Page 254 - Introduction to Paleobiology and The Fossil Record
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ORIGIN OF THE METAZOANS 241
and respiratory organ, the lophophore, are lians and the deuterostomes. The ecdysozoans
characterized by sac-like bodies; but this is no and the spiralians comprise the protostomes
guarantee that these so-called “lophopho- (“first mouth”) where the mouth develops
rates”, brachiopods and bryozoans, are in directly from the first opening, the blastopore,
fact closely related. The hemichordates possess resulting from cell growth and migration. The
a crown of tentacles and some have paired gill deuterostomes (“second mouth”), however,
slits. The echinoderms have an elaborate have a mouth arising from a secondary
water vascular system that drives feeding, opening; the true blastopore often develops as
locomotion and respiration. an anus. Not all phyla fit simply into these
The identification of invertebrate body two major divisions, but using a consensus
plans is a useful method of grouping organ- based on comparative morphology, two main
isms according to their basic architecture. streams emerge: the echinoderm–hemichor-
However, similarities between grades of con- date–chordate (deuterostomous) and the
struction unfortunately do not always mean mollusk–lophophorate–annelid–arthropod
a close taxonomic relationship. Be aware that (protostomous) groupings (Box 10.2).
certain body plans have evolved more than Other studies have laid emphasis on the
once in different groups, Skeletons too, for similarities between the larval stages of organ-
example, have evolved a number of times in isms to investigate phylogenetic relationships.
a variety of forms. Most invertebrates develop fi rst a larval
The skeleton is an integral part of the body stage that may be either planktotrophic, free-
plan of an animal, providing support, pro- living and feeding on plankton, or lecithotro-
tection and attachment for muscles. Many phic, essentially benthic and feeding on yolk
animals such as the soft-bodied mollusks sacs. There is a range of different larval types.
(slugs) possess a hydraulic skeleton in which For example the nauplius larva is most typical
the movement of fluid provides support. Rigid of crustaceans, the planula characterizes the
skeletons based on mineralized material may cnidarians, the trochophore larva occurs in
be external (exoskeleton), in the case of most the mollusks and the polychaetes whereas the
invertebrates, or internal (endoskeleton) struc- shelled veliger also characterizes the mollusks.
tures, in the case of a few mollusks (e.g. Thus those groups (annelids and mollusks)
belemnites), echinoderms and vertebrates. with trochophores may have shared a common
Growth is accommodated in a number of ancestor. Invertebrate larvae are occasionally
ways. Most invertebrate skeletons grow by identified in the fossil record. With the avail-
the addition of new material, a process termed ability of more advanced preparatory and
accretion. Arthropods, however, grow by high-tech investigative techniques, studies of
periodic bursts between intervals of ecdysis or fossil larvae may yet become a viable part of
molting; echinoderms grow by both accretion paleontology.
to existing material and by the appearance of
new calcitic plates.
FOUR KEY FAUNAS
Classifi cation and relationships
The three great evolutionary faunas of the
Classifications based on purely morphological Phanerozoic, the Cambrian, Paleozoic and
data and embryology have met with prob- Modern (see p. 538), developed during a
lems. Diffi culties in establishing homologous timeframe of some 550 myr. Nevertheless, in
characters and homoplasy (see p. 129) have the 100 myr that include the transition
contributed to a number of different phylog- between Precambrian and Phanerozoic life,
enies. The locator tree (Fig. 10.6), however, there were a number of distinctive groups of
outlines some of the main features of animal animals that together paved the way to the
evolution. From the base of the metazoan spectacular diversity we see today in marine
tree, the demosponges and calcisponges are and terrestrial communities. The Ediacara
the simplest animals whereas the cnidarians biota and small shelly faunas, together with
are the most basal eumetazoans. Three robust those that developed during the Cambrian
bilaterian groupings are recognized mainly on explosion and Ordovician radiation, set the
molecular data: the ecdysozoans, the spira- scene for life on our planet.
