Crinoids: Sub-Class Camerata

© Craven & Pendle Geological Society

Amphoracrinus bollandensis

Limestones in Clitheroe have yielded many crinoids with the most numerous specimens belonging to the sub-class Camerata and in particular five genera: Actinocrinites, Amphoracrinus, Platycrinites, Pleurocrinus and Gilbertsocrinus.

The Camerates are a large group of Palaeozoic crinoids comprising of approximately 210 genera. They originated in the early Ordovician and disappeared during the Permo-Triassic extinction event.  The calyx is of variable form but is distinguished by a rigid dorsal cup and a vaulted or domed Tegmen.  The mouth and the food grooves are located beneath the Tegmen.

In Actinocrinites and Amphoracrinus the lowest set of plates in the dorsal cup are the basals which have been fused together to make three plates.  Above this and alternating with the basals are the radial plates of which there are five radials and a sixth plate occupying the posterior interradius.  This sixth plate is referred to as the anal plate or tergal plate.  The next series of plates that make up the cup wall are called Primibrachs which in turn are followed by Secundibrachs and Tertibrachs.

The Camerata were a large and important group from Ordovician to Permian times.  There are both monocyclic and dicyclic forms - a feature that distinguishes one group from another.  Of the two prevalent orders, Monobathrida and Diplobathrida, it is the Monobathrida i.e. Camerata without infrabasals that appear to be the more common.

1. MONOBATHRIDA - Camerata without infrabasals:

Sub-order TANAOCRINA Moore 1952
This sub-order is characterised by crinoids having six plates in the radial circlet.  Five of these are radials, one is tergal.  The base is hexagonal.  Locally two families are prolific - Actinocrinitidae and Amphoracrinitidae.

Sub-order GLYPTOCRININA Moore 1952
There are five plates in the radial circlet.  The tergal plate is found above the radial circlet.  The base is pentagonal.  All the British Carboniferous forms are are included in the Platycrinicae Ubaghs 1953.

1. DIPLOBATHRIDA - Camerata with infrabasals: Eg. Gilbertsocrinus

A dicyclic (with infrabasals) Camerate crinoid is very different in appearance from all the other genera of the Camerate.  the general impression suggests a globular shape consisting of a mass of small plates.  The base, however, may be concave or there may be a deep pit at the bottom of which lie the infrabasals.  The basals form the vertical side of the pit and the succeeding radials are conspicuously spiniferous.  The chief distinction is the presence of long, arm-like appendages which extend outward from the tegmen just above the position where the free arms are given off.  The biological function of the appendages is not fully understood but perhaps has something to do with respiration since they are perforated with a central canal leading to the interior of the theca.

Gilbertsocrinus calyx (bottom right) found near Dunsop Bridge

Two specimens have been found in Coplow with some appendages more or less intact.  The most common genus is that of Gilbertsocrinus konincki Grenfell.  More recently some specimens have been recorded from Salthill and Bellman and seemed to best fit G.konincki and G. mammillaris.