Britannica
Sources used in this answer
- Cambrian Period: Top Questions
- Cambrian Period - Fossil record of the Precambrian-Cambrian transition
- A Journey Through Time Since the Precambrian - Cambrian Period: 541 million to 485.4 million years ago
- Cambrian explosion
- Cambrian Period - Correlation of Cambrian strata
- Cambrian explosion
- Cambrian Period - Boundaries and subdivisions of the Cambrian System
What evidence shows that the Cambrian explosion occurred over millions of years rather than at one instant?
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Cambrian Period
Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Cambrian Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.
The notion that the Cambrian explosion was an instantaneous event is a misconception that arose largely from historical gaps in the fossil record and a lack of knowledge regarding Precambrian life prior to the mid-1900s. Modern research has demonstrated that this transition was a complex, multi-stage process occurring over many millions of years.1
Evidence for this gradual progression includes:
- Successive Biological Phases: The transition is now understood to consist of at least three distinct, informal phases characterized by progressively more diverse and complex communities. This begins with the Ediacaran fauna—soft-bodied organisms appearing over 600 million years ago—followed by a second phase of increasing shelly fauna diversity, and a third phase marked by the appearance and radiation of trilobites.1
- Improved Stratigraphic Records: Since the 1972 project to establish an international boundary stratotype, researchers have identified more complete stratigraphic sections that reveal a clear, sequential history of biotic development rather than an abrupt appearance.1
- Trace Fossils: The study of trace fossils, such as burrowing patterns, shows a gradual increase in the complexity of animal behavior, locomotion, and foraging strategies across the Precambrian-Cambrian boundary.12