Page 408 - Introduction to Paleobiology and The Fossil Record

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 395

noderms at that. They are usually sessile, with 15.3) fixed to the seabed by root-like struc-
characteristic echinoderm pentameral symme- tures or holdfasts. Attached to the top of the
try, rooted by a stalk, for at least part of their stalk is the case containing the main func-
life cycle, to the seabed; but some forms after tional part of the animal called alternatively

a short fixed stage are entirely free living. the calyx, aboral cup or theca. The calyx is
Modern forms live in dense clusters or built of two rings of calcitic plates – the basals
“forests” ranging from the warm waters of and the overlying radials in a monocyclic con-

the tropics to the icy conditions of polar lati- figuration. In a number of taxa, the dicyclic
tudes. The “feather stars” prefer the clear- forms, a second circle of smaller plates, the
water conditions of the continental shelf, infrabasals, interface between the basals and
living in nooks and crevices, and emerging at the stem, providing further articulation. The
night to perch on ridges. The fi xed sea lilies upper, oral surface of the calyx is covered by

occupy the deep-water environments of the a flexible membrane or tegmen and houses a
continental slope. The majority of fossil forms number of important structures. These are the
were almost certainly part of the shallow- mouth, which is usually situated centrally at
water sessile benthos. The success of the cri- the convergence of fi ve radially arranged
noids may be measured by the fact we know feeding grooves; the anus, which is sited pos-
over 6000 fossil species and an age range teriorly with the outlet often modifi ed by an
from the Early Ordovician to the present anal tube enhancing the effi ciency of waste
day. disposal; and the arms or brachials, which
extend upwards from the calyx and together
Morphology and life modes form the crown.
As already noted, two main life strategies
The crinoids consist of a segmented stalk or were pursued by the crinoids (Fig. 15.4). The
stem composed of columnals or ossicles (Box majority of fossil crinoids and about 25 Recent

pinnules

brachials
basals anal tube
radials

infrabasals
calyx

Freely mobile
stem
Antedon

holdfast Stalked attached
Florometra

(a) (b)
Figure 15.4 (a) Morphology of the Ordovician Dictenocrinus. (b) Two main crinoid life strategies,
fixed and mobile. (Redrawn from various sources.)