The strangest of graptolites
Denis Bates and Anna Kozłowska (Poland)
Graptolites (grapto + lithos = writing in the rocks) are common Palaeozoic fossils – found from the Cambrian to the Carboniferous. Usually, they are preserved as flattened and altered remains in mudrocks, particularly the black shales or slates known as “Graptolitic Shales” (Fig. 1), which are widespread around the world, including Wales and southern Scotland.
This article is about a group of graptolites that are extraordinary, even by the usual standards of this strange group of animals. These are the so-called ‘retiolite graptolites’, whose complexity and advanced nature make them a fascinating example of Palaeolithic life.
One of the most famous localities is Abereiddi Bay in Pembrokeshire, the home of the ‘tuning fork’ graptolite, Didymograptus murchisoni (Fig. 1) of Ordovician age. Silurian graptolites from the Rheidol Gorge, near Aberystwyth in mid Wales, also from black shales, are commonly infilled with iron pyrites, preserving their original shape and also with some of their skeleton (Fig. 2). They were once thought to be simple Coelenterata, but, following work by the Polish palaeontologist Roman Kozłowski, are now regarded as belonging to the Pterobranchia, which are related to the Hemichordata.
Occasionally, graptolites are preserved in limestone or chert. Both of these rocks formed or lithified quickly, just beneath the seafloor, and so preserve fossils in almost their original state – rather like insect preservation in amber – uncrushed and perhaps chemically unaltered. Some of the best preserved graptolites come from Arctic Canada and from erratic boulders from the floor of the Baltic Sea.
The graptolites (Figs. 3, 4, 7 to 9, and 11 to 13) can then be chemically isolated from the rock – using dilute acetic or hydrochloric acid for limestones, or the more dangerous hydrofluoric acid for chert. Geniculograptus from the Ordovician Viola Limestone of Oklahoma (Fig. 3) shows the detail of the skeleton, down to individual fibrils less than 1 micron in diameter (Fig. 4).
Graptolites were colonial animals, rather like corals, with the individual cups or thecae each housing an individual zooid, about 1mm long. The initial cup of the colony, the sicula, was presumably made by a sexually produced juvenile zooid, with subsequent thecae being produced by budding. There was a continuous internal space linking the thecae, suggesting that there was also continuous soft tissue linking the zooids together.
The wall of the graptolite skeleton is formed of two different tissues: the fusellum, which is a series of growth bands, seen in Figs. 2 and 3, and a cortex, made of ribbon-like bandages (Fig. 4). The cortex layers were mainly deposited outside the fusellum to strengthen the wall and the material of the wall appears to be collagen, with perfectly preserved fibrils in the case of Fig. 5.
All these graptolites were planktonic, living in the oxygenated surface waters and sinking after death to the seafloor, to be entombed in the accumulating sediment. As each zooid would have been very small, they presumably fed on micro-plankton, filtering it using (it is thought) a pair of feathery arms. Their skeletons come in a variety of shapes (Fig. 6), but are all remarkably regular – uniquely so in the animal kingdom for a planktonic colonial organism. This regularity is probably related to their feeding strategy, with each polyp probably circulating water past the arms by the movement of small hairs, or cilia, to catch the microplankton. The colony would then have moved relative to the water, probably vertically (all plankton tends to do this, on a diurnal cycle).
Retiolite graptolites
While the ordinary graptolite colony was unusual, there are a number of graptolites that took their colony regularity and complexity to a higher level. These have a skeleton which appears to be even more ‘skeletal’ (Figs. 7 and 8) and they may also have had an extra skeletal element, outside the normal one of a string of interconnected thecae (Fig. 9). In a normal graptolite, the wall of the thecae is of a more or less uniform thickness. It forms a three-dimensionally curved structure, rather like that of a motor car, which is more resistant to flexing than if it were planar.
However, the retiolite wall is made of a very thin fusellum, which is then thickened by concentrating the position of the bandages along specific paths, such as round the apertures of the thecae, to form bars or lists. These act rather like the ribs of an umbrella, with the thin fusellum being the fabric. It is remarkable that some specimens, after extraction from the rock, are resilient – they spring back to shape after being flexed. If springy now, they would also have been springy in life. However, in most specimens, only the lists remain intact, the fusellum having been too thin to survive, either during fossilisation or during extraction from the rock.
More remarkable is the extra skeletal element. This appears as a meshwork of lists, which forms a sleeve around the thecae (Figs. 9 and 10). It has a similar structure to the thecae – the lists being formed of bandages, with a very thin fusellum that is not usually preserved. This structure actually starts at the proximal (the first formed) part of the colony, as a circular mesh, referred to as the ancora umbrella (Fig. 8; it looks like an umbrella in some retiolites, at an early stage of growth). In Retiolites (Figs. 9 and 10), which has given its name to the whole group, it can be seen covering the whole graptolite.
The model illustrated was made by Nancy Kirk, who was the first person to recognise the double walled nature of the retiolites. In it, the thecal lists are coloured and the thecal walls are shown as a coloured gauze. Outside, the lists of the sleeve are coloured white. On each side of the specimen, there are a series of rectangular openings (orifices), corresponding to the actual apertures of the thecae inside. Rarely, the fusellar walls of both the thecae and the sleeve are preserved (Fig. 11), and show that they both were continuous, rather than being like a fish net with holes between the meshes.
Stomatograptus, a genus otherwise very similar to Retiolites, has a series of openings, called stomata, along each side (Fig. 12). These are at right angles to the orifices and project outwards, giving the graptolite a cruciform shape.
Another structure, seen in some retiolites, is a terminal structure, called an appendix (Fig. 13). Colonies that have this generally taper distally, so that the overall shape is that of an elongated tear drop. From these structures, we can then see that there were continuous spaces along each side of the graptolite, between the thecae inside and the outer sleeve wall. These spaces also link with the openings of the thecae and also, in some genera, the stomata along each side of the graptolite.
What might have been the function of these series of spaces? The construction of such an elaborate addition to the skeleton of the graptolite must have conferred a big advantage to it and was not simply to provide an extra thickness to the walls – it was too elaborate. It has been suggested by Nancy Kirk that it was related to feeding. If the feathery arms – probably modified – were encased in these spaces, they could have acted as a filter pump. Water, containing food, would have been propelled through the chambers, with the cilia trapping food particles as they passed through. It is not clear in which direction the water flow went, though it is tempting to suggest that it entered through the orifices, and exited through the stomata, or through the distal end.
It is among the last retiolites, in the late Silurian, that we find the forms with an appendix, and therefore a finite end to the colony. These have been described as “superindividuals”, with the whole colony acting as if it were a single organism, each theca having a distinct role – analogous to an organ in a body. The retiolites are truly the strangest and most fascinating, of graptolites – if not of all animals.