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,[4] or the chelicerae of chelicerates,[2][5] although the latter possibility had been discounted by some authors.[6][7] Later observations reject the homology of this two type of "great 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, a 2020 study suggested that the great/frontal appendages of Megacheirans, Isoxyids and Radiodontans to be homologous, and that the presence of a great appendage is the ancestral condition for Euarthropoda.[9]

References
  1. Ortega-Hernández, Javier; Janssen, Ralf; Budd, Graham E. (2017-05-01). "Origin and evolution of the panarthropod head – A palaeobiological and developmental perspective". Arthropod Structure & Development. Evolution of Segmentation. 46 (3): 354–379. doi:10.1016/j.asd.2016.10.011. ISSN 1467-8039. PMID 27989966.
  2. Haug, Joachim T.; Waloszek, Dieter; Maas, Andreas; Liu, Yu; Haug, Carolin (March 2012). "Functional morphology, ontogeny and evolution of mantis shrimp-like predators in the Cambrian: MANTIS SHRIMP-LIKE CAMBRIAN PREDATORS". Palaeontology. 55 (2): 369–399. doi:10.1111/j.1475-4983.2011.01124.x.
  3. Aria, Cédric; Caron, Jean-Bernard; Gaines, Robert (2015). "A large new leanchoiliid from the Burgess Shale and the influence of inapplicable states on stem arthropod phylogeny". Palaeontology. 58 (4): 629–660. doi:10.1111/pala.12161.
  4. Stein, Martin (2010). "A new arthropod from the Early Cambrian of North Greenland, with a 'great appendage'-like antennula". Zoological Journal of the Linnean Society. 158 (3): 477–500. doi:10.1111/j.1096-3642.2009.00562.x.
  5. Dornbos, Stephen; Bottjer, David; Chen, Jun-Yuan (2004). "Evidence for seafloor microbial mats and associated metazoan lifestyles in Lower Cambrian phosphorites of Southwest China" (PDF). Lethaia. 37 (2): 127–137. doi:10.1080/00241160410004764.
  6. Jager, Muriel; Murienne, Jérôme; Clabaut, Céline; Deutsch, Jean; Le Guyader, Hervé; Manuel, Michaël (2006). "Homology of arthropod anterior appendages revealed by Hox gene expression in a sea spider". Nature. 441 (7092): 506–508. Bibcode:2006Natur.441..506J. doi:10.1038/nature04591. PMID 16724066.
  7. Myers, PZ (May 26, 2006). "Chelifores, chelicerae, and invertebrate evolution". Pharyngula.
  8. Peiyun Cong; Xiaoya Ma; Xianguang Hou; Gregory D. Edgecombe; Nicholas J. Strausfeld (2014). "Brain structure resolves the segmental affinity of anomalocaridid appendages". Nature. 513 (7519): 538–42. doi:10.1038/nature13486. PMID 25043032.
  9. Aria, Cédric; Zhao, Fangchen; Zeng, Han; Guo, Jin; Zhu, Maoyan (December 2020). "Fossils from South China redefine the ancestral euarthropod body plan". BMC Evolutionary Biology. 20 (1): 4. doi:10.1186/s12862-019-1560-7. ISSN 1471-2148. PMC 6950928. PMID 31914921.


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