Page 403 - Introduction to Paleobiology and The Fossil Record

390 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
The echinoderms and hemichordates appear
to be two very different groups of animals,
one characterized by fi ve-fold symmetry and
a water vascular system, the other a group of
odd stick-like colonial organisms. Surpris-
ingly, both are closely related to each other
and, moreover, are not so distant from our-
selves, the chordates. Both groups are deu-
terostomes; the first opening to develop in the
embryo is the anus and a second forms the
mouth. The group has a dipleurula larva and
a body cavity that developed from an exten-
sions of the embryonic gut (see p. 240).
Modern morphological and molecular analy-
ses indicate that the echinoderms and hemi-
chordates are in fact sister groups (Smith
2005). A small, extinct group – the Vetulicolia
– so far known only from the Cambrian, has
also been related to the deuterostomes because
of similar gill structures and the absence of
limbs. But although recent finds from Utah
have suggested that this group has more in
common with the arthropods and probably
belongs to the ecdysozoans (see p. 361), the
group remains an enigma (see Box 15.10).
ECHINODERMS
Clearly we stood among the ruins of
some latter-day South Kensington! Here,
apparently, was the Palæontological
Section, and a very splendid array of
fossils it must have been. . . . The place
was very silent. The thick dust deadened
our footsteps. Weena, who had been
rolling a sea urchin down the sloping
glass of a case, presently came, as I stared
about me, and very quietly took my hand
and stood beside me. And at fi rst I was
so much surprised by this ancient monu-
ment of an intellectual age, that I gave
no thought to the possibilities it pre-
sented. Even my preoccupation about the
Time Machine receded a little from my
mind.
H. G. Wells (1898) The Time Machine
Echinoderms today are one of the most abun-
dant marine animal groups, and as fossils they
can sometimes be rather robust, as Weena
from The Time Machine found. Sea urchins
are common in many intertidal environments,
and out in the deep sea the ocean fl oors are
covered by brittle stars and sea cucumbers.
The phylum Echinodermata has an unusual
five-fold symmetry and is uniquely equipped
with a water vascular system in which water
is forced around the plumbing by muscular
action, while tube feet, extending from the
system, are often modified for food process-
ing, locomotion and respiration. The 6000 or
so living echinoderm species include familiar
forms such as sea lilies, sea urchins, sand
dollars, starfish and sea cucumbers (Fig. 15.1).
Although many species today live in the inter-
tidal or subtidal zones, the group is most
diverse in the deep sea. Echinoderms also
occupied a wide range of marine environ-
ments and pursued a variety of life strategies
in the geological past. Fossil echinoderms
are relatively common, and because many
echinoderm skeletons disintegrate rapidly
after death, many limestones are packed with
the distinctive skeletal debris of calcitic
plates.
Apart from the water vascular system, echi-
noderms have a number of other distinctive
features. All members of the phylum have a
mesodermal skeleton constructed from porous
plates of calcite; each plate is usually a single
crystal of calcite and easy to recognize in thin
sections. In addition, the plates have a unique
ultrastructure of rods linked to form a three-
dimensional lattice. This network, or stereom,
is permeated by finger-like pieces of soft tissue
that occupy the spaces, or stroma, in the
lattice. Finally, five-rayed or pentameral sym-
metry, occasionally modified by a secondary
bilateral symmetry, is typical of the echino-
derms. The phylum is generally split into the
mobile, non-stalked eleutherozoans and the
mainly fi xed, stalked pelmatozoans (Box
15.1), but the earliest forms are hard to
classify (Box 15.2).
The multiplated echinoderm skeleton disin-
tegrates very rapidly after death; although
individual plates or ossicles have high preser-
vation potential, the complete skeletons do
not. Nevertheless, occasionally rapid burial or
transportation into anoxic conditions may
result in the preservation of complete echino-
derm skeletons. Starfish beds, usually charac-
terized by accumulations of complete
echinoderms, occur sporadically throughout
the fossil record. The Leintwardine Starfi sh
Bed of the England–Wales border area con-