Fossils explained 7: Crinoid columnals T h e hard skeleton of many marine invertebrates consists of only one or two separate hard parts - for example, the single valve of gastropods and cephalo- pods, and the paired valves of brachiopods, bivalves and ostracods. Because there are relatively few parts upon which the classification of such organisms is based, it is usually easy to name dissociated and even broken valves, whether recent or fossilised. This is not so in the case of fossil echinoderms, which are composed of numerous calcite plates called ossicles. The ossicles of the echinoderm skeleton are held together by ligaments which rapidly rot upon death. The skeleton then disarticulates into its component elements. The dissociated ossicles of one echinoderm group, the crinoids or sea lilies, are particularly com- mon in the fossil record.

A living crinoid has four regions of the body which can readily be identified. Crinoids are attached to the substrate by a root-like structure called a holdfast. This may take a number of forms but is most com- monly similar in appearance to either the root of a tree or to the runner of a strawberry plant. To the root is attached the stem, or column, which generally acts to elevate the crown of the crinoid above the sea floor. It may be regarded as being loosely analogous to an ‘anchor chain’. The stem is composed of separate calcareous plates called columnals. In modern coma- tulid crinoids the stem is composed of only one colurnnal, whereas some Jurassic specimens are pre- served with a column over 15 m in length and com- posed of many hundreds of plates. The stem supports the crown, which consists of the cup, containing the main digestive and nervous organs of the crinoid, and the arms. The arms of crinoids are feeding organs, collecting small food particles in a net-like array of branches. Food is then pushed towards the mouth, which lies on the upper surface of the cup, along a network of food grooves.

For the preservation of whole crinoids, rapid burial

Fig. 1. Ristnucrinus, common in the upper Ordovician of the UK, Scandinavia, France and Spain. Columnals of this genus have a synarrhrial (fulcral) articulating ridge, which in the figure is orientated left-to-right. The lumen is central and the fulcral ridge is flanked by a pair of semicircular, depressed ligament pits.

Fig. 2. Bysnowininus from N o d Wales. This genus has a wide distribution but only becomes common in the UK in the Ashgill (upper Ordovician). The lumen has a distinctive pentastellate outline and articulation is symplexial. 4

b Fig. 3. Myclodaclylus comwlurrcs from the Silurian of N o d America. An unusual’bilaterally symmetrical columnal. Columnals from related species can be found in the Wenlock Limestone.

is necessary to ensure that ossicles are not dispersed by current action following rotting of the ligament fibres. However, such events are relatively rare in the fossil record and by far the greatest proportion of crinoid material is found preserved as disarticulated debris. The crinoid cup usually only has about 10 or 15 ossicles, whereas there are normally many hundreds of plates in both the arms and the stem. Columnals are almost invariably larger and more con- spicuous than the ossicles of the arms, which are called brachials. It is thus the columnals that are most commonly noticed by collectors.

Crinoid columnals have a uniform general struc- ture. There is always an axial canal which runs the length of the stem and in life contains mainly nervous tissues. This gives columnals an appearance not unlike that of a ‘Polo’ mint! The intersection between the axial canal and the articular facet (the surface in contact with the next adjacent columnal) is an opening called the lumen. The outline of the lumen and of the columnal is commonly circular or pentagonal.

100IGEOLOGY TODAY MapJune 1987

Fig. 4. A new species of crinoid columnal from the upper Ordovician Boda Limestone of Sweden, with a unique asymmetrical arrangement of the articular grooves and ridges. 4

However, many Palaeozoic crinoids had columns in which these features show 1-, 2-, 3-, 4- or 6-fold symmetry.

The sculpture of the articular facets between ad- jacent cohnndls is often in the form of alternating radial ridges and grooves which interlock, further increasing the strength of the stem and preventing tearing of ligament fibres by twisting movements. However, the stem still retains some flexibility. In other stems, with somewhat stronger ligamentation, articular facets are planar. A third form of articulation utilises adjacent fulcra1 ridges which give the stem great flexibility.

Crinoid columnals are common fossils from the early Ordovician and throughout the Palaeozoic, becoming less common in the post-Palaeozoic. In the UK the greatest concentrations of crinoidal debris, particularly columnals and short sections of stem, are in the Silurian Wenlock Limestone and the Carbon- iferous Limestone. As yet the systematic study of crinoid columnals is at an early stage. Crinoid classification is based mainly on the features of the crown, and the stem has often been largely ignored in

b Fig. 5. An elliptical columnal with a pentagonal lumen from the Ashgill of North Wales. The lattice- llke microstructure of the columnal is particularly well preserved.

descriptions. The amount of information, particularly in studies of crinoid taxonomy, biostratigraphy, functional morphology and palaeoecology, that the humble columnal can provide is potentially immense and is a field of study ripe for detailed investigation by both amateur and professional alike.

Figures 1-5 illustrate crinoid columnals of various ages and types from several different parts of the world.

STEPHEN DONOVAN Department of Geology

University of the West Indies Jamaica

Scientific drilling of the continents: the prospects for Britain T h e Soviet superdeep borehole Kola SG3 is the most advanced part of a deep exploration programme initiated by the USSR in the early 1960s. To get an impression of its scale, the proportions between the diameter and depth of the hole are the same as that of a piece of thread 1 mm in diameter stretching a distance of 56 m. In 1984, fourteen years after spudd- ing Kola SG3, Soviet scientists revealed numerous unexpected and surprising results from drilling the world’s deepest borehole.

The structure and sequence of rocks at Kola were predicted principally from seismic and surface geo- logical data. The base of the Proterozoic sequence was forecast at 4.75 km depth, the base of the upper crustal ‘granitic’ layer at 7 km depth (thought to be the Conrad seismic discontinuity) and the ‘basaltic’ layer of the lower continental crust below that depth. Drilling was planned to a terminal depth (TD) of 15 km in what was thought to be, on the basis of

refraction seismics, a fairly flat-lying crustal sequence. Unexpectedly, the Russians found from the drilling that the base of the Proterozoic occurred at the seis- mic discontinuity calculated to be at about 7 km and that the Archaean sequence had still not been bot- tomed at 12 km depth. In addition, the dip of the upper crustal sequence was found to be between 40” and 60“.

Equally surprising were results which demon- suated that active fluid processes were taking place in numerous fractured and permeable zones below the relatively impervious uppermost 4.5 km of the Pro- terozoic sequence. The information from the borehole has led the Russians to suggest that the subhorizon- tally-distributed changes in seismic velocity might be caused by levels of metamorphism or levels where metamorphic processes might actually be taking place in the crust. From the reported results it is clear that considerable rethinking and reinterpretation of exist-

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