May 8, 2009 The Neoproterozoic interval of "hidden" evolution refers to a gap of unknown duration between the time when animals first evolved (uncertain) and the oldest known fossil or geochemical evidence of animals (latest Neoproterozoic, about 600-650 million years ago).
Neuweiler et al. now propose to fill this gap. They describe distinctive, microscopic features in early Neoproterozoic limestone (between 779 and 1083 million years old) from the Northwest Territories of Canada, consisting of highly structured zones with multi-generational arrays of carbonate minerals, secondary voids, and internal sediment. Today, such a texture develops when aragonite crystals precipitate on the decaying connective tissue (collagen) of sponges in sediment on the sea floor.
As sponge decay progresses, a complex fabric of calcareous material and voids is produced, which is identical to fabrics in Phanerozoic limestones (less than 542 million years old) containing known sponge body fossils and to the fabrics now reported from the early Neoproterozoic.
Collagenous connective tissue is a fundamental character of all metazoans (animals), and so the presence in early Neoproterozoic rocks of a microscopic fabric directly associated with it implies that metazoan-grade organisms existed at that time.
These purported ancestral metazoans did not have a canal system that would relate them to sponges, the simplest form of animal known today. Instead, they likely represent a structured consortium of protists in a shared collagenous scaffold.
These results push back the earliest geologic evidence for animals by around 200 million years. This timing corroborates results of an integrated phylochronology and supports the concept of a biosphere that persisted through Snowball Earth.
This research was published in Geology by Fritz Neuweiler et al., in May 2009.
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