Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Scalidophora

There are 2 crown pseudocoelomate ecdysozoans phyla, Priapulida from the Chengjiang Fauna and the Burgess Shale (Wills, 1998), and Loricifera (Peel, 2010) from the Sirius Passet Fauna, northern Greenland, are identified from stem lineages in the Cambrian. There is also a third extant phyla, Kinorhyncha, that is not known from the fossil record. Together this extant phyla and 2 extinct phyla are grouped into the clade Scalidophora. There are also stem groups from the Cambrian that are often also identified as Scalidophora. In both the Chengjiang and Burgess Shale assemblages there is an array of forms that have been included in the Priapulida, that range up to 15 cm in length. Priapulids of the present are found in very soft substrates where they are shallow burrowers, and it has been suggested that these larger forms may also have shared this mode of life. The point has been made in a review of Chengjiang pseudocoelomates from the Chengjiang and associated faunas that many features that have been used to identify Cambrian forms may represent plesiomorphies, and if this is the case it may be premature to allocate them to crown taxa. The genomes of extant priapulids are nevertheless highly conserved, and this suggests a low, possibly basal position within the Ecdysozoa, and it has been suggested that their extinct forms from the Cambrian may represent their stem group(s) (Webster et al., 2006).

There are embryos from the faunas of Early Cambrian to the Early Ordovician preserved in phosphatic sediments that have been assigned to the Scalidophora. The genus Markuella contains the best known species which is believed to represent a stem form of either Scalidophora or Priapulida (Dong et al., 2011). It is more common to find late stage embryos preserved, though it has been reported (X.G. Zhang, Pratt & Shen, 2011) that there are some cleavage-stage form that indicates the presence of radial cleavage, as occurs in priapulids. It has been suggested that radial cleavage is the basal cleavage state of ecdysozoans on the basis of molecular phylogenetic relationships (Valentine, 1997).

Sources & Further reading

1. Erwin, Douglas H., & Valentine, James W., 2013, The Cambrian Explosion: The Construction of Animal Biodiversity, Roberts & Co., Greenwood Village, Colorado