Conulariids: Fossilised jellyfish

Maria C Sendino and Paul D Taylor (UK)

Fossils such as ammonites, trilobites, crinoids and shark’s teeth understandably attract the most attention from fossil enthusiasts. However, other groups can provide equally fascinating insights into the history of life and ought not to be neglected. Among these ‘Cinderella fossils’ are conulariids. Found in late Precambrian (Ediacaran) to Triassic marine deposits, conulariids survived for more than 350ma, disappearing about 200 million years ago, at a time when the continents were clustered together into a huge landmass called Pangaea.

However, they are most common in Middle Ordovician to Permian rocks. Almost 400 species of conulariids have been described from around the world, and in some places they are abundant enough to lend their name to particular geological units, for example the Conularia-Sandstone in the Upper Ordovician of Jordan.

Affinities

What are conulariids? Initially, they were thought to be molluscs because of their pyramidal-cone shape that is vaguely reminiscent of a straight nautiloid. Others believed them to be worm tubes. For a long time they were classified as ‘Problematica’, which is a formal way of admitting total ignorance about their affinities.

They have also been placed in a phylum of their own, the Conulariida. This uncertainty results from the lack of preserved soft parts. However, strong evidence has emerged in recent years showing that conulariids belong to the same class – Scyphozoa – as jellyfish and are therefore related to other members of the phylum Cnidaria, including corals and sea anemones. In fact, modern scyphozoans may have a polyp stage in their life-cycle showing similarities to conulariids, notably four-sided symmetry. There are even some species of living scyphozoans that have conulariid-like skeletons made of calcium phosphate.

In all probability, the conulariid animal resembled a sea anemone with tentacles perched on top of the pyramidal shell. Given that they were scyphozoans, conulariids would probably have possessed a swimming medusoid (jellyfish) stage as part of their life-cycle, but this was unmineralized and is not fossilised.

Appearance

The first conulariid genus to be named, Conularia, took its name from the Latin word conulus for cone. In fact, conulariids vaguely resemble fossilised ice-cream cones, apart from having a square rather than a circular cross-sectional shape. Their shape is more accurately described as acutely pyramidal. Fossil conulariids range in length from about 2cm to 30cm, but most measure 3cm to 10cm from the closed pointed end to the aperture at the wider open end.

In a few specimens, a long stalk has been found extending from the apex, whereas in others the apex is rounded. The aperture in some specimens is closed by four, flap-like lappets, but these rarely fossilise. Transverse banding is also a characteristic feature of conulariids. The bands, often defined by ridges or lines of nodes, represent strips of skeleton added at the aperture as the animal grew. Longitudinal bands may also be present, in this case formed by nodes or crests that are aligned parallel to the length of the cone. The most important features apart from the bands are deflections in the pattern of ornamentation along the midlines of each of the four faces of the pyramid.

When preserved in its original state, the thin, delicate skeleton of conulariids is composed of calcium phosphate (apatite), but, in many instances, it has been lost, leaving only a sediment cast or ‘steinkern’. The skeleton varies in colour, from light pink to white, pale blue or black. Viewed under a microscope, it can be seen to comprise numerous layers or laminae. Skeletons tend to be thin in the earliest conulariids but are thicker in younger species, probably as an adaptation against predators. Some conulariid skeletons contain minute pores, although these are far too small to see with an optical microscope. There are also reports of phosphatic pearls associated with conulariids. Occasionally, it is possible to find conulariids with elastically deformed skeletons, bent but not fractured, showing that they were flexible rather than brittle.

Palaeoecology

During life, conulariids were normally attached by means of the flexible stalks mentioned above to substrates, such as a brachiopod shell lying on the sea floor. Strong currents sometimes broke the stalks, casting the animals adrift. This may explain the wide distribution of conulariids, which can be found in many different lithologies, representing a spectrum from tranquil environments with slow deposition, to high-energy environments with rapid sedimentation. While some conulariids were apparently attached by their stalks, others may have lived partly embedded in soft sediments, vertically oriented with the aperture above the sediment surface.

Small sharks, including Symmorium, are thought to have featured conulariids in their diets. Conulariids have been found with repaired injuries, although these structures are uncommon and it is likely that most attacks by sharks and other predators were fatal, fragmenting or destroying the skeleton in the process.

Conulariids commonly lived alone, but they could be also live in aggregations. Notable slabs of aggregated conulariids have been collected from the Carboniferous Calciferous Sandstone Series of Glencarthom in Scotland, one showing seven and another at least 16 individuals radially arranged.

Fossils found associated with conulariids include sponges, hyolithids, bivalves, crinoids, bryozoans and brachiopods, typical constituents of sea floor (benthic) communities of the Palaeozoic. Crinoids and brachiopods were sometimes directly attached to living conulariids.

British conulariid localities

Conulariids can be collected from rocks of various ages in the British Isles. Indeed, the first conulariid ever to have been formally named – Conularia itself – came from the Carboniferous near Keswick in Cumbria and was described by the famous naturalist James Sowerby in 1821. They can also be found elsewhere in the British Carboniferous, for example, in the Redesdale Shale of County Durham and the Coal Measures of Worcestershire and Shropshire (Fig. 6).

The collections of the Natural History Museum in London contain numerous examples of conulariids from the famous Ordovician locality of Girvan in the Southern Uplands of Scotland, most collected by Mrs Elizabeth Gray (1831-1924), an intrepid collector of fossils who was awarded the Murchison Geological Fund for her great services to geology. Conulariids can also be collected from the Silurian Wenlock Limestone of Shropshire and Dudley in the West Midlands. Devonian conulariids may be found in Devon and the same county is a source of reworked Ordovician conulariids in the Triassic Budleigh Salterton Pebble Beds.

Take a look through your collection of Palaeozoic fossils. There may be a conulariid hitherto unrecognized among the brachiopods and corals. If so, you will have an example of a fossil jellyfish!

References

Leme, J. de M., Simoes, M.G., Rodrigues, S.C., Van Iten, H. & Marques, A.C. 2008. Major developments in conulariid research: problems of interpretation and future perspectives. Ameghiniana 45: 407-420.

Moore, R.C. & Harrington, H.J. 1956. Conulata. F54-F66. In: Moore, R.C. (ed.), Treatise on Invertebrate Paleontology. Part F. Coelenterata. Geological Society of America and University of Kansas Press, New York, Lawrence and Boulder.

Slater, I.L. 1907. A Monograph of British Conulariae. Monograph of the Palaeontographical Society, London No. 295, 41 pp., 5 pls.