Graptolites are curious fossils that are common in Lower Palaeozoic rocks where other types of fossils are lacking. The word ‘graptolite’ comes from Greek words that mean ‘writing’ (graptos) and ‘stone’ (lithos), and refer to the fact that graptolite fossils look like pencil marks on stone, partly because they’re flat and partly because of the iridescence of many specimens when freshly exposed. It is generally assumed graptolites were planktonic organisms that occupied an ecological niche like that of modern jellyfish, drifting about the oceans feeding on algae or tiny animals harvested using some sort of filter-feeding mechanism.
The impetus for this article was a quick but successful trip to Abereiddy in Pembrokeshire, Wales, about 2.5km from Britain’s smallest city, St Davids (population: 1,800). I had been to Abereiddy many years before on a geological field trip with Andy Gale, who is currently professor of geology at the University of Portsmouth, but I did not have any clear memory of where the fossils were to be found. But, as it happened, this locality is one of those where the fossils are abundant and easily collected – provided you look at the right sorts of rocks.
Collecting at Abereiddy Bay
Abereiddy is a tiny place, but the bay has become a popular tourist attraction because of a flooded quarry known as the Blue Lagoon. Quarrying for slate ended in 1901 and the sea eventually broke through to the quarry, creating what is, in effect, a small natural harbour. In fact, for a while, fishermen used the quarry for precisely that purpose, but nowadays, the quarry is maintained purely for recreation, with a nice little beach at one end and diving points at the other. Compared to the strong currents that hit Abereiddy Bay, the water within the Blue Lagoon is very calm, which adds to its popularity with families.
The land is owned by the National Trust and is part of the Pembrokeshire Coast National Park. It is a site of special scientific interest (SSSI), so hammering the bedrock (including the cliffs) is not allowed, but it is not really necessary. The rocks in the cliffs on either side of the bay are mostly volcanic and metamorphic, which is of course what attracted the quarrymen looking for slate. However, fossil collectors will want to find the Ordovician shales wedged in between the cliffs, and chunks of shale regularly get worked out of the cliffs and onto the beach. Therefore, all they need to do is walk down to the beach and look over the soft, flaky shales found there, using a hammer only when larger pieces need to be split.
Apart from graptolites, other fossils have been reported from Abereiddy, but apart from trace fossils, these are not common. Among the rarer fossils recorded here are planktonic trilobites called agnostids and, at certain horizons, brachiopods of the type usually referred to as Lingula, although they are almost certainly not closely related to the modern Lingula alive today.
The graptolite that is normally found at Abereiddy Bay is a species known as the Tuning-Fork Graptolite, Didymograptus murchisoni. It existed for only a relatively short period of time, during the later part of the Middle Ordovician, about 470 to 464mya. It is often used as an index fossil for this period of time, which geologists refer to as the Llanvirn stage after a farmer’s cottage on the lane leading to Abereiddy.
In Greek, the word didymus means ‘twin’ and this accurately describes the shape of the graptolite, Didymograptus murchisoni. While most of the fossils will consist of loose bits, many will be found in pairs, joined together at one end by a short stem known as the prosicula. Such specimens really do look a bit like tuning forks, hence their common name.
On closer inspection, each half of the organism is smooth on the outer edge, but jagged on the other – to my eye, they look a lot like electric saw blades. They are narrowest near the prosicula, wider at the other end, and typical specimens are about 5 to 6cm long, although some are considerably bigger than that.
What now looks like a pencil drawing of a saw blade on a typical slab of shale was actually a three-dimensional structure in life. Exceptional specimens preserve their structure and, under a microscope, graptolites reveal a zigzag arrangement of tube-like chambers, open at the saw-tooth edge. In life, these chambers were occupied by a small animal known as a zooid. These were colonial animals that were genetically identical, living together in a shared skeleton in an analogous manner to corals or bryozoans. Nothing of the zooids remains, but their shared skeleton ultimately became preserved as the fossil we call a graptolite.
The prosicula was not merely the joining point between the two halves of the colony – it was also the first chamber made by the graptolite animal. Assuming the organism started off as some sort of free-floating planktonic larva, once it reached a certain point, it secreted an organic skeleton around itself, the prosicula. Budding produced new zooids, one zooid starting one side of the colony and a second zooid on the other side. These zooids would secrete organic material around themselves, adding to the graptolite skeleton, and in turn they would bud off their own zooids, growing the colony.
The fact that graptolites produced a skeleton made of an organic, chitin-like material is noteworthy. In most situations, such a skeleton would be quickly decomposed by bacteria once the graptolite colony died, but if the skeleton fell onto an oxygen-poor sediment, this would not happen. The reason graptolites are so common in shales is that these rocks are formed in anoxic environments that lacked oxygen and consequently any of the bacteria or scavengers that would have destroyed a graptolite skeleton were absent.
However, if there was no oxygen there, how could the graptolite animal have survived? It was simply because the animals did not live on the sediment, but far above it, in the well-oxygenated surface waters where the plankton lived. Modern seas and oceans are actually rather unusual in being thoroughly mixed, with oxygen-rich water working its way down even into the depths. For most of the Earth’s history, there have been large parts of the marine realm that were oyxgen-poor environments.
It does not seem likely graptolites could actively swim and it is generally assumed that these colonial organisms drifted about on ambient water currents. But what kept them afloat? One idea is that they were attached to seaweed and other bits of flotsam. Such a mode of life can be very effective and, if you look at the seaweed floating about in the famous Sargasso Sea, you will see any number of animals clinging onto the seaweed, from colonial organisms such as hydroids and bryozoans through to quite sizeable crabs and snails. Hitchhiking saves on energy, but it is a precarious life – unless you can swim away, if you grow too big for your piece of seaweed or, for some reason, your ride falls apart and decays and you sink along with it, to an almost certain death.
To avoid this problem, many planktonic organisms produce their own floats, and one school of thought suggests that graptolites did precisely this. The jellyfish-like hydroid we call the Portuguese Man o’ War is perhaps the most famous example of a floating animal, using a specific type of gas-filled polyp called a pneumatophore to provide buoyancy. Of course, this polyp is a living thing and dries out in the air, so periodically it flips onto its side, wetting the pneumatophore. Other animals that produce floats include a goose barnacle, called Dosima fascicularis, that builds a polystyrene-like float, and a genus of delicate, violet snails, called Janthina, that produce floats from bubbles. Which of these, if any, give a clue to what graptolites might have done is open to debate, but the float model does account nicely for the fact graptolites are sometimes found arranged in rings, as if they were attached to something at the time when they fell onto the seafloor.