontalmost appendages of radiodonts and euarthropods to be homologous, and that the presence of a raptorial, deutocerebral frontalmost appendage is the ancestral condition for euarthropod itself.[9][10] Under this scenario, the euarthropod Kylinxia zhangi with frontalmost appendages that possess strong resemblance to those of a radiodont may represent an intermediate form between stem and true arthropods.[10]