Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Small Shelly Fossils

Small shelly fossils (small shellies or SSFs) are mostly less than 2 mm in their greatest dimension (see Bengtson, 2005; Bengtson & Conway Morris, 2002). There are some whole shells among the SSFs, such as those of small brachiopods, molluscs and a variety of cones for which their affinity is uncertain, and there are also individual plates, spines and other skeletal elements (sclerites) from larger animals that had been reserved in the same phosphatisation processes. In the living animal the complete skeletal assemblage is its scleritome which is sometimes orders of magnitude larger than the constituent sclerites. It has been shown by a new compilation of origination patterns of the small shellies that they are apparently at the base of the Cambrian, their diversity increasing steadily through Stage 1 (Maloof, Porter et al., 2010). The authors¹ suggest that it was probably during Stage 2 that they attained their greatest diversities, and during Stage 3 they were still fairly diverse, which is when the record of the Cambrian Explosion is at its height (Porter, 2004).

The presence of many different clades is suggested by morphological diversity small shelly sclerites from the Early Cambrian, the most common of which are lophotrochozoans. Deuterostomes are present in the explosion faunas of Stage 3 and Stage 5, which are exceptionally preserved, though they are largely absent. Some scleritomes are comprised of more than 1 type of sclerite, thereby complicating the interpretation; however there are now a variety of scleritomes that are well preserved, in which the sclerites are in the living positions, which preserves the body plan of a number of sclerite-bearing animals. An example is Microdictyon, a type of sclerite that was found and described in small shelly assemblages a number of years before the discovery of the complete animal. Microdictyon is now known to have had the plates embedded in the body wall above the limbs of a branch of animals that is now extinct that is related to onychophorans and arthropods, the lobopodians (Liu, J. et al., 2008; Ramskold & Chen, 1998), therefore they belong to the superphylum Ecdysozoa. Ecdysozoans tend to be relatively rare among the classic small shelly assemblages, except for the sclerites of lobopodians, though Ecdysozoans have the richest, most disparate fossil record of any superphylum from the Cambrian.

For other scleritomes discoveries of a few associated sclerites can only hint at the existence of a scleritome. An example is a group, cambroclaves, for which the full scleritome is not known, all that is known of their scleritome is small groups of their sclerites that were found associated together at a location in Kazakhstan, which permitted a partial reconstruction of their geometry. It is suggested by these sclerites, which are mostly less than 10 mm long, that at least some surface of the animal was covered by a mosaic of many sclerites. Interpretation of growth and possible life habits of this form is allowed by these small insights (see Conway-Morris & Bengtson, 1990). At the inferred posterior end of each sclerite the spines bend forwards (or possibly backwards), rather than outward, which suggests the animal crawled or burrowed, possibly using the spines to gain purchase during movement, and possibly not for protection. It is suggested the sclerites appear to have been composed originally of low-magnesium calcite and interlocked so it seems growth might have needed to have been accommodated by the secretion of new files of sclerites in the zone of growth. The authors¹ suggest it is plausible cambroclaves may have been benthic bilaterians, though before they can be included with certainty in the metazoan tree will require the finding of more fossils that provide more information about them.

It is clear the SSFs are a large, phylogenetically disparate assemblage, though many SSFs are still not well known, representing a thriving fauna, the preservation of which was restricted to a narrow taphonomic window (Porter, 2004). The appearance of different SSF groups generally correlates with abundance changes in the sedimentary record of phosphatic deposits. From Stage 4 and onward such deposits in the sedimentary record become relatively rare which presumably reflects a change in the chemistry of seawater. The diversity of small shelly fossils declined as the phosphatic deposits became increasingly rare, and by Stage 5 SSFs are essentially gone from the fossil record. As SSFs are not found in many phosphatic deposits from the Ediacaran, the authors¹ suggest it is likely that during most of the Ediacaran small shelly faunas were rare of completely absent, evolving mainly early in the Cambrian. The disappearance of the SSFs is associated, at least partially, with an episodic extinction (Zhuravlev & Wood, 1996), though the clades some of them belonged to are still represented in some later deposits.

Sources & Further reading

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