Wednesday, 24 September 2014

A possible Cnidarian from the Late Ediacaran of Newfoundland.

The fossils of the Ediacaran Period record the first widespread macrofossils in the rock-record. Many of these fossils do not appear to belong to any modern group, but instead are thought to belong to an extinct taxa (sometimes known as ‘Vendobionts’), which may-or-may-not be related to modern Animals, though some fossils have been linked to Sponges (a group which also has pre-Ediacaran potential members), Anthozoan, Hydrozoan and Scyphozoan Cnidarians, Ctenophores, Placozoans, early Molluscs and even Ascidian Chordates. Most of these potential members of modern groups are from the latest part of the period, between 555 and 541 million years ago, when these groups are predicted to have existed, but remain rather controversial.

In a paper published in the Proceedings of the Royal Society SeriesB; Biological Sciences on 27 August 2014, Alexander Liu of the Department of Earth Sciences at the University of Cambridge, Jack Matthews and Latha Menon of the Department of Earth Sciences at the University of Oxford, Duncan McIlroy of the Department of Earth Sciences at the Memorial University of Newfoundland and Martin Brasier of the Department of Earth Sciences at the University of Oxford and the Department of Earth Sciences at the Memorial University of Newfoundland, describe a new Ediacaran fossil from the 560 million year old Fermeuse Formation on the Bonavista Peninsula, Newfoundland, which they interpret as an impression of a fibrous, muscular, quadrilaterally symmetrical organism.

The new fossil is named Haootia quadriformis, where ‘Haootia’ means ‘Demon’ in the indigenous Beothuk language of Newfoundland, and ‘quadriformis’ means ‘fourfold-form’ in Latin. Haootia quadriformis has a 56 x 37 mm discoid structure interpreted as a basal disk, and a 49 x 72 mm roughly rectangular body, covered with linear ridges interpreted as muscle fibres. At each of the corners bundles of fibres bifurcate (split in two) several times.

Digitized images of Haootia quadriformis, emphasizing the convergence of fibrous linear features at the corners of the body, and the symmetry ofthe fossil. (a) Photograph of the specimen as it appears in situ. (b) Interpretive sketch of the non-retrodeformed specimen. Labels indicate: (a) muscle bundles, (b) expanded bundles, (c) ‘contracted’ bundles, (d) twisting fibres, (e) superimposed fibres and (f ) disc. Liu et al. (2014).

This is very different from the typical anatomy of a frondose Ediacaran, which usually have a circular basal disk with a single leaf-like frond, and which have never been found to have anything resembling muscle fibres. Quadrilateral symmetry is not found in other Ediacarans, but it is the most common body-plan within both modern and ancient Cnidarians, and thought to be the ancestral state within the group.

Artistic reconstruction of Haootia quadriformis. Liu et al. (2014).

Not only does Haootia quadriformis have a bodyplan that strongly suggests Cnidarian affinities, it actually resembles the structure of a group of modern Cnidarians, the Staurozoans, which have a cup-like body attached to a circular basal disk, with branching arms at each corner. While the specimen does not preserve enough features for Liu et al. to place it within this group, would be an exceptionally large member of the group, and has multiply branching limbs at each corner while modern members have limbs that branch only once, the presence of a member of this group within strata of this age (about 560 million years old) would not fall outside our understanding of the history of the group. Staurozoans are placed within the Medusozoa (the group of Cnidarians which includes Jellyfish), which are predicted by molecular data to have split from their closest relatives, the Octocorallia, about 571 million years ago. Within the Medusozoa, the Staurozoans are thought to be the group which split from all other (known) members the earliest, making the presence of such an organism in 560 million year old sediments quite possible.

The extant staurozoan Lucernaria quadricornis, exhibiting abody plan similar to that hypothesized for Haootia quadriformis. Liu et al. (2014).

See also…


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