Limestone: Death assemblages

This blogpost is part of my ‘limestone lockdown’ project. For an Introduction to the project, and a guide to the growing list of related posts, see Limestone in the Lake District: an Introduction – and the ‘categories’ list in the right-hand bar.

The boulders are the fossilised graveyard of hundreds, perhaps thousands, of the shells of brachiopods, that lived then died about 300 million years ago. These tangled remains of former lives are what geologists call a ‘death assemblage’ [1]. A variety of brachiopod species, large and small, are crumpled together within a layer of limestone less than a metre thick. This scene of mass deaths is sandwiched between other layers that are almost devoid of fossils, in a small outcrop in the ‘limestone country’ to the East of Tebay and at the edge of the Westmorland Dales.

Layer with brachiopods above a layer without

Just a few miles away is Little Asby, where eroded plates of ancient limestone decorate the plateau of the fell-top. At the edge of this limestone pavement, the fell-side drops steeply down to the valley of the looping meanders of Potts Beck – listed, delightfully, as a ‘misfit’ river (apparently this is a technical term!) on the CumbriaGeoConservation website (search for Little Asby and Potts Beck). Thistles and grasses are interrupted here and there by shallow but vertical escarpments, and on the top of the hill there are piles of stones and the vegetated hollows of former sink-holes. One, edged with a circle of jumbled rocks, makes us wonder if it was at one time a clamp kiln for ‘burning’ limestone [2].

We work our way down the side of a dank gully, picking our way across slippery soil and screes of flat, sharp-edged stones that plink underfoot, and then into a rockfall of boulders. Mosses flourish, ferns peep out from crevices; another rain shower blows down the valley, slicking the slippery surface. Then my husband points to a rock from which fossil corals appear to be bursting out, fanned like the drones of a miniature bagpipe. We scramble around the gully, examining small rocks and clutching at large boulders to keep our balance in the wind and rain – the limestone is packed with fossils, mainly corals but brachiopods and crinoid stems too. There are corals in side view, their thin cylinders packed side by side, and in cross-section, with skeletal filaments radiating across the core; some are less than a centimetre across, others are huge solitary specimens ten to fifteen centimetres in diameter. As the rocks have split, so the skeletons have been sliced through in longitudinal, transverse and even sagittal section; they are dramatic, rough and grey and, of course, very dead. Picture, instead, the animals alive and feeding, a rich and colourful profusion of life, tentacles swaying in the currents.

This patch of corals and other invertebrates lived and fed and reproduced in a warm shallow sea, that fringed a land-mass that lay South of the Equator. There were brachiopods, twin-shelled animals lying partly submerged in the sediment; crinoid ‘lilies’ (echinoderm animals related to sea-urchins) with stalks supporting a crown of feathery arms; corals (cnidarians, related to sea-anemones) either solitary or grouped together in colonies; and bryozoans, colonies of tiny, tentacled animals each enclosed in its own box-like skeleton. And because these invertebrate animals were fixed in place, their life-style was that of filter-feeders – they were adapted to strain the water either through wide gills that flickered with minute hair-like cilia, or past hydraulic tube-feet with suction-pads, or tentacles with an armoury of sting-cells and sticky mucus. Planktonic plants and animals, and small particles of organic matter were filtered from the water and captured by the blanket of animals that coated the sea-floor.

Carboniferous sea. (C) Dr Elizabeth Pickett [7]

But during millions of years the relative levels of sea and land repeatedly changed and the environment was slowly but drastically altered. It is possible that there were 40-50 cycles of inundation and retreat by the ocean during the 60 million or so years of the Carboniferous period [3]. These numbers, these periods of time, are impossible to imagine, their reality seems meaningless – but by concentrating on the living creatures themselves, the animals and marine algae, it becomes easier to comprehend what was happening. The brachiopods, crinoids, corals and the like were supported by skeletons made of crystals of calcium carbonate, and when they died their bodies settled on the sea-bed and the soft tissues decayed or provided feasts for bacteria and detritivores. Particles of calcium carbonate were mixed with the sediment to form limey mud. Or more acute catastrophes might have occurred – a storm might uproot a colonised area and sweep the creatures into a pile; the edge of a channel might collapse and smother the filter-feeders; mud might be swept down by flash-flooding rivers: protected by sediment, the skeletons of the animals would remain intact, piled together in death.

Such mass killings are much more frequent than you would expect. In 1860, naturalist Philip Henry Gosse wrote: “After the intense and protracted frost of February 1855 the shores of South Devon were strewn with dead and dying Anemones … which rolled helplessly on the beach” [4]. An earthquake in New Zealand’s South Island in 2018 caused pau or ormer shells to be lifted out of the ocean and deposited in piles, and in February 2021 mounds of burrowing bivalve Molluscs, the razorfish Ensis mixed with some large clams, were washed up at Mossyard Bay near Gatehouse of Fleet on the Galloway coast, the razorfish shells still glossy, their pale muscular feet and siphons extended in death: cause of death very likely the low sea temperatures, and especially the freezing of the sands and mud during low Spring tides. In contrast, there were possibly 1 billion mussels, starfish, sea-anemones and other marine invertebrates of the Pacific shores that died during the ‘heat dome’ that hit western Canada in this same year [5] We are able to see these results of local crises, but there will be many more marine events where the animals die sub-tidally and are buried out of view.

Unlike the sad and stinking mounds of dead animals on the shore, the ‘death assemblages’ and other fossil-packed rocks like those near Little Asby are thrilling to discover. By looking at these skeletal remains we can try to imagine those ecosystems and changing environments, both local and large-scale, short-term and long-term, that occurred in those far-distant times.

See also ‘Skeletons: Sea-sorted’

[1] Dale Chadwick (2019) Death assemblages of marine shells. Earth Sciences Picture of the Day

[2] David Johnson (2018) Lime kilns. History and Heritage. Amberley Press

[3] Peter Wilson (2010) Lake District Mountain Landforms Scotforth Books

[4] P H Gosse (1860) Actinologia Britannica: a history of the British sea-anemones and corals. p214 The Dahlia Wartlet Anemone. London, Van Voorst

[5] California heat dome, July 2021, in The Guardian

[6] See also a very informative video by Sylvia Wooodhead, about the Westmorland Dales limestone pavements

[7] My thanks to Dr Elizabeth Pickett for allowing me to use her painting of the ‘Carboniferous sea’. Elizabeth was a Consultant in the Westmorland Dales geology project, and in addition to her research produced information and diagrams for several information boards at limestone pavements and quarries. See for example, the video [6] at eg 16’40”

This entry was posted in fossils, geology, LIMESTONE, limestone fossils and tagged , , , . Bookmark the permalink.