Fuxianhuia

Taxobox
name = "Fuxianhuia protensa"
fossil_range = Lower Cambrian

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regnum = Animalia
phylum = Arthropoda
classis = Yunnanata†
ordo = Fuxianhuiida
familia = Fuxianhuiidae
genus = "Fuxianhuia"
species = "F. protensa"
binomial = "Fuxianhuia protensa"
binomial_authority = Hou, 1987

"Fuxianhuia protensa" is a Lower Cambrian fossil arthropod known from the Chengjiang Fauna in China. Its purportedly primitive features have led to its playing a pivotal role in discussions about the euarthropod stem group. Nevertheless, despite being known from many specimens, disputes about its morphology, in particular its head appendages, have made it one of the most controversial of the Chengjiang taxa, and it has been discussed extensively in the context of the arthropod head problem.

Morphology

Complete "Fuxianhuia" specimens are approximately 3 centimetres long. The anterior of "Fuxianhuia" is marked by an oval sclerite, from which fifty-two stalked eyes emerge. Inserting directly behind this sclerite, on the head shield proper, are six stout antennae. When the head of "Fuxianhuia" was originally described, twelve additional head appendages, the "sub-chelate" pair were also described. [Chen, J.-y., Edgecombe, G. D., Ramsköld, L. & Zhou, G.-q. (1995). Head segmentation in Early Cambrian "Fuxianhuia": implications for arthropod evolution. "Science" 268, 1339–1343.] These are geniculate, backward-pointing appendages that lie in a highly stereotypical position (i.e., their position does not vary much from one specimen to another). Partly because of this, and partly because of their rather indistinct morphology, their status as appendages has been questioned. Indeed, on the grounds that these structures seem to lie between two cuticular layers, Waloszek and colleagues have suggested that they are not appendages at all, but rather gut diverticulae; [Waloszek, D., Chen, J., Maas, A. & Wang, X. (2005). Early Cambrian arthropods – new insights into arthropod head and structural evolution. "Arthropod Structure and Development" 34, 189–205.] a reassignment that has however not been universally accepted. [Scholtz, G. & Edgecombe, G. D. (2006). The evolution of arthropod heads: reconciling morphological, developmental and palaeontological evidence. "Development, Genes and Evolution" 216, 395–415.] Ventrally, a large plate has been interpreted as a hypostome.

The head shield overlaps a tapering series of 12–17 trunk tergites, which lead into a set of limb-bearing segments comprising the thorax. The limbs are simple in form, consisting of a smooth oval exopod and a stout, annulated endopod. There is no one-to-one correspondence between the thoracic tergites and the limbs, but, rather, there appear to be two or three limbs per tergite.

Behind the thorax is a narrower abdominal region consisting of 14 tergites that bears no appendages. The abdomen is terminated by a telson-like spine.

Classification

"Fuxianhuia" was first described from incomplete material, [Hou, X.-g. (1987). Three new large arthropods from Lower Cambrian, Chengjiang, Eastern Yunnan [in Chinese] . "Acta Palaeontolologica Sinica" 26, 272–285.] and its true nature did not become apparent until the head and limbs were discovered. Its articulated head region, lack of tergite-segment correspondence and undifferentiated limbs have all been taken to indicate a very basal position in the arthropods, [Hou, X.-g. & Bergström, J. (1997). Arthropods of the Lower Cambrian Chengjiang fauna, southwest China. "Fossils & Strata" 45, 1–116.] [Budd, G. E. (2002). A palaeontological solution to the arthropod head problem. "Nature" 417, 271–275. ] [Chen, J.-y., Edgecombe, G. D., Ramsköld, L. & Zhou, G.-q. (1995). Head segmentation in Early Cambrian "Fuxianhuia": implications for arthropod evolution. "Science" 268, 1339–1343.] even though an early cladistic analysis suggested, rather, that it was a stem-group chelicerate. [Wills, M. A. (1996). Classification of the arthropod "Fuxianhuia". "Science", 272, 746–747.] The presence of a distinct anterior sclerite bearing the eyes has been taken to suggest that a distinct acron once existed in front of the euarthropod head. [Chen, J.-y., Edgecombe, G. D., Ramsköld, L. & Zhou, G.-q. (1995). Head segmentation in Early Cambrian "Fuxianhuia": implications for arthropod evolution. "Science" 268, 1339–1343.]

"Fuxianhuia" is not a unique arthropod: two other taxa, "Chengjiangocaris" and "Shankouia" are clearly closely related, [Waloszek, D., Chen, J., Maas, A. & Wang, X. (2005). Early Cambrian arthropods – new insights into arthropod head and structural evolution. "Arthropod Structure and Development" 34, 189–205.] although they differ in some details, such as the limbs. A relationship with the Burgess Shale taxon "Canadaspis" has also been suggested. [Budd, G. E. (2002). A palaeontological solution to the arthropod head problem. "Nature" 417, 271–275. ]

Notes

Further reading

*Hou, Xian-Guang; Aldridge, Richard J., Bengstrom, Jan; Siveter, David J.; Feng, Xiang-Hong 2004; The Cambrian Fossils of Chengjang, China, Blackwell Science Ltd, 233 pp.