Geology of the Cambrian Explosion

Burgess Shale: Olenoides serratus from the Burgess Shale. Note the dark lines extending from the shell; these are rarely-preserved soft tissues, an indication of the unique preservation conditions of the Burgess Shale. Photo by Steven Newton.Burgess Shale: Olenoides serratus from the Burgess Shale. Note the dark lines extending from the shell; these are rarely-preserved soft tissues, an indication of the unique preservation conditions of the Burgess Shale. Photo by Steven Newton.

The Cambrian Explosion (or Radiation) refers to a period of rapid diversification of the fossils found between ~530-520 Ma (millions of years ago). During this geologically brief period, fossils record a blossoming of animal life into complex organisms. Although the phyla of most of these animals appear to have originated before the Cambrian Explosion, it was during this time that they announce themselves in rocks worldwide.

Fossil locales such as Chengjiang and the Burgess Shale show a world of animals hunting and killing and defending themselves with ever-more complicated claws and teeth and eyes and armor. This is different from the soft-bodied world that existed prior to the Cambrian. The world following the Cambrian Explosion is our world.

Trace Fossils: Planolites trace fossils from the Middle Member of the Deep Springs Formation, just above the Cambrian-Precambrian boundary, White-Inyo Mountains, California. Photo by Steven Newton.Trace Fossils: Planolites trace fossils from the Middle Member of the Deep Springs Formation, just above the Cambrian-Precambrian boundary, White-Inyo Mountains, California. Photo by Steven Newton.

The Cambrian/Pre-Cambrian boundary is set at 542.0 ± 1 Ma by the first appearance of Trichophycus pedum, a mud burrower with complex movement. Animals existed prior to this time, but did not exhibit complex foraging behaviors.

The earliest animal fossils are the Ediacaran Fauna, which immediately preceded the Cambrian. There is considerable scientific debate about the nature of the Ediacarans. Most of these soft-bodied organisms lacked clear mechanisms of locomotion. The exact relationship of Ediacarans to modern animal phyla remains unclear.

Following the Pre-Cambrian-Cambrian boundary at 542 Ma, the subsequent twelve million years saw only modest increases in animal diversity. Rocks from this period record rich trace fossils, but few mineralized skeletons. The rapid diversification of animals began during the Tommotian period (530 Ma), and lasted 5-10 million years.

Molecular research suggests animals diverged into major groups far earlier than is shown in the fossil record. For example, Blair and Hedges (2005) put the split between echinoderms and hemichordates at 876 Ma, and Douzery et al. (2004) calculate the diversification of eukaryotes between 950-1,259 Ma. Thus, the major animal phyla may originate far earlier than the Cambrian Explosion. However, strata from these periods show no conclusive animal fossils at all. This issue is the subject of ongoing research.

Timeline:

  1. 850-630 Ma: Cryogenian Period

    extensive world-wide glaciations

  2. 630-542 Ma: Ediacaran fossils
  3. 542.0 ± 1 Ma: Pre-Cambrian/Cambrian boundary
  4. ~530 Ma: the Cambrian Explosion (Tommotian Age)
  5. 525-520 Ma (Chengjiang fauna) and 505 Ma (Burgess Shale)

    Fossil locales rich in complex animals, such as arthropods and chordates

References

  • Blair, J. E., and Hedges, S.B., 2005. “Molecular Phylogeny and Divergence Times of Deuterostome Animals.” Mol. Biol. Evol., 22 (11): 2275-2284, 27 July 2005.
  • Douzery, E.J.P., et al., 2004. “The timing of eukaryotic evolution: Does a relaxed molecular clock reconcile proteins and fossils?” Proc. National Academy of Sciences, 101 (43): 15386-15391, 26 October 2004.
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