Page 441 - Introduction to Paleobiology and The Fossil Record

428 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

The backbone is the key. Human beings are fossils, such as Sacabambaspis from the Ordo-
vertebrates, and so are horses, sparrows, alli- vician of Brazil (Fig. 16.1a), had the begin-
gators, turtles, frogs and trout. What they all nings of a bony skeleton, but only on the
share is their bony internal skeleton, and, in outside of the body, and there is no trace
particular, vertebrae – the individual elements preserved of an internal mineralized skeleton.
of the backbone. The skeleton consists of a The rigid armor is made up of lots of little
backbone, a skull enclosing the brain and tooth-like structures, each equivalent to an
sense organs, and bones supporting the fi ns individual shark scale, but united by continu-
or limbs. Vertebrates are important today ous sheets of bone arranged like plywood.
because humans are such a successful species, This shows how adaptable the vertebrate
and also because of the huge diversity and skeleton can be, and this is perhaps why ver-
abundance of species of bony fi shes, birds and tebrates became such a diverse and abundant
mammals. Other groups, such as insects group. The internal skeleton of vertebrates
and microbes, are even more abundant and has a unique property – it allows them to
diverse, but vertebrates include the largest
animals on land, in the sea and in the air.
Vertebrates are a subgroup of the Phylum
Chordata, a major deuterostome clade.
Current views and debates about the nearest
relatives of vertebrates are considered in
Chapter 14. In this chapter, we look at the
origin of vertebrates, the evolution of fi shes
from the Cambrian to the present day, and the
Paleozoic tetrapods. The end-Permian mass
extinction reset the clock for vertebrates on
land, so we save the dinosaurs and their allies
and the mammals for Chapter 17. If the ver-

tebrate skeleton is so significant, what is so
special about it?
20 mm
ORIGIN OF THE VERTEBRATES (a)
The skeleton
The skeleton of vertebrates is made from bone
and cartilage. Bone consists of a network of
collagen fi bers on which needle-like crystals
of hydroxyapatite (a form of apatite, calcium (b) pectoral fin
phosphate, CaPO 4 ) accumulate. Hence bone
has a flexible component and a hard compo- dorsal plate rostral plate

nent, which explains why bones may undergo dorsal spine orbital plate
a great deal of strain before they break, and
also why bones do not break along simple
brittle faces. Cartilage is a fl exible, gristly
tissue, usually unmineralized, and containing ventral plate branchial plate
collagen and elastic tissues. In humans, most (c) 10 mm
of the bones are laid down in the early embryo
in the form of cartilage, and this progressively Figure 16.1 Early jawless fi shes: (a)
mineralizes by deposition of apatite. In adult Sacabambaspis from the Mid Ordovician of
humans, cartilage can be found in fl exible Brazil, the oldest well-preserved fi sh; (b) the
parts like the ears and the nose, as well as at osteostracan Hemicyclaspis from the Devonian;
the ends of the ribs and some limb bones. and (c) the heterostracan Pteraspis, also from the
The first vertebrates probably had a carti- Devonian. (a, b, based on Gagnier 1993; c,

laginous skeleton. Some of the oldest fi sh based on Moy-Thomas & Miles 1971.)