Fossils older than the base of the Cambrian – 542 million years ago – are not exactly abundant, so it was interesting to see not one, but two interesting papers in the latest issue of Geology that describe fossils from the Neoproterozoic period, from 1000 to 542 million years ago.
The first paper reports the discovery of 565 Ma trace fossils found at Mistaken Point in Newfoundland. Mistaken Point is the location of a nice section across the Cambrian boundary, and hosts the oldest known fossilised Ediacaran macrofauna (at least 10 million years older than the ones I’ve seen in Namibia).
Ediacaran fossil, Mistaken Point. Source
Ediacarans have generally been interpreted as immobile, bottom-dwelling filter feeders, but Liu et al. have discovered a horizon in the sequence that appears to show that something was moving around:
Source: Liu et al., Figure 2
These trail-like features are found on the top of a fine green mudstone unit, laid down in deep water, that is capped by a volcanic tuff (which probably helped in their preservation). Liu et al. found 70 tracks about 1cm wide and up to 17 cm long. The crescent shaped ridges within the grooves, and their marginal ridges indicate that they have been formed by sediment being pushed aside and piled up by something moving through it. The lack of any consistent orientation, and the fact that a fair number are curved, indicating a change in the direction of motion, makes it unlikely that they are formed by something being passively dragged along by a current.
It is not known what manner of creature might have made the trails, as none are preserved at the same stratigraphic level; the only possible clue is that the tracks sometimes end in circular impressions which might mark where the creature that made the trails was resting before or after moving. Whilst the authors say that the possibility of them being made by giant single-celled protists cannot be discounted, they remark that modern sea anenomes can leave similar trails..
It seems, then, that an anenome-like creature (in terms of body form and/or mode of life, at least, if not direct descent) is their favoured culprit.
The second paper concerns these unusual microfossils from the Tindir group of northwestern Canada:
Scale bar is 15 μm. Source: Macdonald et al., Figure 1
The little mineralised scales are quite interesting; the appearance of mineralised body parts is also associated with the Cambrian diversification, although the whys and wherefores are still disputed. Modern scale-forming groups of micro-organisms are also all eukaryotes, which suggests (although does not prove) that these critters were too. So from an evolutionary perspective, they’re quite interesting; unfortunately the sequence they were found in was not particularly well-dated, so based mainly on fossils like this they were placed close to the Cambrian boundary. With some more thorough mapping and geochemical correlations, however, Macdonald et al. have shown that the fossil-bearing formations were deposited prior to glacial deposits linked to the period of extreme “Snowball Earth” glaciations between about 750 and 635 million years ago. This means that the microfossils pictured above must be at least 750 million years old. Since the Snowball Earth theory proposes an extreme winnowing of most life due to the whole Earth being frozen over, this is a valuable glimpse at what might have occupied the pre-Snowball world, and tests the biological part of the hypothesis.
Taking a wider perspective, what both of these papers demonstrate is that whilst the the dawn of the Cambrian clearly marked the diversification of mobile, active animals and biomineralisers, the story of their first origins appear to have begun earlier, possibly much earlier; something to bear in mind when we are trying to link biological changes on the ancient Earth to wider geological events.
Liu, A., Mcllroy, D., & Brasier, M. (2010). First evidence for locomotion in the Ediacara biota from the 565 Ma Mistaken Point Formation, Newfoundland Geology, 38 (2), 123-126 DOI: 10.1130/G30368.1
Macdonald, F., Cohen, P., Dudas, F., & Schrag, D. (2010). Early Neoproterozoic scale microfossils in the Lower Tindir Group of Alaska and the Yukon Territory Geology, 38 (2), 143-146 DOI: 10.1130/G25637.1