Wiwaxia corrugata is an extinct organism from the Middle Cambrian period (505 ma) and is another creature from this period that has been hard for paleontology to classify.
Although initially described in 1895 (G.F. Matthew) from a single discovered spine, the first complete specimen of Wiwaxia was not discovered until 1911 by Charles Doolittle Walcott in the Burgess Shale deposits of British Columbia. Debate over its classification has been ongoing ever since.
Wiwaxia is covered in rows of imbricating, chitinous sclerites with additional rows of dorso-lateral spines, which give it a distinctive armoured appearance. These sclerites are comparable with some forms of flattened hair (Chaetae) occurring in polychaete worms and so Wiwaxia has been classified as an annelid (Butterfield, 1990). However, the ventral surface of Wiwaxia is comprised of a single muscular foot (lacking any clear segmentation associated with annelids) with radula-like mouthparts, both of which are more suggestive of the Mollusca phylum (Conway-Morris 1985, Caron et al. 2006). Wiwaxia could even belong to an unknown and now extinct phylum. On balance however, Wiwaxia is currently regarded as an extinct stem-mollusc. (M. Smith 2014) - closely related to the common ancestor of modern molluscs.
The sclerites that cover Wiwaxia vary in shape, number and size depending on specific anatomical region and upon the age of individual organisms - as Wiwaxia displayed infant to adult ontogenesis (development). Generally the scleritome is made up of 8 seperate transverse rows of sclerites with each row made up of four distinct regions occupied by clusters of sclerites. In addition to these are an anterior cap of leaf shaped sclerites and a distinctive dorso-lateral array of spines (12).
Each of Wiwaxia's sclerites has a striated, ribbed surface which has led to speculation that the scleritome of Wiwaxia was iridescent and colourful. The use of colour for selection, camouflage or warning is thought to have been well established by this period.
For many years the only known specimens of Wiwaxia were from members of the Burgess Shale locale. More recent discoveries have been made from further afield, in the Chezch Republic and China (Zhang, Smith, 2015) illustrating the adaptive success of Wiwaxia during this period.
The problems of phylum categorisation for Cambrian organisms was famously covered by Steven J. Gould in his book "Wonderful Life". Many of the questions presented there are still being debated. How did phylums become so distinct and well developed so early on in the history of animal life on the planet? How many phylums evolved at this time? How many phylums survived and why? These are all still largelyunresolved questions.
With thanks to Dr Jean-Bernard Caron for assistance with anatomical detail.