Wednesday 15 February 2012

A partial Marrellomorph Arthropod from Western Queensland.

The Marrellomophs were early Arthropods, best known from the Burgess Shale in British Columbia, but with specimens being known from the UK, Morocco and China as well. They are not particularly well understood, as they were soft bodied and only entered the fossil record in sites of exceptional fossil preservation (Fossil Lagerstätte), but it is thought they may have formed an important part of Early Palaeozoic ecosystems, and that they can shed light on the evolution of they earliest Arthropods.

In a forthcoming paper in the journal Acta Palaeontologica Polonica a team of scientists lead by Joachim Haug of the Department of Geology and Geophysics at Yale University describe the discovery of a partial specimen of a Marellomorph Arthropod from Mount Murray in Western Queensland.

The new specimen has been named as Austromarrella klausmuelleri, meaning Klaus Müller's Southern Marrella. Klaus Müller was the discoverer of the Orsten Fossil Assemblage in Sweden (more on this bellow), and Marrella was the first genus of Marrellomorph described (from the Burgess Shale, by Charles Dolittle Walcott) and which gives the group its name. It is identified on the basis of a partial limb fragment 970 μm (0.97 mm) in length, from an 'Orsten-type' nodule studied by electron-microscopy at the Central Unit for Electron Microscopy at the University of Ulm.

Scanning electron microscope images of Austromarrella klausmuelleri. (A) Median or lateral view (without the whole specimen it is unclear which). (B) Anterior or posterior view. (C) Median or lateral view; the other side to 'A'. (D-F) Details of spine, as indicated on 'B'. From Haug et al. (2012).

'Orsten-type' preservation refers to the type of preservation first described from the Orsten Fossil Assemblage of Sweden, in which small, early Palaeozoic fossils, generally presumed to be interstitial life-forms, are preserved as highly detailed three-dimensional phosphate casts. This is attributed to the activities of bacteria that coated the original organisms (of which nothing remains), and preserved themselves via a phosphatization process. This type of preservation is not found in more recent deposits, where sediments are constantly reworked by larger organisms, but provides a unique view of the interstitial fauna of the Cambrian. Interstitial animals that live in between the particles of marine sediments, they are little understood, but are the most abundant animals on Earth (everything you ever read telling you that insects were the Earth's most abundant animals was wrong, several different groups of interstitial animals are now known to vastly outnumber them) and are similarly important in ecological terms. Such animals have almost no fossil record, so Orsten-type deposits are highly valued by palaeontologists.

Austromarrella klausmuelleri is interpreted to have been an interstitial animal 4-9 mm in length. This expands the ecological niches known to have been used by Cambrian Marrellomorphs, since the Burgess Shale and other specimens, while not large, are interpreted as living on the surface of the sea floor or in the water column. It is quite possible that this single specimen is not a mature animal, and that part of its life cycle was spent in a different environment, but there is no way of resolving this without further specimens.