A rich variety of life inhabited the Carboniferous tropical seas and the commonly encountered fossils include brachiopods, crinoids, corals, sponges, bryozoans and algae. You are most likely to spot brachiopods, corals and crinoids. Look out for the curved shapes of brachiopod shells in cross-section; intricate patterns made up of many small circles or tubes which are colonial or solitary corals and cylindrical or disc-like segments which are crinoid stems. Often being able to see fresh sections of limestone can be difficult and many fossils can be found in the stream beds where they have eroded from outcrops of limestone further upstream. Crinoid fossils are particularly common and are the main constituent of crinoidal limestone.
Fossil crinoid ossicles and crinoid stem with many fused together plates (known as ossicles). Loose block from Surrender Mill.
Fossils may be scattered through the rock or concentrated in layers which would once have been flat-lying reefs and shell beds in a shallow sea. Some fossils are in ‘life position’ whereas others were tumbled around by the sea after they died.
Loose block of crinoidal limestone from upstream of Wain Wath Force. From Underset Limestone (also known as Four Fathom Limestone).
Like modern-day coral reefs, the abundant remains of fossil corals in the Carboniferous limestone suggest the former existence of warm, clear, shallow and well-lit tropical seas. Corals have a variety of branching and encrusting shapes that provide homes for other creatures and act as baffles to trap sediment. Different kinds of fossil corals occur at different levels in the limestone, allowing geologists to distinguish between older and younger beds.
A tabulate coral fossil which we think may be Syringopora geniculata, within Carboniferous Limestone near Keld.
Solitary rugose fossils of Dibunophyllum bipartum, stream section, Old Gang Smelt Mill.
Close up image of fossil rugose solitary corals. Showing the typical horn shape and cross section.
The rugose solitary corals (Dibunophyllum) are formed of coarse-grained sparry calcite and some appear to be linked, note that all the corals in the top left of the section are circular and roughly equidistant from each other indicating they are a life position relative to one another. The larger corals, over 1cm in diameter are variably oriented. Some of the shell and coral fragments are partially filled by fine-grained carbonate rich mud, and the remainder of the filling is a sparry calcite.
Brachiopods have become all but extinct in modern seas and oceans, but in the geological past they flourished at the shallow margins of oceans, especially in the Carboniferous. At first they appear little different from familiar modern-day sea shells, but they are in fact quite distinct, with different shell and soft-part anatomy. Many brachiopods lived openly on the sea floor. Two important groups of brachiopods in the Carboniferous are strongly radially ribbed forms, called ‘spiriferids’, and large, less strongly ribbed forms with relatively plano-convex valves, called ‘productids’.
Limestone sample packed with large brachiopod shells. Upper Swaledale, near Keld in the River Swale.