Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Annelida

In the Chengjiang Fauna these are poorly represented by body fossils, though in the Burgess Shale, there are several distinctive annelid body types that have been recovered (Conway Morris, 1979b). These have gross morphology that appears quite as complex as crown clades, though none have been assigned to extant groups. There are a variety of phosphatic tubes, which taper more or less and are open at both ends, among the SSFs. In north Greenland assemblages such tubes have been found in the living position, embedded vertically in the substrate, with the larger end upward (Skovsted & Peel, 2011). It is believed that in life an animal living in the tube was probably a suspension feeder, and it has been suggested that the tubes were formed by stem annelids (Skovsted & Peel, 2011). There were also 2 genera of the unsegmented Sipuncula, which nested within marine annelids, and have been found in the Chengjiang Fauna (Huang et al., 2004). Both of these genera have morphological features that are typical of extant forms, especially in the family Golfingidae, and have no striking synapomorphies. Therefore this group has maintained an extremely conservative morphological mode for 520 My at least, though the sample is still quite small and it cannot be known for certain that disparate sipunculid morphologies have never existed. The morphological conservatism that is displayed by many clades over hundreds of millions of years is emphasised by the early establishment of this unusual annelid architecture.

Sources & Further reading

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