Neale Monks (UK)
The rocks we all know in Britain and Eire because the Carboniferous Limestone were laid down between 363 and 325 million years ago, during a period when global sea levels were particularly high, a condition that geologists refer to as a transgression. The climate was tropical, and the warm, shallow seas that covered much of the British Isles teemed with life. Consequently, the Carboniferous Limestone is often highly fossiliferous, and good exposures can yield vast numbers of crinoids, brachiopods, corals, bryozoans and other types of marine fossil.
Despite being known as the Carboniferous Limestone, one thing notably absent from this formation is coal. Coal is made from the fossilised remains of trees, and the forests and freshwater swamps where those trees grew could only develop once sea level had dropped. Coal-bearing sediments weren’t laid down until the second half of the Carboniferous Period, when sea level was relatively low.
| International stratigraphy |
|---|
| The International Commission on Stratigraphy refers to the interval of time between 359 and 299 million years ago as the Carboniferous Period, but, historically American geologists recognised two periods instead: the Pennsylvanian and the Mississippian. These were roughly equivalent to what geologists elsewhere considered the Lower and Upper Carboniferous, so the ICS has standardised the Pennsylvanian and Mississippian as the two epochs within the Carboniferous. |
However, it isn’t quite as simple as sea level dropping in the middle of the Carboniferous and all the subsequent sediments of the period being terrestrial in nature. What tended to happen was that periods of low sea level alternated with periods of higher sea level. Therefore, formations like the Coal Measures are characterised by bands of terrestrial sediments (such as coal) interspersed with estuarine and marine sediments, such as shales and limestones.
So, while the marine fauna of the Carboniferous Period is best seen in the Carboniferous Limestone, to some degree, it can also be seen in later marine sediments, such as those in the Coal Measures. Indeed, many types of marine invertebrate persisted with little change well into the Permian. However, by the end of this Period, the situation had changed dramatically, and many of the marine invertebrates that were abundant during Carboniferous times became extinct. Among the victims were several groups of crinoids, brachiopods and corals, to name just a few.
Permian limestones are poorly exposed in the British Isles and, consequently, it is difficult to examine marine invertebrate faunas, such as those that existed immediately prior to the Permo-Triassic mass extinction event. This gives the Carboniferous Limestone a special significance, as it gives scientists and amateur geologists by far the best and most convenient way to examine the full diversity of Late Palaeozoic marine faunas.
Carboniferous Limestone exposures
The Carboniferous Limestone is one of the most widely distributed formations in the British Isles, with only the north of Scotland, central Wales, and southern and eastern England being far removed from convenient exposures. Several major geomorphological features are partially or entirely made of it, including the Mendips, the Peak District, the Pennines, and the Midland Valley of Scotland. The great coalfields of Wales are bounded to the north and south by Carboniferous Limestone, and virtually all of the central plain of the island of Ireland is a single, huge expanse of this sedimentary rock. If you look at a geological map of the British Isles, you can scarcely miss the fact that Carboniferous Limestone covers more of these islands than any other formation.
Limestone cliffs of the quarry marking the boundary.
While there are numerous coastal exposures, many of the best places to see the Carboniferous Limestone are inland, at places such as disused quarries cut into the hillside. Inland exposures have a tendency to be overgrown, but, if you’re prepared to walk up the hills (where the weather keeps any vegetation down to a minimum), some of these old quarries can be highly productive. As ever, collecting should be done in a safe and responsible manner and, with inland sites especially, care should be taken not to trespass on private land.
Notable coastal exposures in the UK include Weston-super-Mare and Portishead in Somerset, Great Ormes Head in Conwy, Aberlady in East Lothian, and St Monans in Fife. Excellent inland outcrops can be found in the Peak District, at places such as Castleton and Parkhouse Hill, while geologists in Scotland will find similar exposures and quarries all over the Midland Valley. I have particularly fond memories of collecting at Trearne Quarry (Ayrshire) and on Bishop’s Hill (on the Fife/Kinross border) during my student days in Scotland.
Some Carboniferous Limestone localities are at Sites of Special Scientific Interest (commonly known as SSSIs). Collecting at such sites may be prohibited entirely (as at Castleton) or limited to loose material (as at Aberlady). Working quarries are among the best places to collect fossils, but, for safety reasons, these are closed to casual collectors. However, geological clubs often organise field trips to quarries and these are well worth attending.
Collecting and preservation
Carboniferous Limestone is a hard, dense rock and, consequently, rather difficult to work with. Hammering out individual fossils is not easy and, because the rock often splinters, any such work will demand appropriate safety gear, in particular, safety goggles. For casual collectors, it is usually best simply to go through loose rocks and scree, picking out pieces with interesting fossils embedded in them. Trying to clean these up is difficult and requires a bit of practise if you are to avoid shattering the fossil completely. It certainly isn’t something to do in the field. Instead, wrap each slab or fossil in newspaper to prevent scratching during the journey home, and get on with collecting more material.
The durability of limestone does mean that, once you get your fossils home, they are essentially zero maintenance. Unlike fossils preserved in chalk or as pyrite, limestone fossils are robust and tolerant of changes in temperature and humidity.
Corals
Corals are among the most common fossils in the Carboniferous Limestone and, at some locations, they can be by far the most dominant specimens. For instance, at Weston-super-Mare, large solitary corals (of the genus Caninia) are very common. Like many modern corals, Caninia corals grew upwards, but, as they grew, they sometimes toppled over. Such a coral would start growing in the new ‘up’ direction, and end up with a skeleton that looks like a crooked tube, with each kink reflecting a time when it tipped over and had to start growing in a new direction.
Other corals were colonial and formed reefs. Lithostrotion junceum is one very common example of a reef-building coral and can be found dominating entire horizons of Carboniferous Limestone, as can be seen at Aberlady and St Monans.
However, note that, while Carboniferous corals (indeed, Palaeozoic corals generally) might resemble modern corals, they are not closely related. The Palaeozoic corals are mostly members of the orders Rugosa and Tabulata, groups that became extinct by the end of Permian. Almost all modern corals belong to the order Scleractinia (which first appeared in the Middle Triassic), the few exceptions — perhaps surprisingly — being closely related to things like jellyfish and anemones.
Crinoids
Whereas corals dominate the faunas at some Carboniferous Limestone localities, crinoids dominate at others. Any Carboniferous Limestone exposure in the Peak District, Pennines or Midland Valley is likely to provide the collector with lots of crinoid material, usually in the form of disarticulated skeletal elements known as ossicles. Indeed, certain limestones are so rich in crinoid material that they are called crinoidal limestones and, when polished, make a decorative, marble-like building stone.
Loose ossicles look to us a bit like fat Polo mints, but medieval fossil collectors compared them to rosary beads and commonly associated them with saints and other holy people. In the north of England, crinoid ossicles were known as St Cuthbert’s beads, while in Germany they were named after St Boniface.
Occasionally, crinoids can be found in their articulated form and then they can be easily compared with a group of echinoderms still alive today and known as sea lilies. Modern sea lilies are divided into two types – those with stems and those without. The stemless forms are the ones found in shallow water habitats, such as sea lochs and coral reefs. The look a bit like flowers, but the petals are in fact five arms equipped with feathery projections used to trap the plankton on which they feed.
Only in the deep sea do stemmed sea lilies persist, and their discovery in the late nineteenth century was something of a surprise to palaeontologists. The assumption had been that stemmed sea lilies had died out some time during the Mesozoic. However, when the Victorians were dragging up transatlantic cables for repair, they were astonished to find stemmed sea lilies attached to them, seemingly unchanged for millions of years.
Since then, biologists have spent time using submersibles and robots to watch stemmed crinoids in their natural habitat, and fascinating animals they turn out to be. Among other things, it was discovered that they move about! Therefore, when looking at fossil crinoids, it is important to remember that, however much they look like plants, they were in fact animals – and quite a bit more active than you might imagine!
Bryozoans
Although often overlooked by collectors, bryozoans are very common in the Carboniferous Limestone and are well worth getting to know better. Like their modern relatives, these were colonial organisms, somewhat like corals in shape and lifestyle, though very different in terms of anatomical detail.
In particular, one bryozoan genus will be familiar to anyone who has collected fossils from the Carboniferous Limestone, and that is the genus Fenestella. These bryozoans formed lace-like colonies that apparently formed quite large, whorl-shaped structures attached to firm substrates. Their fossils are often very abundant at some horizons, particularly at levels where crinoids are also common.
Related to Fenestella is the famous bryozoan genus Archimedes. As its name suggests, the central part of the colony formed a robust, screw-shaped structure around which the rest of colony extended. Unfortunately, this genus doesn’t occur in the Carboniferous Limestone, though it is quite common in equivalent strata elsewhere in the world.
Brachiopods
Like bryozoans, brachiopods are equipped with a lophophore (that is, a feeding mechanism consisting of a ring of tentacles surrounding the mouth) rather than gills. Therefore, despite their superficially bivalve-like appearance, they aren’t molluscs. Brachiopods were very common during the Carboniferous Period, and are easily found at most Carboniferous Limestone exposures. The most commonly encountered species are those belonging to the orders Strophomenida and Productida.
At this point, it’s probably worth stating that brachiopod taxonomy has been heavily revised in the last few years, and geologists still argue about the precise relationships between the various brachiopod groups. For a long time, strophomenids and productids were both considered part of the one order Strophomenida, along with a number of other distinct groups. That usage has now been abandoned and several new orders are recognised, reflecting a clearer understanding of their distinct evolutionary histories.
None of this really matters much to the field geologist of course! Broadly speaking, the brachiopods you are most likely to find in the Carboniferous Limestone are the flattened, scallop-like species (which may be either strophomenids or productids) and the deep, bowl-like species (which are usually productids). The flat forms, such as Chonetes, are thought to have lived either resting on the substrate or attached to things like crinoid stems. It has been proposed that those living on the substrate were mobile to some degree and, like scallops, were able to clap their valves together vigorously to swim away from danger.
The deep-bodied forms, like Productus,could have been infaunal to some degree. If they did live in the substrate, they could have used their long spines to keep them fixed in the sand, while having a ‘spout’ at one of the shell so that water could enter and leave the body chamber, even if the shell was partially covered with sediment. That said, many of these deep-bodied forms seem to have lived in aggregations rather like reefs, rather like modern oysters and mussels.
Some of the productid brachiopods attained considerable size and, at certain horizons of the Carboniferous Limestone, specimens of Gigantoproductus can be found measuring well over 20cm in width.
Other types of brachiopod can also be found, notably spiriferids and rhynchonellids. These two groups of brachiopod are closely related and shared the same spiral-shaped lophophore design. While they tend to have a lozenge or pyramidal-shape, the most obvious feature that distinguishes them from strophomenids and productids is the shape of the margin of the shell. On strophomenids and productids, the margin of the shell is essentially flat, like that of a scallop. However, the margins of spiriferid and especially rhynchonellid shells are thrown into a series of folds, so that their openings appear to be guarded by a series of interlocking teeth. The Latin name of one common genus of Carboniferous brachiopod, Pugnax, underlines their rather fearsome appearance!
Although spiriferids died out by the end of the Jurassic, rhynchonellids are still around, so we can at least say something about their ecology. Living rhynchonellids have a short stalk, called a pedicle, which attaches them to solid objects, including other types of shellfish. As they cannot move, it is important that they attach themselves to something that won’t be smothered with sediment. Therefore, many modern brachiopods like to fix themselves to steep rocky walls, such as those found in sea lochs, while others attach themselves to animals, such as mussels, that are able to crawl about on top of the mud.
Trilobites
Trilobites are not particularly common in the Carboniferous, though occasional specimens of genera such as Griffithides and Phillipsia can be found, from time to time.
There is some debate as to how diverse and abundant trilobites had become by Carboniferous times. On the one hand, all of the Carboniferous trilobites belong to just two families within a single order, the Proetida. Compare this with the early Ordovician heyday of the trilobites when there were over 60 different families scattered across no fewer than nine different orders. On the other hand, while taxonomic diversity might not seem very high, the actual variation in terms of ecological niches was quite impressive. There were blind trilobites living in deep water, and shallow water species equipped with big eyes, all the better for spotting predators. Trilobite expert, Richard Fortey, has argued that, far from being in decline, trilobites were in fact recovering from the mass extinctions they had suffered at the end of the Devonian and, throughout the Carboniferous, they were enjoying something of a renaissance. That said, most of the trilobites present during this period are small, woodlouse-like animals that were very different from the larger and more flamboyantly armoured specimens that can be found in older sediments.
Cephalopods
Nautiloids and primitive ammonites can be found in the Carboniferous Limestone, but they are usually very rare except at certain localities, such as Castleton in the Peak District. Preservation tends to be rather poor, but at least some species occur with sufficient frequency that they have been used for the purposes of stratigraphy. Ammonites belonging to the order Goniatitida are particularly favoured in this regard, and are most readily distinguished by examination of the suture line. On goniatites, the suture line is bent into a series of folds that alternate between sharply pointed creases going in one direction and gently curved ones going in the other. Goniatites crenistria and Goniatites striatus are among the species most frequently encountered in the Carboniferous Limestones.
Conclusion
The Carboniferous Limestone is one of my favourite geological formations. It’s a highly accessible formation within easy reach of many parts of the UK and Ireland, and coastal exposures are comparatively family friendly, provided you’re content to pick over the scree. But what really makes the Carboniferous Limestone special is the sheer abundance of the animal life revealed. Though admittedly biased towards animals with hard shells and exoskeletons, the collector will still be rewarded with a very fair impression of the rich diversity of life that teemed around a Carboniferous reef.
Further reading
K Edwards & B Rosen 2004, From the Beginning
R Ellis 2003, Aquagenesis
R Fortey 1993, The Hidden Landscape: A Journey into the Geological Past
R Fortey 2001, Trilobite! Eyewitness to Evolution
J D Lawson and D S Weedon 1992, Geological Excursions around Glasgow and Girvan
N Monks & P Palmer 2002, Ammonites
M J Rudwick 1970, Living and Fossil Brachiopods
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