Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Disparity

The extraordinary range of morphologic innovation is the most remarkable pattern that emerges from any analysis of the diversification of metazoans that occurred in the Early Cambrian. This range is evident at many different scales, from the obvious appearance of groups that were morphologically distinctive to a diversity of anatomical details. An example is the eyes. It had been expected that the complexity and sophistication of eyes had improved through the Phanerozoic, until the recovery of arthropod eyes that had been exquisitely preserved from the Emu Bay Shale, Kangaroo Island, South Australia, which illustrated that highly advanced eyes with more than 3,000 ommatidial lenses had evolved at some point early in the history of the Clade (M.S. Lee et al., 2011). A surprising feature of these ancient eyes is that they preserve a “bright zone” within the ommatidia that has higher light sensitivity, and possibly acuity. It is demonstrated by sophistication of these eyes that in some groups vision was an important adaptation.

One way in which the Cambrian faunas differed from those of the present is that many morphologically distinctive groups (high disparity) had relatively few species (low diversity), when compared to similar groups of the present. At the level of fundamental body plans that originated in the Early Cambrian, as well as at the levels of morphological disparity within most of these body plans, this pattern is robust. The architectural theme of a body plan is recorded by the earliest known phase of morphological evolution, which has been found to hold for a number of groups, the subsequent post-explosion evolution being largely a process of exploiting these themes; i.e., in most new major clades the early species were distributed widely across the eventual morphological range of the group – its morphospace boundaries – with the later evolved taxa tending to largely fill the space rather than by extending it significantly. Furthermore, subsequent morphology tends to be narrowly bounded as well when the early morphospace is narrow.

This pattern, that has been known for many years, was originally discovered by the use of the higher Linnaean categories, phylum, class, and order as metrics of morphological novelty (e.g., Erwin, Valentine & Sepkoski, 1987; Valentine, 1980) – though with the publication of the book Wonderful Life (Gould, 1989). At the time Gould wrote there was a high point of interest in extinct higher Linnaean ranks. The spread of phylogenetic methods has resulted in greater emphasis being on topological relationships of stem lineages within phylogenies that are well-constructed. According to the authors¹ the Use of Linnaean ranks as proxies for disparity has paved the way for morphometric studies within new cladistic frameworks. It has been shown that some Linnaean taxa are polyphyletic, especially when groups with distinct morphologies are involved. E.g., how does an order within Crustacea compare to an order within Bivalvia (Forey et al., 2004; A.B. Smith & Patterson, 1988)? Alternatively, there are other groups that were originally identified as clades by cladistic methods that have proven to also be polyphyletic, and patterns that have been revealed in rank-based studies have been shown to be generally accurate with the introduction of more quantitative approaches. As it has been pointed out (Foote, 1997), the issue is whether Linnaean categories are scientifically useful as proxies. It is indicated by a carefully defined Linnaean taxon that the rises(s), decline(s), or both of a particular collection of synapomorphies with a body plan, together with its successes and failures, in itself has its uses.

According to the authors¹ Gould’s book has helped to spur the development of quantitative morphological disparity estimates, which largely replaces simple Linnaean taxa counts as disparity metrics (Erwin, 2007; Foote, 1997). There are presently a variety of quantitative approaches that are available for the measurement of disparity which rely on both discrete character measurements (as is used in phylogenetics) and on features that are continuously variable, such as shape, by the use of geometric morphometrics. When the patterns of disparity change within the phylogenetic framework that resulted are evaluated, it also allows the examination of whether there is a phylogenetic relationship between similar morphologies or are simply similar due to convergence. To test hypotheses of evolutionary transformation it is essential to use these approaches. As almost all the quantitative studies are best applied whiten a common body plan, where there are homologous characters, is one of the challenges of disparity studies. To examine the disparity patterns among the Ecdysozoa as a whole, for example, is more challenging.

There are 3 potential relationships that have been identified between taxonomic diversity and morphologic disparity (Foote, 1993, 1996):  

1.      Constrained morphologic increases with taxonomic diversification;

2.      Disparity increases rapidly, with early steps being large with smaller steps later in diversification; and

3.      Increases in disparity and diversity that are concordant, that pattern that had been argued by Gould to be the null      hypothesis of most evolutionary biologists; described as (‘a bit of a straw man’).

It is suggested by the first possibility that for further increases in the disparity of a group the increases were constrained by either development or ecology. In the 2^nd case, it is suggested that the morphologic distance between sister taxa was larger in the clade’s history and smaller later. According to the authors¹ there are 2 possible alternative explanations for such a pattern: either the size of the morphologic transitions between taxa decreased over time, which could possibly the result of developmental increases of limitations of morphologic innovation, or as the result of a decline in the success of morphologic transitions, likely for ecological reasons. In the 3^rd case taxonomic diversity is a good proxy for morphologic disparity, with no change in the distance of morphologic transitions with time. As discussed in the case of arthropods, a larger volume of morphospace is found in a single sample of arthropod disparity from the Cambrian Shale than is found in a representative sample of extant arthropods (Briggs, Forty & Wills, 1992). A long series of discussions resulted from this pioneering study (as reviewed in Erwin, 2007). Since the early 1990s there have been many quantitative studies of morphologic disparity, and a pattern of great, or even maximum disparity early in the histories of clades, has been identified with many, though not all. The authors¹ say that it is suggested by their qualitative reading of the diversification patterns of Cambrian clades, which they discussed, that these groups from the Cambrian explosion generally followed this same pattern of disparity.

Sources & Further reading

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