Some background: The Ediacaran organisms (https://en.wikipedia.org/wiki/Ediacaran_biota) existed for around 60 million years, beginning approximately 600 million years ago. It seems that they were mostly immobile, like plants and sponges, and they're known only through the fossil record. Though many scientists think that they were some sort of animal, rather than plant, their exact nature is still subject to speculation. They flourished in the period immediately preceding the so-called Cambrian explosion, and appear to have died out in the Cambrian period, in which pretty much all contemporary animal body plans first appeared, including the bilateral form that characterizes the majority of modern animals.
"Was early animal evolution co-operative?" |
ScienceDaily (http://www.sciencedaily.com/releases/2015/12/151206165259.htm)
QuoteThe fossil group called the Ediacaran biota have been troubling researchers for a long time. How do these peculiar organisms relate to modern organisms? In a new study, published in Biological Reviews, researchers from Sweden and Spain suggest the Ediacaran reveal previously unexplored pathways taken by animal evolution. They also propose a new way of looking at the effect the Ediacarans might have had on the evolution of other animals.
The fossil record of animals starts for sure by about 540 million years ago, but their origins before this point have remained obscure. Darwin himself worried about this problem at length in the "Origin of species." But after Darwin was writing, a famous group of fossils were discovered called the Ediacaran biota, named after a remote mine in South Australia where many were found. They are now known to be widespread around the globe from the interval of time just before the animal fossil record starts.
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.
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Traditionally, it has been thought that the more advanced animals, many of which are mobile and can burrow energetically through the sediment, were kept in ecological obscurity by the largely immobile Ediacarans, just as the mammals were by the dinosaurs; and it was not until the Ediacarans all went extinct that the mobile animals could diversify in the so-called "Cambrian explosion."
Budd and Jensen propose a new view of this relationship however, inspired by the interaction between the vegetation and animals in the modern savannah environments of east Africa. In their new 'savannah' hypothesis, they propose that concentration of nutrients both above and below the sediment-water interface were enhanced around the stationary Ediacarans, and the creation of these resource "hot spots" created a very diverse environment, ideal for both diversification and for investment of energy into movement. 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.
[Continues. . . (http://www.sciencedaily.com/releases/2015/12/151206165259.htm)]
The full paper is available for free: "The origin of the animals and a 'Savannah' hypothesis for early bilaterian evolution" |
Biological Reviews (PDF) (http://onlinelibrary.wiley.com/doi/10.1111/brv.12239/epdf)
QuoteAbstract
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.
[Added paragraph breaks. - R]