Re the Ediacaran period (before the Cambrian, 635–541 million years ago), from :
But what are these peculiar organisms? Their very strange morphology has made relating them to modern organisms very difficult, and they have been suggested to be related to anything from plants, fungi and lichens through to recognisable animals such as worms and arthropods.
In a major review of the Ediacaran fossils recently published in Biological Reviews, Graham Budd, professor of palaeobiology in Uppsala University, Sweden, and Sören Jensen, researcher at Badajoz University, Spain, suggest that most of the Ediacarans are very basal representatives of animal lineages, and as such are likely to reveal the hitherto very obscure pathways taken by animal evolution. This goes some way to explain why they happen to appear just before clearly recognisable animals do in the fossil record, and raises the question of what the ecological relationship between the two biotas is.
It’s been assumed that the mostly rather static Ediacarans block our view of mobile life forms that probably hid much of the time.
The new thesis suggests that the typical Ediacaran animals created an environment around them that enabled the more mobile creatures to evolve.
Rather than the Ediacarans and later animals being direct competitors then, the Ediacarans themselves created a permissive environment that was ideal for higher animals to evolve in. This idea fits well into a modern view of evolution, called “ecosytem engineering” whereby key species (such as beavers) influence the environment in order to create new evolutionary and diversity opportunities for other species. Perhaps then, the Ediacaran taxa weren’t impediments but the drivers of the evolution that was eventually to lead to all the rich animal diversity we see today. More.
A promising suggestion, though there are lots of questions it doesn’t answer.
See also: Animal with muscles found from 560 mya (Ediacaran period)
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Here’s the abstract:
The earliest evolution of the animals remains a taxing biological problem, as all extant clades are highly derived and the fossil record is not usually considered to be helpful. The rise of the bilaterian animals recorded in the fossil record, commonly known as the ‘Cambrian explosion’, is one of the most significant moments in evolutionary history, and was an event that transformed first marine and then terrestrial environments. We review the phylogeny of early animals and other opisthokonts, and the affinities of the earliest large complex fossils, the so-called ‘Ediacaran’ taxa. We conclude, based on a variety of lines of evidence, that their affinities most likely lie in various stem groups to large metazoan groupings; a new grouping, the Apoikozoa, is erected to encompass Metazoa and Choanoflagellata. The earliest reasonable fossil evidence for total-group bilaterians comes from undisputed complex trace fossils that are younger than about 560?Ma, and these diversify greatly as the Ediacaran–Cambrian boundary is crossed a few million years later. It is generally considered that as the bilaterians diversified after this time, their burrowing behaviour destroyed the cyanobacterial mat-dominated substrates that the enigmatic Ediacaran taxa were associated with, the so-called ‘Cambrian substrate revolution’, leading to the loss of almost all Ediacara-aspect diversity in the Cambrian. Why, though, did the energetically expensive and functionally complex burrowing mode of life so typical of later bilaterians arise? Here we propose a much more positive relationship between late-Ediacaran ecologies and the rise of the bilaterians, with the largely static Ediacaran taxa acting as points of concentration of organic matter both above and below the sediment surface. The breaking of the uniformity of organic carbon availability would have signalled a decisive shift away from the essentially static and monotonous earlier Ediacaran world into the dynamic and burrowing world of the Cambrian. The Ediacaran biota thus played an enabling role in bilaterian evolution similar to that proposed for the Savannah environment for human evolution and bipedality. Rather than being obliterated by the rise of the bilaterians, the subtle remnants of Ediacara-style taxa within the Cambrian suggest that they remained significant components of Phanerozoic communities, even though at some point their enabling role for bilaterian evolution was presumably taken over by bilaterians or other metazoans. Bilaterian evolution was thus an essentially benthic event that only later impacted the planktonic environment and the style of organic export to the sea floor. (Public access) – Graham E. Budd, Sören Jensen. The origin of the animals and a ‘Savannah’ hypothesis for early bilaterian evolution. Biological Reviews, 2015; DOI: 10.1111/brv.12239