Page 145 - Introduction to Paleobiology and The Fossil Record

132 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

shark salmon frog lizard chicken mouse

3. warm-bloodedness
1. fins 5. diapsid skull
9. elongated neck vertebrae
6. loss of larval stage
8. amniote egg
2. legs
4. bone
7. lungs or swim bladder
10. marginal teeth
(a)
shark salmon frog lizard chicken mouse
0
Cenozoic

Cretaceous
100

Jurassic

200
Triassic
Time (Ma) Permian Diapsida

Carboniferous Amniota
300

Devonian Tetrapoda
400
Silurian Osteichthyes
Gnathostomata
(b)

Figure 5.10 The relationships of the major groups of vertebrates, tested using six familiar
animals. (a) Postulated relationships, based on the analysis of characters discussed in the text.
(b) Phylogenetic tree, showing the cladogram from (a) set against a time scale, and basing the
dating of branching points on the oldest known fossil representatives of each group.

are many controversies as systematists try to
find agreement on disputed parts of the tree The molecular revolution

of life. In some cases, the shape of the tree is The second approach in reconstructing the
clear because each node is diagnosed by many tree of life is based on comparison of mole-
apomorphies, but in others the clades and cules. With the birth of molecular biology in
nodes are hard to pinpoint. Perhaps in those the 1950s and 1960s, it became clear that
cases, evolution happened so fast that apo- homologous proteins share similar structures
morphies were not established, or perhaps in different organisms. For example, many
they have been overwritten in time. animals share the molecule hemoglobin, a