Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Diversity

It has become evident that the SSFs represent a diverse array of clades, as partial or whole scleritomes from these fossils have been recovered and reconstructed, from stem molluscs to stem lobopods, and probably stems of other groups. A recent analysis from southern China has provided a census from a region (G.X. Li et al., 2007), though there is as yet no database that catalogues the entire diversity of the Cambrian. From Cambrian Stage 1 relatively few genera of body fossils have been identified, all of which are small shellies. During Stage 2 there is a dramatic increase in the number of genera of small shellies, where they still make an overwhelming proportion of the taxa. Stage 3, which includes the Chengjiang Fauna, has the highest diversity, 421 genera. Though the number of SSFs has dropped from the high levels of stage 2, they still comprise 15 % of the fauna, 65 genera. By Stage 4 the trilobite and other arthropods dominate.

It is suggested by 2 recent global syntheses of biomineralised clade occurrences in the Early Cambrian, which it may be possible to discern 2 separate biomineralisation pulses, which are possibly tied to seawater chemistry changes (Kouchinsky et al., 2012; Maloof, Porter et al., 2010). The first pulse is proposed by these papers to have occurred during stages 1 and 2, mainly during the first 10 My of the Cambrian. The appearance of total-group Lophotrochozoa, with the exception of the Chancelloriids that are phylogenetically ambiguous, though it does include molluscs and halwaxiids, hyoliths, tommotiids, and a range of other components of the SSF, dominates the first pulse. Among the earliest hard skeletal parts that have been recovered are Protoconodonts, which have been suggested to have likely been the spines chaetognaths used in predation. All these forms are present near the negative carbon isotope excursion that is associated with the boundary between the Ediacaran and the Cambrian. Brachiopods and related groups, as well as the better skeletonised sponges and corallomorphs make their first appearance during Stage 2.

The earliest representatives of the ecdysozoans and deuterostomes, dating from the Neoproterozoic, and occurred earlier in the Cambrian, but it wasn’t until Stage 3 of the Cambrian that the first biomineralised representatives of these 2 clades appeared. Included among these forms are trilobites and other arthropods, echinoderms, and chordates that were not mineralised. According to the authors1 this pattern is consistent with fluctuations in the magnesium/calcite ratio driving the changing skeletal mineralogy (Porter, 2004; Zhuravlev & Wood, 2008). The earliest known forms that were biomineralised were aragonitic. The second biomineralisation phase occurred following a -10 ‰ δ13C isotopic excursion, and involved the first appearance of skeletons with high magnesium levels and a lowering of the Mg/Ca ratio. It is emphasised that caution is necessary in interpreting these results (Maloof, Porter et al., 2010). It is possible that these phases may simply reflect the considerable differences in the dominant preservation style between the 2 intervals: during Stages 1 and 2, the phosphatisation of shells which was followed in Stage 3 by the exquisite preservation of soft parts of many different groups in the Chengjiang Fauna and similar lagerstätten. The sort of rigorous analysis of patterns of origination that would provide statistical limits on the reliability of these origination phases was not included in either of these studies.

Sources & Further reading

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

Author: M. H. Monroe
Last Updated 12/05/2014
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