16. Fossils Underfoot

The flooring blocks of the Leonard Wolfson Auditorium, café, Academic Wing, and adjacent Stallworthy Grove are made of 340 million-year old Carboniferous Limestone from the Threecastles Quarry at Kilkenny in Leinster Province, south-east Ireland. At the time these limestones accumulated, the British Isles formed part of the continent of Laurasia, made up of North America and Europe west of the Urals, and lay well to the south of their present position. There was no North Atlantic Ocean between Europe and North America at that time.

Photo: George Mather

The limestone, commonly known as Irish Blue Limestone, is made up of grains of calcite – a form of calcium carbonate – derived from the skeletons of a range of marine organisms. It accumulated on a shallow sea floor in turbid waters. The colour of the limestone reflects the presence of land-derived clays. The blocks in the Stallworthy Grove and those flooring the café come from a level in the sequence where stable sea floors supported a rich fauna, dominated by corals, brachiopods, and crinoids, their calcite skeletons white against the blue-black limestone matrix. In contrast, the flooring blocks by the Lodge desk and outside the entrance come from a different part of the sequence, with few obvious fossils. They have a subtle mottled appearance, the result of the carbonate sediment that was ultimately transformed into limestone having been churned over by successive generations of soft-bodied organisms, whose only record is their burrows.

To return to the fossils. The corals present belong to two extinct groups, known as tabulate and rugose corals. Unlike modern reef-building corals, they probably lacked the symbiotic organisms (zooxanthellae) that limit present day reef development to the photic zone (the zone of light penetration). Several types are present. Massive colonies, built of close-packed polygonal corallites belong to the genus Palaeosmilia. Each corallite was secreted by a single polyp, a sea anemone-like individual. A second rugose coral, Lithostrotion, is made up of cylindrical corallites, and appears as areas of circular sections around a centimetre in diameter, with radiating septa within. Areas of much smaller circular sections belong to a second species of Lithostrotion. Colonies of the tabulate coral Michelinia have polygonal corallites, but lack septa.

Image taken from a nineteenth-century Palaeontological Society monograph.

Distinctive circular to elliptical structures consisting of one or two rings of white shell up to ten centimetres across, and single U-shaped structures, are sections through the shells of a group of brachiopods known as productids. Their shells consisted of two bowl-shaped valves, with a narrow cavity between (now filled with blue-black limestone) that housed their soft tissues. These animals lived with the convex lower valve resting on, or in, the sea floor, and filtered sea water for nutrient particles, as do extant bivalves such as mussels and oysters. They occur singly, concentrated into patches, or stacked, like saucers, as a result of being swept together by bottom currents. Diamond-shaped sections, filled by blue-black limestone, are less frequent. They are cross-sections of a second group of brachiopods known as spiriferids.
A range of much smaller brachiopods with thinner shells are also present.

Much rarer are sections of cylindrical crinoid stem ossicles, around a centimetre in diameter. Crinoids – or sea lilies – are representative of the phylum Echinodermata, which includes sea urchins and starfish. But rather than being free-living, they were attached to the sea floor by a long stem of ossicles made of calcite, and held together by soft tissue in life. The most obvious examples are in the slabs outside the Lodge

Jim Kennedy