Biotic Changes Across The Precambriancambrian Boundary

Diversity curves of metazoan genera show a fall at the Vendian-Tommotian boundary (Sepkoski 1992: figure 11.4.2). This data set includes genera from all depositional environments, and the fall has been interpreted as reflecting a mass extinction. Seilacher (1984) suggested that Vendian biota mark not simply a nonskeletal start to metazoan evolution but a distinct episode to the history of life, terminated by a major extinction. He later suggested that they were quilted constructions that represented an evolutionary experiment that failed with the incoming of macrophagous predators (Seilacher 1989).

There is, however, also a remarkable change in the style of preservation of many of the body fossils in passing across the Precambrian-Cambrian boundary (cf. Seilacher 1984). The Vendian shallow-water sequences are dominated by relatively large forms, commonly exceedingly well preserved in three dimensions and found within fine-to-coarse-grained, well-washed, matrix-poor sandstones (figure 13.3). Such three-dimensional preservation is almost unknown in the Phanerozoic (cf. Seilacher 1984, 1989). By that time, these high-energy sandstones commonly lack body fossils and are dominated by trace fossils, many of which are produced within or between beds. Explanations for the three-dimensional preservation of Vendian body fossils include early mineral precipitation within the matrix (Jenkins 1992), low rates of microbial decomposition (Runnegar 1992), absence of scavengers (Conway Morris 1993), and the supposed existence of mineral crusts formed by cyanobacterial mats (Gehling 1991).

The parallels between the three-dimensional preservation of these body fossils in the Vendian and the trace fossils produced within similar sandstones in the Phanero-zoic is remarkable but is consistent with the conclusions of Crimes and Fedonkin (1996) that many of these three-dimensionally preserved Vendian body fossils actually formed by growth within the sediment by a process of plasmic permeation. Such animals would then undoubtedly suffer from the incoming of macrophagous predators in the Phanerozoic as envisaged by Seilacher (1989). They seem to have responded by onshore-offshore migration, and a few appear in deeper-water environments during the Cambrian (Conway Morris 1993; Crimes and Fedonkin 1996). Crimes (1994) has argued that Vendian trace fossils also include many unusual and short-ranging forms. The trace fossil diversity data (Crimes 1992: figure 2; Crimes 1994: figure 4.1) do not, however, support a mass extinction, nor indeed is this indicated by the body fossil data set of Sepkoski (1992: figure 11.4.1) when considered in terms of families, orders, or classes. Evidence for such an event is perhaps best shown when the fauna is divided into "Ediacaran, "Tommotian," and "Cambrian sensu stricto" (Sepkoski 1992: figure 11.4.2). There is, however, increasing evidence that some, or perhaps many, elements of the Ediacara fauna continue through the Nemakit-Daldynian (see Brasier 1989) and into later Cambrian strata (Conway Morris 1992;

Figure 13.3 Three-dimensional nature of Pteridinium from the Kliphoek Member of the Neopro-terozoic Nama Group (South Namibia). A, Field photograph at Plateau Farm, near Aus; B-F, specimens lodged in a small museum at Aar Farm, by permission of Mr. H. Erni. All scale bars 2 cm.

Crimes et al. 1995; Crimes and Fedonkin 1996). Additionally, the data are imprecise because of correlation problems at this level.

Although an overall reduction in diversity cannot be discounted, the picture is far from clear, and, interestingly, Jablonski (1995) places the first of his "Big Five" mass extinctions at the end of the Ordovician. One might also anticipate that any extinction event could have greater consequences in shallow water than in the more constant slope environments considered here. In contrast, the dramatic increase in diversity of both body and trace fossils in the earliest Cambrian strata is obvious (Sepkoski 1992;

Crimes 1994) and has led to the concept of "explosive evolution." There are a significant number of short-ranging forms in the late Precambrian, but the Cambrian is dominated by much longer-ranging forms of Phanerozoic type, and this has prompted Crimes (1994) to suggest that the major change is a biological one in which a period of early evolutionary failure, as represented by a high proportion of short-ranging forms, is replaced by evolutionary success.

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