The Oldest Muscles In The Fossil Record

The 560 million year old fossil of Haootia quadriformis

The first animals were little more than strands of jelly anchored to the sea bed, incapable of movement except when the ocean currents swayed them. Known as the Ediacara biota, the first members of this group were immensely primitive, extracting nutrition from the surrounding water by filter feeding. Examples of these early Ediacaran species include Charnia and Fractofusus which lived approximately 600 million years ago.

Yet 560 million years ago the Ediacarans had evolved to such a point that they were unrecognisable in comparison to their primitive ancestors. Their bodies were symmetrical rather than fractal. They possessed differentiated heads, tails and segments, and some even had mandible-like structure used for feeding.

By far the greatest leap forward was the development of organs which allowed them to move. Trails and tracks preserved in sedimentary rocks around the world are the tell tale signs of this mobility revolution. This shows that Ediacarans must have developed muscles and muscle tissues to affect motion. Direct evidence of this has been found by researchers from the University of Cambridge. Uncovered from 560 million year old rocks in Newfoundland, the fossil represents an entirely new species and genus, named Haootia quadriformis. Believed to be a cnidarian, a group of organisms which encompasses corals and jellyfish, it was a simple creature.

'The problem is that although animals are now widely expected to have been present before the Cambrian Explosion, very few of the fossils found in older rocks possess features that can be used to convincingly identify them as animals,' said Dr Alex Liu from Cambridge's Earth Science Department. 'Instead, we study aspects of their ecology, feeding or reproduction, in order to understand what they might have been.' The fossil of Haootia, however, displayed a feature which showed beyond a shadow of a doubt that it was an animal, namely a series of parallel lines running across its body which the Cambridge researchers have interpreted as muscle fibres.

An artist's impression of Haootia quadriformis

The fossil indicates that Haootia lived anchored to the sea bed and so it is unlikely that the fibres were used for the purpose of locomotion. Instead it may have used them to contract its net-like body to wash nutrient-rich water onto its feeding apparatus in a fashion similar to modern jellyfish, or even as a defensive mechanism to reduce its size.

'The evolution of muscular animals, in possession of muscle tissues that enabled them to precisely control their movements, paved the way for the exploration of a vast range of feeding strategies, environments, and ecological niches, allowing animals to become the dominant force in global ecosystems,' said Liu.

Molecular evidence suggests that animals first evolved around 700 million years ago during an ice age referred to as the Snowball Earth Glaciation. Further studies suggest that the protein actin, which forms an integral part of muscle tissue in animals evolved around 600 million years ago. Therefore it is possible that muscles could be older still, although this fossil provides a benchmark which sits reasonably closer to this figure.

Haootia is a prime example of how the Ediacara biota laid the biological foundations for the Cambrian Explosion, but what is equally remarkable is the degree of preservation this fossil displays. Most Ediacarans are preserved in coarse grained sediments which reduces the detail of the fossils. As a result their biology is mysterious. What Haootia shows is that examples of detailed soft body preservation do exist from the Precambrian. Continued efforts should unearth fossils with enough preservation to unlock the position of the Ediacara biota in the tree of life, an endeavour which has so far eluded palaeontologists since their discovery over 70 years ago.