The disparids: Weird and weedy crinoids of the Palaeozoic

Stephen K Donovan (Netherlands) and David N Lewis (UK)

Palaeozoic crinoids are uniformly beautiful and come in many shapes and sizes, but almost all fall into one of three principal groups. The camerates are the largest and most robust, commonly incorporating the lower part of the arms into an enlarged cup with a plated roof (tegmen), producing a structure that is commonly reminiscent of a golf ball. The cup may be monocyclic (one circlet of basal plates supporting the radials; see Glossary (below) for explanation of specialist terms) or dicyclic (two basal circlets, that is, infrabasals and basals, supporting the radials). The arms of camerate crinoids bear multiple, fine branchlets called pinnules that must have formed an efficient ‘net’ for feeding on plankton.

The second major group, the cladids (plus the closely related flexibles) are dicyclic, lack an armoured tegmen and, except for some advanced (Upper Palaeozoic) forms, lack pinnules. The flexibles may also show a camerate-like feature with small plates separating the arms.

And then there are the disparids. The disparids were the ‘weeds’ of the Palaeozoic crinoids; generally smaller and less impressive than other crinoids, but including some unusual, even bizarre forms. Herein, we introduce the disparids of the British Palaeozoic, examining their form and function, and where to collect them.

The disparid cup was commonly small, always monocyclic and lacked an armoured tegmen, but had a prominent anal sac or tube in some groups. The arms were usually slender, lacked pinnules and were branched or unbranched, and the stem was usually circular in cross section, but may have had a distinctive morphology that permits identification without the cup. A morphologically distinct example of an early British disparid is the Middle Ordovician Iocrinus (Fig. 1).

Although the crown is large, with arms that are longer than the column, the cup forms only a small part of it and, in the figured species, shows strong folding of the plates. The arms branch several times, but even they are not as long as the complex anal tube or sac, with the anus at or near the top. The stem consists of the long column, with pentagonal to pentastellate columnals (see Glossary below), with a distinct arrangement to the articular facet, and an attachment structure (incomplete in the figured example) that formed a permanent bond by growing in a spiral around a suitable substrate such as a bryozoan.

The Upper Ordovician Claviculacrinus scoticus (Fig. 2) is, perhaps, a more ‘typical weedy’ disparid. The whole crinoid is gracile, lacking distinctive features like the large anal sac of Iocrinus. The column is circular in section, the cup small and the arms long, slender and branching several times. The Middle and Upper Ordovician disparids of the British Isles include Iocrinus and related species, weedy relatives like Claviculacrinus without a large anal sac or rare calceocrinids (see below). In the main, their Silurian relations were somewhat more flamboyant.

Apart from the calceocrinids, Iocrinus and other related disparid taxa had disappeared by the Silurian. They were replaced by two other groups, both morphologically very distinct from each other and Iocrinus – the myelodactylids and pisocrinids. They are found together with calceocrinids in the Much Wenlock Limestone Formation and represent a truly weird association of disparid crinoids, as explained below (Figs. 3 and 4).

First of these groups is the myelodactylids, such as Myelodactylus (Figs 3A, C and 4B), are best recognised by features of their peculiar stem. This is composed of bilaterally symmetrical columnals with a broad, U-shaped section that may resemble a cottage loaf in outline, which is arranged as a doubly re-curved coil near the small and apparently insignificant crown. Long, unbranched roots (radices) are developed as two parallel ‘curtains’ on one (inner) side of the long distal column. These did not act to anchor the column but, in many specimens, are preserved enclosing the proximal  column and the crown (Fig. 4B).

A complete specimen can look more like a Catherine-wheel firework than a fossil. They are perhaps the most unusual group of Palaeozoic crinoids, with a geometry quite unlike extant members of the group. This has resulted in a wide range of ideas concerning their functional morphology. Our favoured interpretation is of a recumbent lifestyle, with the crown enclosed within two ‘curtains’ of radices, which lacked musculature, but would have been protective and probably absorbed nutrients from seawater.

Also lying on the sea floor were the second of the two groups, the rather different calceocrinids (Fig. 3B, D and E). Their stems had a terminal attachment structure and the column was recumbent, lying straight and flat on the sediment surface. The crown was bilaterally symmetrical with three arms: a central one – either unbranched or symmetrically branched – flanked by two asymmetrical lateral arms showing multiple branches that were mirror images of each other.

The anal tube was at the opposite side of the crown to the central arm, both situated in the plane of symmetry. Perhaps, most peculiar of all, the cup was hinged and the crown could be folded down against the column by a muscle. It was re-elevated by a ligament. In essence, the calceocrinids were a group of crinoids that followed a life style more reminiscent of the articulated brachiopods – the crown was erected to feed, but could be folded tightly against the column, like a closed shell. Unlike myelodactylids, they lay on their ‘backs’ (that is, on the column) rather than their ‘sides’. British Silurian calceocrinids include Calceocrinus, Chirocrinus (Fig. 3B) and Synchirocrinus (Fig. 3D and E).

These genera are differentiated by the progressive increase in the number of major branches of the asymmetrical lateral arms from Calceocrinus (least) to Synchirocrinus (most). Two calceocrinid species are also known from the Upper Ordovician Lady Burn starfish bed, near Girvan, Strathclyde in Scotland, and calceocrinid basal circlets are present, but rare, in the Lower Carboniferous of Lancashire and elsewhere.

If the myelodactylids and calceocrinids were weird, the small and slender pisocrinids (Fig. 4A and C to F) were almost ‘normal’ in appearance. The column was certainly used to elevate the crown above the sea floor in this group and was slender, resembling a string of pearls in some species. The cup was pea-like or conical. The anal opening above the cup was inconspicuous. Two genera occur in the Silurian of the British Isles, Pisocrinus, with arms that were unbranched (Fig. 4C and E), and Cicerocrinus, with simply branched arms (Fig. 4A, D and F). The oldest pisocrinid to have been described is Eocicerocrinus sevastopuloi from the Upper Ordovician Lady Burn starfish bed in southwest Scotland.

From their first appearance in the Ordovician to the last species in the Late Permian, there were disparids like the pisocrinids and Claviculacrinus that are close to what might be described as a common, simple morphology. That is, the column elevated the crown, the cup was small and the arms were long and slender, either branched or unbranched. For example, there are several similarities in gross morphology between Wenlock Pisocrinus and Lower Carboniferous Synbathocrinus , although they are separated by about 100 million years and are not closely related within the disparids.

While the weirds make fascinating objects for functional study, some of the weeds provide unique palaeoecological evidence (Fig. 5). The stems of living, stalked crinoids are capable of surviving after detachment of the crown following self mutilation (autotomy) or predation, feeding by the absorption of nutrients through the skin (ectoderm). Locally common fragments of crinoid column derived from the gracile, disparid crinoid Cincinnaticrinus, from the Upper Ordovician of Kentucky, Ohio and Indiana, have rounded ends reminiscent of the overgrowths seen in some modern crinoids following decapitation. These were originally interpreted as globular distal terminations of mature individuals that have become separated from their attachment structures.

However, by comparison with overgrowths of similar morphology in living, decapitated crinoids, some or all of these specimens most probably represent overgrowths of the column following predation or autotomy. These specimens suggest that predation may have been an early phenomenon in the history of the crinoids. The most probable predators in the Upper Ordovician were vagile cephalopods, which are common fossils in the Upper Ordovician of the Midwest of the USA, rather than the rarer fishes.

Where to find them

Iocrinus is a distinctive genus of British Ordovician disparid, including at least four species. The most common British species is the Middle Ordovician Iocrinus shelvensis. It is similar to the species pictured in Fig. 1, but has a smooth cup and column with a pentagonal section. It is locally common in the Weston Formation of the Shelve Inlier in Shropshire.

Small, spherical to conical cups of the pisocrinid Pisocrinus pilula (Fig. 4E) are common at many Wenlock localities close to the Welsh Borders, both in the Much Wenlock Limestone Formation and Wenlock shales (Donovan et a., 2008). The same sites also yield fragments of the distal column of the myelodactylids Myelodactylus ammonis (Bather) and M. Fletcheri (Salter) (Fig. 3C), which are bilaterally symmetrical and have a cross section reminiscent of a cottage loaf.

Another pisocrinid, Pisocrinus cf. campana, is locally common at the base of the slightly older Wether Law Linn Formation (Upper Llandovery) of the North Esk Inlier of the Pentland Hills, southeast of Edinburgh. Several localities Clarkson et al. (2007) – R 82, R263, R265 and R260 – R 82, R263, R265 and R260 – yield common external moulds of the bead-like columnals, small dorsal cups and unbranched, blade-like arms.

One of our favourite crinoid sites is Salthill Quarry, Clitheroe in Lancashire. In a crinoid-rich fauna, the conical cups of Synbathocrinus conicus are uncommon, but distinctive (Donovan et al. 2003). Triangular basal circlets of the calceocrinid Halysiocrinus are even rarer and can be identified by the presence of the hinge along the longest side. These are among the youngest known disparids from the British Isles.

Stephen Donovan works in the Department of Geology at the Nationaal Natuurhistorisch Museum, Leiden in The Netherlands, and David Lewis is recently retired, but still undertakes research in the Department of Palaeontology at the Natural History Museum in London.

References

Clarkson, E. N. K., Harper, D. A. T., Taylor, C. M. & Anderson, L. I. (eds). 2007. Silurian Fossils of the Pentland Hills, Scotland. Palaeontological Association Field Guides to Fossils, 11, 218 pp.

Donovan, S.K. 1989. Pelmatozoan columnals from the Ordovician of the British Isles, part 2. Monographs of the Palaeontographical Society, London, 142 (no. 580), 69‑114.

Donovan, S.K. 1996. Fossils explained 15: Palaeozoic crinoids. Geology Today, 11 (for 1995), 196‑199.

Donovan, S.K., Lewis, D.N. & Crabb, P. 2003. Lower Carboniferous echinoderms of northwest England. Palaeontological Association Fold-Out Fossils, 1, 12 pp.

Donovan, S.K., Lewis, D.N., Crabb, P. & Widdison, R.E. 2008. A field guide to the Silurian Echinodermata of the British Isles: Part 2 – Crinoidea, minor groups and discussion. Proceedings of the Yorkshire Geological Society, 57, 29-60.

Donovan, S.K. & Schmidt, D.A. 2001. Survival of crinoid stems following decapitation: evidence from the Ordovician and palaeobiological implications. Lethaia, 34, 263-270.

Glossary
articular facets – the adjacent faces of columnals or any other plates that are not permanently sutured together, commonly permitting some relative movement.
autotomy – self mutilation. Crinoids and other crinoids are able to alter the physical state of the ligaments that hold certain parts of the skeleton together so that they drop off. This is well known from the arms of extant crinoids, which can subsequently be re-grown.
basal plates – in a dicyclic cup, the lowest plate circlet (see ‘circlets’), called the infrabasals, sit on the column and support the basals, which support the radials. In a monocyclic cup the basals are the lower circlet, resting on the column and supporting the radials. Basals are offset from infrabasals and radials, and are present in all crinoids..
circlets [of plates] – the crinoid cup is composed of two or three (very rarely four) circlets of plates forming a rigid structure that is more or less tubular to globular; that is, plates are arranged in rigid rings. Each circlet is commonly formed of five plates, although these show considerable variation between genera.
column – the part of the stem between the attachment structure (‘root’) and cup. In most crinoids the column elevates the cup and its arms above the sea floor.
columnals – the stacked, disc-like plates (also called ossicles) that form the column.
flexible crinoids – an important group of crinoids that evolved from the cladids. Flexibles are distinguished by the dicyclic cup and arms which together are usually small, short and rotund. Arms abut or not; the former are commonly interlocked. Arms closely curve inwards at the tip.
articular facets – the adjacent faces of columnals or any other plates that are not permanently sutured together, commonly permitting some relative movement.
lateral arms – the anal series and three arms of a calceocrinid are arranged about a plane of bilateral symmetry. The anal series and a bilaterally symmetrical arm lie on the lane of symmetry. On either side of this plane is a bilaterally symmetrical arm; these arms are mirror images of each other.
proximal column – the part of a crinoid column immediately beneath the cup.
radial plates – the highest circlet of plates of the dorsal cup, supported by the basals and supporting the arms. Present in all crinoids.
radice – root-like branches, either as the distal termination of the column or arising from one side of it. Radices may be unbranched, as in Myelodactylus, or branched.
vagile – free moving, commonly applied to benthic organisms that are not attached.