Page 265 - Introduction to Paleobiology and The Fossil Record

252 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 10.6 Roughness landscapes

There have been a number of explanations for the rapid explosion of life during the Early and Mid
Cambrian involving all sorts of developmental (genetic), ecological and environmental factors. Why,
too, was this event restricted to the Cambrian? Was there some kind of developmental limitation,
an ecological saturation, or were there simply no further ecological opportunities left to exploit?
One interesting model that may help explain the ecological dimension of the event involves the use

of fitness landscapes. The concept is taken from genetics but can be adapted to morphological infor-
mation (Marshall 2006). Biotas can be plotted against two axes, each representing morphological
rules that can generate shapes. The Ediacara fauna has only three recognizable bilaterians, so the
landscape is relatively smooth with only three peaks. On the other hand the Cambrian explosion
generated at least 20 bilaterian body plans and a very rough landscape rather like the Alps or the
Rockies (Fig. 10.15). What roughened the landscape, or why were there more bilaterians in the
Cambrian fauna? Much of the bilaterian genetic tool kit was already in place in the Late Proterozoic
and the environment was clearly conducive to their existence. The “principle of frustration” (Mar-

shall 2006), however, suggests that different needs will often have conflicting solutions, ensuring that
the best morphological design is rarely the most optimal one. Is it possible that, with the rapid
development of biotic interactions such as predation, many morphological solutions were developed,

some less than optimal but nevertheless driving a roughening of the fitness landscape. Thus “frustra-
tion”, the multiplication of attempted solutions to new opportunities, led to the roughening of the
Cambrian landscape and may have been an important factor in the Cambrian explosion.

(a) (b) (c)
1
Fitness 2 1 2 3 1 2 3
Ediacaran 3
Morphogenetic rule 2

Morphogenetic rule 1
6 7
1 2 8 1 2
Increased number 4 5 3
of frustrated needs
roughens landscape 11 5
3 4 8
6
10 12 13
3 18 2 19 9 7
5
9
Fitness
13
Cambrian 6 17 8 15 16 1 10 14 16 17 9 10 11 12
7
14 4 20 11 15 13 14 15
12 Morphogenetic rule 2 20 17 16
18
19
18
Morphogenetic rule 1
19
20
Figure 10.15 Comparison of Ediacara and Cambrian landscapes: (a) fitness landscapes; (b)

locally optimal morphologies (Nicklas’ plants); and (c) locally optimal morphologies (bilaterian
animals). (Based on Marshall 2006.)