Take a sharp-eyed walk along the beach at Cocklawburn, just south of Berwick-upon-Tweed, or on the beach just down from St Mary’s parish church on Lindisfarne and with a bit of luck you may find a small disc rather like a tiny petrified polo mint. If you are particularly lucky it will be a well-preserved specimen. A closer look will reveal that the hole in the middle of the “polo-mint” is not circular, but a minutely detailed pentagon. These small discs which bear a resemblance to rosary beads are known locally as St Cuthbert’s beads. The saint must have been particularly heavy handed with his rosary prayers given the numbers of these intriguing objects to be found. They can be discovered not only near to Lindisfarne but also occur commonly in the numerous limestones along the Northumberland coast and north into Fife. They may also be spotted inland throughout the limestones to be found in the middle Carboniferous succession in the Tyne valley and south into the Pennines. A close inspection of the Great Limestone by volunteers on this year’s only geo-walk to Haltwhistle Burn revealed several small discs of this sort.

These beautiful objects are just one part of a particular fossilised animal called a crinoid. Crinoids have a venerable history, over 300,000 times longer than Bede might have supposed, with the first recorded fossils from the Ordovician Period some 480 million years ago. Their close relations in the animal world are more familiar to us and reveal something of the nature of the crinoidal lifestyle.   

Jack Matthison’s Bank is one of my favourite places to be on Lindisfarne, with the enormous sweep of the bay running right up by the massive dune system that reaches north to Cocklawburn with Berwick-upon-Tweed not far beyond. This is a place I have often come across the fragile creamy-white shells of the aptly named sea-potatoes (albeit they can be found on many sandy beaches around the country). Sea potatoes (Echinocardium cordatum) are one of several types of sea urchin found in Northumberland. Looking closely on the top of the animal will reveal a pin-pricked tracery with five-fold symmetry. Similarly, when rock pooling, if you are lucky enough to find a starfish (or even a brittle star) these too usually have a set of five arms. The five-fold symmetry of the starfish, brittle stars, sea urchins and St Cuthbert’s beads is a family brand (carried by most but not all) which gives a clue to their membership of the Echinoderm family. 

The sea urchins, starfish and brittle-stars are predators, wandering the ocean floor to hunt for their invertebrate prey. They get around using hundreds of tube feet covering their apex, akin to the Luggage in the Discworld novels of Terry Pratchett. Rather than muscles or the sinister magic of the luggage, their little feet are operated by a hydro-vascular system with water being pumped in and out of the tube-feet to make them work. These echinoderms also have a “bite” to them. In the case of sea-urchins they have a set of five modified plates which are operated with muscles as teeth to give a powerful bite. This whole structure has become known as Aristotle’s Lantern in consequence of a mistranslation of Aristotle’s description of these animals in his “History of Animals”. Starfish on the other hand have the knack of externalising their stomachs so that they can digest their prey in situ. This gives them the advantage of being able to eat prey larger than their mouths.

Then there are the crinoids. They took a turn down an evolutionary path where they ended up as the couch-potatoes of the echinoderm family. Rather than hunt, some crinoids have developed a skeleton with root like structures at their base (to hold them in the sediment) and a long flexible column made of lots of little discs like a mini Greek column. The animal itself lives like Simon Stylites within a swelling at the top of the column. This swelling, the cyst, has a mouth pointing upwards and is surrounded by arm like structures and it just waits for the food to be delivered by the currents above the seabed. In this upside-down echinoid, the tube feet are to be found in the arm like structures where they are used to help collect the passing food.

To be fair to these creatures, most modern crinoids are only sessile for a part of their lifecycle. They are free swimming as larvae, then as juveniles they go through the couch phase before becoming free-swimming once more as adults.  Their elegant form also belies their lifestyle, their arms can form intricate, feather like structures and the whole animal has a plant-like form. This elegance is reflected in their names, with one group of free-swimming crinoids known as feather stars and the sessile forms commonly referred to as sea-lilies. The name Crinoid derives from this sessile form, coming from the Greek meaning “lily-like”.   

The Carboniferous Period is known for the large number of crinoids preserved as fossils. Rocks laid down in a marine environment are almost invariably limestones and this is where crinoids are often found. The rocks of the middle part of the Carboniferous which underly Hadrian’s Wall between Brampton and Heddon-on-the-Wall have frequent limestone layers in them. Limestones are resistant to weathering and commonly form low lying crags. They are also the raw material for making lime and consequently have been quarried extensively. This means limestones are more commonly exposed and easier to find than many rock types in the Hadrian’s Wall landscape. When you come across a limestone, it’s worth looking for fossils, especially crinoids, an example of which is seen in this month’s mystery rock. Happy fossil-hunting!

@Northumbrianman

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Breaking the soil open with my hand-fork and gently, slowly, pulling each of the mini-minimalist Christmas trees that were the horsetails invading my allotment, I found their roots. The long thin segmented rhizome penetrated the soil like lightning: dark brown with patches of shiny white, with wriggly roots radiating from the joint of each segment. Frustratingly frequently the top broke off at one of the segment joints leaving more rhizome and root to keep the insurgence going. This gardeners’ nightmare was well designed, simple, and effective at maintaining its presence in the soil and fast to invade with its network of rhizomes. Not much wonder then that it has survived as a group of plants (the Equisetidae) from the Devonian Period over 360 million years ago, when plants were becoming a major part of the landscape. 

They thrived in the Carboniferous coal swamps too, becoming one of the most common understory plants, preferring wet environments as indeed they do now. Carboniferous gardeners would not have been happy. There were many more groups of these plants then, from the elegant Sphenophyllum to the more familiar looking Calamites. Calamites had a similar looking form to modern horsetails, with needle-like leaves radiating from the stem nodes. It was, however, much larger, growing up to 10m tall with secondary growth giving it a woody stem with the requisite strength to support its stature.

Calamites weren’t the only familiar plants to be found in the coal swamps. Ferns which wouldn’t look out of place in a modern shade garden could be found in amongst the undergrowth. However, alongside them would have been two groups of plants, one that is now extinct and the other which is uncommon and markedly different from its ancient ancestor.

One of the problems in identifying fossil plants is that they are quite fragile and break up and decay readily – consider looking at your compost heap and trying to visualise the constituent plants from the prize cottage garden that was degraded to make this organic matter. What is preserved tends to be fragmented so that it is difficult to tell which bits belong to each other, much like trying to do a jigsaw puzzle with most of the pieces missing and no picture on the box lid. Early in the research history of Carboniferous plants, the Carboniferous became known as the period of ferns. It wasn’t until more fossils were discovered and with more detailed observations that it became clear that a number of the ferns were a completely different group which produced seeds. These seed ferns (Pteridosperms) as they became known have fern-like leaves but are not ferns. The pteridosperms are now extinct, appearing with the Equisitidae (horsetails) in the Devonian Period; by the end of the Cretaceous Period they had mostly disappeared.

The other group is now represented by club-mosses, quillworts and firmosses; large by moss standards (albeit they aren’t mosses) but diminutive by general plant standards, with the largest reaching circa 2m in height. These are the lycopods and in the Carboniferous, just like Calamites, were much bigger than their modern relations (this gigantism and its cause is another story for another blog). Lycopods formed the canopy in the Carboniferous swamp forests reaching over 30m in height with trunks 2 metres wide at the base. As with the seed-ferns, palaeobotanists initially identified different parts of the same tree as different species. So it is that the bark of one group of these giant lycopods, or scale-trees as they are also known, are given the species name of Lepidodendron. On the other hand, the root structures of these lycopods – both for Lepidodendron and Sigillaria (the other common Carboniferous lycopod) – have been given the species name of Stigmaria.

The giant lycopods, unlike modern trees, are tubular with a simpler vascular structure (the veins that carry the water and nutrients around the plant). They do, however, share the same remarkable polymer used to give plants strength, lignin. Evolving the ability to make lignin was one of the most important steps to enable plants to colonise the land.  Lignin’s strength and resistance to decomposition had major consequences as the mass-trespass of the land-surface got underway in the Devonian Period and expanded into the Carboniferous Period. The plants grew and then died, and the presence of lignin meant that the carbon in the ex-plants was held within the sediments.

When the layers of decaying plant matter were sufficiently thick, as in the Carboniferous deltaic swamps, this resulted in coal formation. This in turn meant that by the early Carboniferous the plant proliferation had reduced the CO₂ content of the atmosphere, in turn moving the global climate to something more temperate (complete with fresh new ice-caps) as this greenhouse gas was removed.

Coal is an extraordinary material and there is much more to say about it. Having explored its formation rooted in the plants it was made from as well as its time and place in our evolutionary and geological history, what else is there to look at? A more detailed exploration of the process of turning plants into coal, the sediments that are associated with the coal seams and what this tells us about these ancient environments (including the animals that cohabited with the plants) would all be interesting narratives to follow. So too is an exploration of the way that coal has been exploited, not least by the Romans, and the consequences this exploitation has had on human development and the environment we live in. But these are all stories for another day.

@Northumbrianman

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