Monday, May 24, 2010

The 'Scutella Bed'

Specimen of Scutella subrotunda (Leske, 1778) from the British Natural History Museum, E16593

Leske's original description of the sea-urchin Scutella subrotunda, 1778


The rocks of the Maltese Islands formed beneath the sea in the period spanning from the late Oligocene to the late Miocene, from 28 to 5 million years ago.

By looking at the rocks and the fossils they contain, it is possible to deduce which kind of environment existed all those years ago, by comparing them to present marine habitats and environments.

The Lower Coralline Limestone (known in Maltese as Żonqor) is the first exposed rock of the islands. It was the first to form, 28 to 23 millions of years ago, in the Oligocene period.

Towards the end of the Oligocene, the consistency of the Lower Coralline Limestone changed considerably, passing from frequently semi-crystalline, very compact rock (Attard member) to about ten metres' thickness of relatively less rigid, coarser rock (Il-Mara member), with cross-bedding characteristics. This probably represents very suddenly shallowing water, and may suggest tectonic activity.

This changed environment meant that the seabed was now subjected to stronger currents than before, therefore waves would drag large amounts of debris from deeper waters and deposit them onto these shallow reef-like platforms nowadays called the Maltese Islands.

This debris consisted mostly of organisms and their skeletons, the most common of which was the now extinct sea-urchin Scutella subrotunda (Leske, 1778). The large calcium carbonate testas (usually fragmented, but not exclusively) of huge populations of this echinoid compose one or more layers (up to five) marking the end of the Lower Coralline Limestone phase, to the extent that they have been named 'Scutella Beds' by the British geologist T. A. B. Spratt.

Scutella Bed in Xlendi member Lower Coralline Limestone, Spinola Bay

Scutella fossil in Xlendi member Lower Coralline Limestone, Dwejra (Gozo)

References:

Leske, N. G., 1778. Additamenta ad Jacobi Theodori Klein naturalem dispositionem Echinodermatum et lucubratiunculam de aculeis echinorum marinorum. Lipsiae, Leipzig, 278 pp.
Smith, A. B. (ed.), 2005. The Echinoid Directory. [WWW document, link: http://www.nhm.ac.uk/research-curation/projects/echinoid-directory/index; last accessed: 24.V.2010]
Spratt, T. A. B., 1843. On the geology of the Maltese Islands. Proceedings of the Geological Society of London, 4 (2): 225-231.
Spratt, T. A. B., 1854. On the geology of Malta and Gozo [2nd edition]. Malta, 16 pp.

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