The Cambrian explosion

More and more data are helping to explain the event at the base of the Cambrian where mineralized skeletons evolved. A few sections worldwide record the full timespan from just before to just after the Precambrian-Cambrian boundary. As these are investigated, a detailed picture of events associated with this time period is emerging.

The two most important questions about the boundary are:

1 What caused the appearance of skeletons? Was it a biological phenomenon or one related to changes in the oceans or atmosphere?

2 Did modern groups of multicellular animals exist and were they diverse prior to the boundary, or does this event signify not just the appearance of skeletons but also the appearance of the modern cohort of animal phyla?

The answer to the second question is that there is good evidence for a period of diversification of metazoans during the Precambrian. All molecular clocks suggest a Precam-brian origin for the group, and many different phyla evolved mineralized skeletons in a short period of time. In addition, early Cambrian faunas are provincial, with species being specific to particular geographic regions. Evolution must have occurred before mineralization for this provincialism to have developed. The answer to the first question is more problematic, and has yet to be resolved. The main issues are outlined in Table 16.1.

Table 16.1 Summary of significant events associated with the Cambrian explosion.

Possible drivers for the Cambrian event

Acting to allow biomineralization or to increase diversity?

Evidence

Analysis

Mass extinction close to the

Precambrian-Cambrian boundary

D

Abrupt increase in organically derived carbon (12C), which is interpreted as a mass-release of frozen methane gas

A causally similar mass extinction at the end of the Permian caused a faunal turnover, but biomineralization is not clearly explained by this driver

Increase in oxygen levels

B/D

Indirect, coming from inferring a linear increase in atmospheric oxygen from around 2.5 billion years to the present, and the observation that large animals need more oxygen than smaller ones

At present this driver is impossible to measure or assess

Continental break-up

D

The supercontinent of Rodinia broke up in the late Precambrian, and this would have increased the amount of shallow marine shelf space for organisms to inhabit

A similar break-up during the Mesozoic seems to have facilitated a significant increase in the diversity of marine organisms

Biological arms race

D

The evolution of biomineralized predators would have forced the rapid evolution of defense mechanisms in prey and vice versa

This is clearly going to be part of the answer, but it does not explain the origin of biomineralization in the first place

Changes to animal development

B/D

Changes to regulatory genes, or genetic complexity, may have enabled the evolution of more disparate groups close to the boundary

Hox genes are shared by all metazoans and so must have evolved in the Precambrian. Other suggestions are persuasive but currently untestable

Increasing food supply from the plankton

D

Although biomass in the plankton has remained effectively constant through the Phanerozoic, its size increased around the boundary, which may have increased its utility to benthic organisms

Larger plankton are found from early Cambrian times onwards, but whether this was a cause of diversification, or a response is unclear

Changes in ocean chemistry

B/D

Phosphate deposits worldwide indicate a rapid upwelling of nutrients, which may have been caused by an overturn of the world ocean as a result of mixing with brines from inland drainage basins

Some stratigraphic analyses place this event in the early Cambrian, or relate it to the changes in carbon isotope coming from the possible melting of frozen methane

B, biomineralization; D, diversity.

B, biomineralization; D, diversity.

The speed of events around the Precambrian-Cambrian boundary were originally thought to be extremely rapid, hence the term "Cambrian explosion". However, further study has shown that diversity increased in a series of steps during the early Cambrian, and that these occurred over around 15 million years (Fig. 16.1). One of the reasons for the apparently great speed of change is that this was a time of rising sea levels, so that only offshore sections recorded the whole event, with deposits further into continental interiors only beginning to record biological changes in the marine realm as they were flooded by the sea.

Cambrian Ma paleogeography

510-

Trace fossils

Sketch boundary section from Siberia. Note datable volcanic horizon and facies change at boundary

Hsds

Small shelly fossils

Archaeocyathids

Trilobites

Trace fossils appear in the late Precambrian, most are simple forms

Small shelly fossils characterize the early Cambrian. Different skeletal types appeared over about 10 million years

Many of these shells are small parts of larger organisms whose skeleton was constructed of overlapping parts like chain mail

Phosphatic shells

Calcareous shells

Agglutinated shells

Siliceous shells

Archaeocyathid reefs flourished briefly then disappeared

Trilobites and other arthropods became dominant as the Cambrian progressed

This later fauna had better developed, protective skeletons and tended to be more mobile

By the Middle Cambrian, most modern phyla were present in the fauna and diversity had reached about 80 orders

Rare Ediacaran animals survived into the Cambrian

Fig. 16.1 Schematic diagram showing important elements of the biological world across the Precambrian-Cambrian boundary.

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