Page 413 - Introduction to Paleobiology and The Fossil Record

400 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
appendages. Over 30 genera have been
described from rocks of Early Cambrian to
Late Silurian age. The origins of the other
blastozoan classes are probably to be found
within this heterogenous group; for example
the aberrant Late Cambrian eocrinoid Cam-
brocrinus has been cited as an ancestor for the
rhombiferan cystoids. Whereas many eocri-
noids were high-level suspension feeders with
the first columnal-constructed stems, some lay
reclined or recumbent on the seabed. The
Ordovician Cryptocrinus, for example, has a
globular theca with a more irregular arrange-
ment of plates.
Paracrinoids
The paracrinoids (Fig. 15.9b) are a small, odd
group of arm-bearing echinoderms that have
globular thecae and numerous irregularly-
arranged plates together with two to fi ve arm- (interambs), are armed with spines and abut
like, food-gathering structures. They are so
different that some scientists have suggested
that they represent a separate subphylum.
The group is restricted to North America,
where they are common in the Middle
Ordovician.
Echinoidea
Echinoids, the well-known sea urchins and
sand dollars, have robust, rigid endoskeletons,
or tests, composed of plates of calcite coated
by an outer skin covered by spines. The tests
are usually either globular or discoidal to
heart-shaped (Smith 1984). Echinoids are
most common in shallow-water marine envi-
ronments where they congregate in groups as
part of the nektobenthos. Their classifi cation
(Box 15.5) is based on the arrangement of
plates and their mouth structures.
Echinoids have a long history from their
fi rst radiation in the Ordovician (Paul &
Smith 1984). Two of the most signifi cant evo-
lutionary events in the history of the subphy- tooth, operating like a mechanical grab and
lum were marked by sudden divergences from
the regular morphology to generate irregular
burrowing echinoids. The fi rst, in the Jurassic,
led subsequently to a range of irregular bur-
rowers, and the second, during the Paleocene,
to the quasi-infaunal sand dollars. Both events
were probably rapid and permitted major
adaptive radiations of parts of the group into
new ecological niches.
Basic morphology
The exoskeleton or test of most regular echi-
noids, for example the common sea urchin
Echinus esculentus, is hemispherical and dis-
plays all the main features of the group (Fig.
15.11). The lower, adapical or oral, surface is
perforated by the mouth whereas the upper,
apical or aboral, surface has the anal opening.
The sea urchin is part of the active mobile
benthos, in contrast to the sand dollars which
were quasi-infaunal.
The test is built of a network of many hun-
dreds of interlocking calcite plates organized
into 10 segments, radiating from the oral
surface and converging on the aboral surface.
Five narrower segments or ambulacral areas
(ambs) carry the animal’s tube feet and are in
contact with the ocular plates. The ambs
alternate with the wider interambulacral areas
against the genital plates. Together the ambs
and interambs comprise in total 10 areas and
20 columns, which make up the corona – the
majority of the test.
The central part of the aboral surface has
a ring of five genital plates, each perforated
by a hole to allow the release of gametes; the
madreporite is commonly larger than the
other genital plates and has numerous minute
pores interfacing, beneath, with the water
vascular system. These alternate with the
ocular plates, terminating the ambulacral
areas, and each houses further outlet holes for
the water vascular system. This part of the
apical system surrounds the periproct, or anal
opening, which is partially covered by a
number of smaller plates attached to a mem-
brane. On the underside of the test, the peri-
stome, containing the mouthparts, is also
covered by a membrane coated with small
plates. The mouth holds a relatively sophisti-
cated jaw apparatus comprising fi ve individ-
ual jaws each with a single, curved, saber-like
forcing particles into the animal’s digestive
system. The great ancient Greek naturalist
Aristotle, who described the structure fi rst,
compared it to a “horn lantern with the panes
of horn left out”, and the echinoid jaw is
often called Aristotle’s lantern. In crown-
group forms, muscles attached to the lantern
are anchored to the perignathic girdle, devel-
oped around the edge of the peristome.