Page 419 - Introduction to Paleobiology and The Fossil Record

406 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
Aulechinus from the Upper Ordovician of
southwest Scotland is one of the most primi-
tive echinoids and the fi rst with only two plate
columns in the ambulacral areas. During the
Paleozoic there was generally an increase in
the number and size of ambulacral areas and
the sophistication of Aristotle’s lantern,
although most genera remained relatively
small (Fig. 15.15).
There was a significant decline in echinoid
diversity during the Late Carboniferous. By
the Permian only half a dozen species are
known, and they belonged to two primary
groups: detritus feeders and opportunists.
Large proterocidarids were highly specialized
detritus feeders, and the small omnivorous
Miocidaris and Xenechinus were opportun-
ists. Two lineages, including Miocidaris, sur- nized: the basal Somasteroidea, the Asteroidea
vived the end-Permian extinction event to
radiate extravagantly during the early Meso-
zoic, thus ensuring the survival of the echi-
noids. Following the end-Permian extinctions
the regular echinoids diversifi ed during the
Late Triassic and Early Jurassic with more
advanced regulars dominating the early Meso-
zoic record. The irregulars appeared during
the Early Jurassic and substantially increased
in numbers during the period. Diversity was
severely reduced by the Cretaceous-Tertiary
extinction event but both the regulars and
irregulars recovered rapidly during the early
Cenozoic.
Asteroidea
Starfish are common on beaches today, and
their biology has made them hugely success-
ful. Some feed by preying on shellfi sh and
other slow-moving shore and shallow-marine
animals. Their feeding mode is unusual but
deadly: they simply sit on top of their chosen
snack, turn their stomachs inside out and
absorb the flesh of their victim. The majority
are benthic deposit feeders that ingest prey or
filter feed. Starfish are also unusual in that
they have eyes at the ends of their arms – these
are actually light-detecting cells, not true eyes,
but the adaptation is novel nonetheless.
Asteroids appeared first during the Early
Ordovician. The subphylum contains two
main groups: the asteroids or starfish and the
ophiuroids or brittle stars. These animals have
a star-shaped outline with usually fi ve arms
radiating outwards from the central body or
disk. The water vascular system is open. The
mouth is situated centrally on the underside
of the animal on the oral or dorsal surface
whereas the anus, if present, opens ventrally
on the adoral surface. The asterozoans are
characterized by a mobile lifestyle within the
benthos, where many are carnivores. Astero-
zoan skeletons disintegrate rapidly after death
due to feeble cohesion between the skeletal
plates. Thus, recognizable fossils are relatively
rare. Nevertheless there are a number of star-
fish Lagerstätten deposits where asterozoans
are extremely abundant and well preserved.
Distribution and ecology of the main groups
Three classes of asterozoans have been recog-
or starfi sh and the Ophiuroidea or brittle stars.
The Somasteroidea include some of the earliest
starfish-like animals, described from the
Tremadocian of Gondwana. These echino-
derms have pentagonal-shaped bodies with the
arms initially differentiated from around the
oral surface. In some respects this short-lived
group, which probably disappeared during the
Mid Ordovician, displays primitive starfi sh
characters intermediate between a pelmato-
zoan ancestor and a typical asterozoan descen-
dant. Typical asteroids have fi ve arms radiating
from the disk, which is coated by loosely fi tting
plates permitting considerable fl exibility of
movement (Fig. 15.16). Additional respiratory
structures, called papulae, project from the
celom through the plates of the upper surface.
This backup system aids the high metabolic
rates of these active starfi sh.
The fi rst true starfish were probably derived
from the somasteroids during the Early Ordo-
vician and were relatively immobile, infaunal
sediment shovelers. Some of the fi rst starfi sh,
for example Hudsonaster, from the Middle
Ordovician, have similar plate confi gurations
to the young growth stages of living forms such
as Asterias. Although relatively uncommon in
Paleozoic rocks, the group was important
during the Mesozoic and Cenozoic and is now
one of the most common echinoderm classes.
The ophiuroids first appeared during the
Early Ordovician (Arenig), but the group, as
presently defined, may be paraphyletic. Clas-
sification is based on arm structure and disk
plating. The ophiuroid body plan is distinc-
tive, with a subcircular central disk and fi ve