FOSSIL INVERTEBRATES/Crinoids 343 Figure The main morphological features of an extant stalked crinoid (isocrinid) Some of the arms have been omitted for clarity ligaments connect opposing faces of adjacent columnals, with longer through-going ligaments linking groups of columnals together Autotomy – the voluntary shedding of distal parts of the stem to allow the animal to drift to a new site – can occur at a junction between adjacent linked groups of columnals through a change in the properties of the catch connective tissue Hence, discarded columnals can contribute to the future fossil record long before the animal itself has actually died! Articulating surfaces of adjacent columnals show a range of morphologies, most typically a radial pattern of crenulations (Figure 2A), which interlock with opposing crenulations on the adjacent columnal to resist shearing and torsional stresses These crenulations may be arranged in more complex, often pentaradiate, configurations (Figure 2B) Another distinctive type of columnal articulation developed independently in the Silurian to Permian Figure The main types of columnal articulations encountered in crinoids: (A) multiradiate is mm across; (B) pentaradiate is 12 mm across; and (C) synarthrial The columnal in (C) is from a larval isocrinid and is less than 0.7 mm across; the stereom is clearly visible