This month’s blog from our Community Geologist, Dr Ian Kille focuses on animal fossils, particularly Crinoids, and Mystery Rock 7 which featured in last month’s WallCAP newsletter. If you’d like to receive our monthly newsletter and get involved with our Stone Sourcing activities, sign up as a volunteer here.
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!