Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Other Likely Spiralians

There are some enigmatic Cambrian explosion taxa with unusual morphologies, by the standards of crown groups, that may have been Spiralians, as they do not seem to have moulted, are quite unlike deuterostomes, and don’t resemble lophophorates. Odontographus, is such a taxon from the Burgess Shale, which ranged to more than 12 cm long, and has a ventral surface that is at least party annulated, on which were serrated marginal features that have been interpreted as gills that appear to have been placed 1 within each annulus (Caron et al., 2006). According to the authors¹ it is tempting to interpret the annuli as delineating coelomic compartment margins as occurs in annelid-style architectures, though annulated integuments are also present in forms lacking eucoeloms, in which case they are interpreted as locomotory adaptations. Wiwaxia is another enigmatic fossil, which at about 3 cm is smaller and its dorsal surface is covered by chaetae that compare closely to certain annelid chaetae (Butterfield, 2006); and Orthozanclus, which at 10 cm is even smaller (Conway Morris & Caron, 2007). Both Odontographus and Wiwaxia have feeding structures that are composed of 2 or more transverse chevron-shaped tooth rows, which are possibly replacement rows, in a ventral apparatus, which suggests they fed on such things as algal mats or sheets on the floor of the ocean. The feeding apparatuses are similar enough to suggest the possibility of a common ancestor (see Butterfield, 2006). These apparatuses have been compared to feeding apparatuses present in most classes of molluscs that have belts of transverse tooth rows. Rasping feeding structures are also possessed by some crown-group annelids, though again they are not identical to those found in these fossils. It has been suggested that Wiwaxia may be a stem annelid and that Odontographus may be a stem mollusc, or a stem that branched earlier in the lophotrochozoan tree.

Halkieria (Conway Morris& Peel, 1990)

Found in Stage 3 deposits from north Greenland, which ranges up to 8 cm, it has been allied to Wiwaxia based on its general form and flattened mineralised sclerites, interpreted as originally aragonite (Porter, 2008). It has dorsal shell-like structures at both anterior and posterior ends of Halkieria which show some resemblance of general shape to brachiopod shells. It has been postulated that halkieriians are a stem group of lophotrochozoans from which brachiopods branches (Conway Morris & Peel, 1995). According to this hypothesis brachiopods evolved by folding and anterior-posterior compression of the Halkierid body plan, though according to the authors¹ basic brachiopod anatomy would not be accounted for by simple folding. The microstructure of the sclerites of Halkieriid also somewhat resembles that of Chancelloriids which are sponge-like animals (Porter, 2008). According to the authors¹ the current evidence for affinities to brachiopods seems very weak; who suggest that the microstructural features used to link these fossils to crown groups may possibly be plesiomorphic. A re-study of Halkieria (Vinther & Nielsen, 2005), concluded that it is an early mollusc for which they erected a new class, the Diplacophora, to contain it. The halwaxiids is a group that was used to unite Halkieria, Wiwaxia and Orthozanclus which were placed within the Molluscan stem (Conway Morris & Caron, 2007).  It was found that the canals on sclerites of halkieriids were uniquely similar to canals in Polyplacophoran (Mollusca) valves, which suggests a position as a stem group mollusc, which branched below Polyplacophora and Aplacophora, its likely sister group. Much work needs to be undertaken on the relations among these groups.

Sources & Further reading

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

2. Halkieria is a Mollusc