TEMPORAL DISCONTINUITY IN THE IMPACT OF IMPACTS: THE PRE-PHANEROZOIC FOLLOWS DIFFERENT RULES

Extraterrestrial impacts are thought to contribute to macroevolution by way of mass extinction and the consequent removal of ecological incumbents. The model is supported by compelling examples from the Phanerozoic fossil record, to the extent that incumbency has become the default explanation for major adaptive radiation. The Cambrian explosion, for example, is regularly linked to (putative) extinctions of Ediacaran macro-organisms which, in turn, were preceded by a major extraterrestrial impact (Acraman) and a post-impact radiation.

The model is appealing, but there is good reason to doubt that it holds for the pre-Phanerozoic. Where, for example, are the Ediacaran incumbents holding Cambrian-type bilaterians at bay? And there is limited evidence of extinction associated with the Acraman impact. In both these cases, the patterns of occurrence are better interpreted as evolutionary additions rather than replacements – a pattern exemplified by terrestrial land plants through the Phanerozoic.

Nor is the incumbency model supported by the pre-Phanerozoic fossil record. In sharp contrast to their Phanerozoic counterparts, Proterozoic species are all but impervious to extinction, with readily identifiable forms typically exhibiting age ranges of 500 to 1000 million years – despite a presumably equivalent rate of extraterrestrial impact. The reason for this resilience/stability is almost certainly related to contemporaneous ecosystem structure: Proterozoic diversity appears to have been too low, its constituent populations too large, and its ecosystem structures too simple to usefully displace incumbents. Mass extinction, by extraterrestrial impact or otherwise, was an invention of the Phanerozoic.