Fuxianhuia - Morphology

Morphology

Complete Fuxianhuia specimens are approximately 3 centimetres long. The anterior of Fuxianhuia is marked by an oval sclerite, from which 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. 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; a reassignment that has however not been universally accepted. 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.