Page 375 - Introduction to Paleobiology and The Fossil Record

362 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
Chelicerate:
Trilobitomorph: scorpion
trilobite
Crustacean: Crustacean:
ostracode barnacle
Figure 14.1 Some of the main arthropod groups: a variety of forms based on a simple body plan of a
tough exoskeleton and jointed limbs.
ARTHROPODS: INTRODUCTION
Arthropods are a very common and spectacu-
larly diverse group of legged invertebrates
accounting for about three-quarters of all
species living on the planet today, largely
because of the phenomenal abundance of the
insects. The basic body plan – conspicuously
segmented, with jointed appendages adapted
for feeding, locomotion and respiration –
together with a tough exoskeleton, fi rst
appeared during the Early Cambrian and has
since been exploited by a huge variety of living
and fossil arthropods that pursue many life-
styles. All members of this phylum have both
segmented bodies and appendages (Fig. 14.1);
moreover the animal is differentiated into a
head, thorax and abdomen, with often the
head and thorax fused to form the cephalo-
thorax. The possession of mandibles, or hard
mouthparts, equipped many arthropods with
the ability to process a wide variety of
foods.
The arthropod exoskeleton is constructed
mainly from the organic substance chitin.
This is often hardened or sclerotized by
calcium carbonate or calcium phosphate, so
the potential for preservation is excellent
across the group. The exoskeleton acts as a
base for the attachment of locomotory
muscles, permitting rapid movement, and is
Crustacean:
decapod
Crustacean:
decapod
Insect:
palaeopteran Insect:
coleopteran
not usually mineralized. Although many
arthropods undergo metamorphosis, virtually
all the main groups grow by molting or
ecdysis; first the endoskeleton is dissolved and
second the old exoskeleton is detached along
sutures while the new exoskeleton is gener-
ated. Exuviae, or cast-off coverings, are all
that remain of the previous skeleton or cuticle
of the animal: one arthropod can thus produce
many potential skeletal fossils in its lifetime.
During a geological history of at least 540
million years, the fi ve subphyla of arthropods
(Box 14.1) have adapted to life in marine,
freshwater and terrestrial environments. For
a long time their closest living relatives were
thought to be the segmented annelid worms,
but new studies show that the closest sister
group of arthropods is a clade of unsegmented
worms that includes the priapulids and the
nematodes or round worms. Their segmenta-
tion may thus either have arisen independently
to that of the annelids, or may have been
inherited from a very deep ancestor to both
groups.
EARLY ARTHROPOD FAUNAS
A huge variety of bizarre arthropod types
formed much of the basis for the Cambrian
explosion (see p. 249). Over 20 groups of
arthropod have been described from the Mid