Page 442 - Introduction to Paleobiology and The Fossil Record

FISHES AND BASAL TETRAPODS 429
grow very large because the skeleton can grow
with the animal. An external skeleton cannot
grow so fast, and is less adaptable in support- In the Late Cambrian and Ordovician, the
ing a large volume of soft tissues. Further, the
external skeleton is vulnerable to damage and
either has to be repaired by extending fl eshy
parts outside the shell (mollusks, graptolites)
or by molting the skeleton (arthropods), a
wasteful process that uses up energy and
leaves the animal vulnerable until the new
exoskeleton hardens. By contrast, the verte-
brate skeleton is maintained and remodeled
constantly within the body, and can act as a
support for small, medium, large and massive
organisms.
Jawless fi shes: slurping rather than biting
Two key defining characters of vertebrates are
the head and neural crest tissues. Our head is
so essential that we rarely stop to think that
actually only vertebrates have heads – indeed
vertebrates are sometimes called craniates,
meaning “with a skull”. Mollusks, worms,
brachiopods and echinoderms do not have
heads – we might call the front end of a worm
its “head”, but it really is not any more than
its front end. The vertebrate head is unique in
providing an organized structure that con-
tains the brain, the major sense organs and
the mouth.
The vertebrate head is formed from cells
derived from the neural crest, a second key
apomorphy of vertebrates. The neural crest
appears in the early embryo as a strip of cells
lying just below the outer skin, the ectoderm,
of the embryo, above the line of where the
backbone will develop. As tissues begin to
differentiate in the early embryo, cells derived
from the neural crest spread through the
embryo and stimulate the development of
muscles, nerves and blood vessels along the
trunk and around the heart and gut, but a
major target is the head region. The cranial
neural crest cells give rise to bones, cartilage,
nerves and connective tissue in the head and
neck region, forming the face, teeth, eyes,
inner ear, the thymus, thyroid and parathy-
roid glands, and the gills and gill arches of
fi shes.
The first vertebrates had no jaws (Fig.
16.1). Until recently, these fi rst fi shes were
said to be Ordovician in age, but controver-
sial new specimens from the remarkable fossil
sites at Chengjiang in China (Box 16.1) have
pushed the range back to the Early Cambrian.
commonest vertebrates were the conodont
animals. Fishes became common and diverse
during the Late Silurian and Devonian.
The jawless fishes are sometimes referred
to as ostracoderms (Box 16.2). Ostracoderms
were jawless, they were generally armored,
although some were not, and they had their
heyday in the Devonian. Osteostracans like
Hemicyclaspis (Fig. 16.1b) have a semicircu-
lar head shield bearing openings on top for
the eyes and nostrils, as well as porous regions
round the sides that may have served for the
passage of electrical sense organs, perhaps
used in detecting other animals by their move-
ments in the water. Heterostracans like Pter-
aspis (Fig. 16.1c), are more streamlined in
shape, and were perhaps more active swim-
mers. Both forms have their mouths under-
neath the head shield, and they probably fed
by sieving organic matter from the sediment.
These armored jawless fishes died out at the
end of the Devonian, and their place was
taken over by fi shes with jaws.
Jawless fishes still exist today, the 50 or so
species of lampreys and hagfi shes, eel-shaped
animals. Hagfishes scavenge on dead fl esh,
while lampreys are often parasitic. Although
they have no jaws, their mouths are fi lled with
tooth-bearing bones, and these are used to
grip prey animals and to rasp off lumps of
flesh. Salmon and trout are commonly caught
in the American Great Lakes with huge circu-
lar craters in the sides of their bodies, where
flesh has been torn out by a sea lamprey.
Conodonts: animals of mystery
The commonest early vertebrates were the
conodont animals (Sweet & Donoghue 2001).
For over 150 years conodonts had been a
mystery, known only from their jaw elements
– no one knew which animal had produced
them.
Conodonts were fi rst identifi ed by the
Latvian embryologist and paleontologist
Christian Pander in 1856. They occur as
phosphatic tooth-like microfossils, termed
elements. Three main conodont groups have
been established (Fig. 16.3): (i) protocon-
odonts such as Hertzina are simple cones with
deep basal cavities; (ii) paraconodonts like