For this month’s blog our Community Geologist, Dr Ian Kille, gives us an insight into Mystery Rock 8 from October’s newsletter, by delving into rock pools and exploring what fossil corals can tell us about the past and also the future of our planet. If you’d like to receive our monthly newsletter and get involved with our Stone Sourcing activities, sign up as a volunteer here.
As a childhood pastime rock-pools offered a wealth of entertainment. One of the tricks I was shown, was to search out green and red blobs hanging on the side of the pools. These sea anemones were sleeping out the low tide till it turned and brought new and exciting things for them to eat. They could be persuaded to come out and play by patiently lapping the water in the pool. This mimicked the waves caused by the incoming tide and was recognised by the sea- anemones as the signal to wake up and get feeding. As the wavelets gently 
washed over them, they turned from unassuming blobs into beautiful florets of many tentacled medusas, sometimes in vivid colours. As a reward they were given scraps of limpet levered off the rock and which their tentacles grabbed, sometimes brushing with a strange stickiness at your fingers as they pulled in the food.
Anemones contain all the hallmarks of the Cnidarian phylum to which they belong. They have radial symmetry and a mouth surrounded by tentacles. Most importantly for their classification they have specially adapted stinging cells (cnidocytes) which reside in their tentacles. Jellyfish, box-jellyfish and siphonophores (which includes Portuguese Man-of-War) are all Cnidarians. More closely related are corals which sit alongside anemones in the sub-class of Hexacorals along with several lesser known animals which all fit under the umbrella of the class Anthozoa.
Corals take the Cnidarians from mostly-blobs-of-jelly status (albeit many are exotic and beautiful blobs of jelly and some are even dangerous), to that of brilliant structural engineers. The corals, most of all the creatures in the Cnidarian phylum, make themselves something firm to sit upon. These cnidarian couches are not only fascinating and beautiful structures but are important both to the fossil record and to our climate.
The reason they are important to the fossil record is principally because their skeletons are hard and therefore more frequently preserved in the fossil record. As with all fossils they have an evolutionary tale to tell and we can see the way they have adapted to a wide range of marine environments 
over the millennia. Some fossil coral species had a relatively short evolutionary history, which, for the Carboniferous period, made them of use in dating the rocks in which they are found. Corals are symbiotic with the photosynthetic organisms Zooxanthellae. This means they have to live in the light, photic zone near the sea surface. This was also true back in the Carboniferous day, and means that the presence of fossil corals can tell us 
that the ancient sedimentary environment was shallow and marine. In addition to all this, corals skeletons display external growth lines which vary in the same way that tree rings do. Corals like it when its warmer and grow faster, so much so that you can not only see seasonal changes in growth, but at a microscopic level you can see daily changes. A dedicated researcher counted these diurnal changes and found that there were about 400 of them per year. This is an interesting corroboration that the speed at which the earth spins is slowing down, so that in the Carboniferous period there would have been shorter days and more of them in the year.
The structure of coral skeletons is complex. A good place to start is with the solitary corals, the ones that want to be alone, and that are found in the Carboniferous limestones. There are many limestones, particularly in the middle Carboniferous which crops out under the middle section of Hadrian’s Wall between Heddon-on-the-Wall and Brampton. This month’s mystery rock is an example of a solitary coral which belongs to the now extinct order of Rugose corals. These mini ice-cream cones are also known as horn corals and there are many varieties of them, distinguishable from each other by the internal structures in their skeletons. The radial element in the skeletons are called septa – the number and shape of which varies between species. The number of septa also increases as the animal becomes older progressively growing its skeleton. Additionally, the coral builds a series of horizontal layers as it grows, called tabulae, which increase the strength of the skeleton. Some corals also grow small, curved plates called dissepiments, which link and strengthen the septa and tabulae creating an intricate lace-like texture when seen in section.
These same structural elements apply to other members of the Rugose order which are more sociable in their habit. Some live in close-knit communities with interwoven long tube-like structures with simple septa and tabulae. Siphonodendron is an example of this type of coral which looks somewhat like a pile of spaghetti. When living it would have formed large structures with the same sort of shape as bracket fungus. Surprisingly, there is an example 
of this coral preserved in the only piece of limestone I have so far found in Hadrian’s Wall, as part of the bridge structure at Willowford crossing. Other species became so friendly that their skeletons have grown together producing polygonal patterns much like a honeycomb.
As with modern day corals it is likely that these formed reef-like structures. Examples of this can be found in the middle limestone group exposed just south of Berwick upon Tweed. Here communal corals are found in profusion mixed with a variety of other corals and other creatures, notably crinoids. Then as now, coral reefs are a diverse habitat supporting a profusion of animals and plants.
These are clearly eco-systems that we should be looking after and not just the modern reefs. Fossil reefs, and all the other limestones formed either by biological action or through precipitation are reservoirs of carbon in the form of calcium carbonate (CaCO3). The process of digging up the limestone and turning it into cement is a major 
source of CO2. Ironically, modern coral’s ability to fix that CO2 into carbonate and perpetuate the cycle is hindered by the acidification of sea water caused by the oceans dissolving excess atmospheric CO2. Unchecked, the combination of increasing sea temperature and ocean acidification caused by man-made CO2 emissions will kill more corals and shut down another mechanism for natural carbon capture.
Change, however, is an essential part of the way that life on earth has evolved. It will be interesting to see how different 
species adapt to this rate of change, especially our species who of all the animals are able to research how the world works and may be able to take action. Thinking geologically is an excellent way of getting a perspective on this. It shows us the way in which the earth works in timescales which are far larger than our daily lives. It allows us to make predictions for the future to allow us to make good decisions beyond the 5-year terms of politicians. It also gives us a perspective on our place in earth’s evolution – the rugose corals of the Carboniferous were around on earth for 200 million years, some 30 times as long as us late arriving hominids. Time spent amongst the rocks leads to a sense of awe, of the scale and beauty and diversity of the world. It also allows us to wonder at the way it is possible to persuade a sleeping sea anemone to wake up and wave its tentacles at you.