Tuesday, 17 May 2022

Evolution News - Ecology Before the 'Cambrian Explosion'

A group of Ediacaran specimens of Fractofusus and Plumeropriscum from the “E” surface, Mistaken Point Ecological Reserve, Newfoundland, Canada.
Credit: Charlotte G. Kenchington (CC BY 4.0)
Creationists are fascinated by the so-called Cambrian Explosion, believing it to have happened over a very short time span and claiming it to be evidence of spontaneous creation of a wide variety of body plans. Of course, it took place over tens of millions of years and was preceded by the Ediacaran biota in which a number of novel body plans evolved in the second major wave of multicellular evolution.

Now, According to a study by Rebecca Eden, Andrea Manica1 and Emily G Mitchell of the Zoology Department, Cambridge University, UK, published today in the open access online journal, PLOS Biology, these early animals formed complex ecological communities more than 550 million years ago, setting the evolutionary stage for the Cambrian explosion.

The Ediacaran period is itself divided into three periods of evolutionary radiation, named after the locations where representative fossils have been found: the Avalon, White Sea and Nama radiation. The Avalon (575-565 million years ago), shows relatively limited ecological and morphological diversity, unlike the White Sea radiation which shows a large increase in morphological diversity, including probable bilateralism and with it an increase in ecological diversity and environmental influence. Some this was followed by a mass extinction leading to a loss of taxonomic diversity but the evolution of biomineralization, such as shells, in the third, Nama, radiation.

Biomineralization implies the need for protection and that implies the prior evolution of motility and predation and the beginnings of evolutionary arms races between predator and prey, and with that would have come environmental pressure to evolve sensory organs such as eyes. Predation also requires digestive and excretory systems.

According to information supplied by PLOS ahead of publication:
The first animals evolved towards the end of the Ediacaran period, around 580 million years ago. However, the fossil record shows that after an initial boom, diversity declined in the run-up to the dramatic burgeoning of biodiversity in the so-called “Cambrian explosion” nearly 40 million years later. Scientists have suggested this drop in diversity is evidence of a mass extinction event roughly 550 million years ago – possibly caused by an environmental catastrophe – but previous research has not investigated the structure of these ancient ecological communities.

We found that the factors behind that explosion, namely community complexity and niche adaptation, actually started during the Ediacaran, much earlier than previously thought. The Ediacaran was the fuse that lit the Cambrian explosion

Emily Mitchell, corresponding author
Department of Zoology
University of Cambridge, Cambridge, UK.
To evaluate the evidence for an Ediacaran mass extinction, researchers analyzed the metacommunity structure of three fossil assemblages that span the last 32 million years of this geological period (between 575 to 543 million years ago). They used published paleoenvironmental data, such as ocean depth and rock characteristics, to look for metacommunity structure indicative of environmental specialization and interactions between species. The analysis revealed increasingly complex community structure in the later fossil assemblages, suggesting that species were becoming more specialized and engaging in more inter-species interactions towards the end of the Ediacaran era, a trend often seen during ecological succession.

The results point to competitive exclusion, rather than mass extinction, as the cause of the diversity drop in the late Ediacaran period, the authors say. The analysis indicates that the features of ecological and evolutionary dynamics commonly associated with the Cambrian explosion — such as specialization and niche contraction — were established by the first animal communities in the late Ediacaran.
The team's findings are published today in PLOS Biology:

The first animals appear during the late Ediacaran (572 to 541 Ma); an initial diversity increase was followed reduction in diversity, often interpreted as catastrophic mass extinction. We investigate Ediacaran ecosystem structure changes over this time period using the “Elements of Metacommunity Structure” framework to assess whether this diversity reduction in the Nama was likely caused by an external mass extinction, or internal metacommunity restructuring. The oldest metacommunity was characterised by taxa with wide environmental tolerances, and limited specialisation or intertaxa associations. Structuring increased in the second oldest metacommunity, with groups of taxa sharing synchronous responses to environmental gradients, aggregating into distinct communities. This pattern strengthened in the youngest metacommunity, with communities showing strong environmental segregation and depth structure. Thus, metacommunity structure increased in complexity, with increased specialisation and resulting in competitive exclusion, not a catastrophic environmental disaster, leading to diversity loss in the terminal Ediacaran. These results reveal that the complex eco-evolutionary dynamics associated with Cambrian diversification were established in the Ediacaran.

According to this research, the Cambrian biota did not explode but emerged from the Ediacaran biota with a further radiation of body plans, some of which had already become established in the Ediacaran, as were complex ecosystems and inter-relationships between species. Not an explosion of spontaneously created species occurring almost overnight, as creationists like to claim, but a continuum, building on earlier evolutionary results, including complex communities, motility and arms races - major drivers of evolutionary radiation.

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