Page 115 - Introduction to Paleobiology and The Fossil Record
P. 115
102 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
been promoted by storms, providing fresher- In marine environments acceleration of the
water conditions for short periods of time. height, complexity and stratifi cation of benthic
tiering was later matched by increases in the
depth and sophistication of infaunal tiering
Ecological patterns and trends through time
as, particularly in Mesozoic and Cenozoic
During the last 600 myr, both animal and faunas, many more organisms adopted bur-
plant communities expanded and diversifi ed rowing lifestyles and the benthos switched
(Box 4.5). In simple terms the number of from filter to deposit feeding with signifi cantly
Bambachian megaguilds multiplied through more predators. The Cambrian evolutionary
the Cambrian (nine megaguilds), Paleozoic fauna occupied, more or less, only the surface
(14) and Modern (20) evolutionary faunas. of the seabed, but by the Ordovician crinoids
The focus in the Cambrian was on marine had developed tiers over a meter above the
animals that were either attached or mobile seabed and burrowing had already com-
with suspension- or deposit-feeding strategies, menced into the sediment. Terrestrial environ-
such as the eocrinoids and trilobites. The ments, initially dominated by small green
morphologies of individual organisms were plants, various arthropods and snails, together
rather plastic as were their community com- with diverse amphibian faunas in the Mid to
positions and structures. Relatively few class- Late Paleozoic, changed signifi cantly during
level taxa were included in each ecological the Mesozoic, with the diversifi cation of veg-
box (Fig. 4.21). By the Ordovician, however, etation and eventually fl owering plants, and
the number of megaguilds had expanded, terminating, for now, in the high and elabo-
with an overall numerical dominance of sus- rate canopies we see today in the tropical rain
pension feeders, such as the brachiopods, forests (see p. 505).
bryozoans, corals and crinoids. The Paleozoic The Modern fauna was also characterized
fauna was characterized by sedentary organ- by something rather special, an arms race
isms. The Modern fauna, by contrast, was (Harper 2006). During the so-called Meso-
dominated by deposit-feeding, essentially zoic marine revolution, predators, such as
mobile animals bound into a process of esca- bony fishes, crustaceans, marine reptiles and
lation, or ever-increasing competition, and the starfishes began to develop better and better
first intense arms race on the planet. The term ways of crushing or opening shells. The
arms race is used by ecologists to describe Modern world was a much more dangerous
ever-intensifying interactions between preda- place and in order to survive, potential prey
tors and prey, for example. had to develop thicker, more elaborately
Throughout the Phanerozoic there seems to ornamented shells with smaller apertures
have been an offshore movement in marine (Box 4.6) and devise more cunning evasive
faunas. New communities and taxa may have strategies such as greater mobility or deeper
occurred in nearshore, high-energy environ- and deeper burrowing. Unfortunately expo-
ments first, before migrating into deeper sure to intense predation and a much more
water. Thus older, more archaic groups tended bioturbated seafloor was no place for many
to characterize deeper-water habitats. For groups of epifaunal animals such as the bra-
example during the Ordovician radiation (see chiopods, some groups of bivalves and echi-
p. 253), typical members of the Paleozoic noderms. But as prey developed more armor
fauna (brachiopods, bryozoans and crinoids) and better evasive strategies, the hunters
expanded and migrated into deeper-water developed better weaponry. Together this
habitats, while their place in shallow water escalation and increased tiering set the
was taken by components of the Modern Modern fauna quite apart from those of the
fauna (bivalves and gastropods). But why? Cambrian and Paleozoic. Perhaps the whole
Are nearshore habitats particularly harsh, ecosystem functioned in a different way,
driving innovative communities and taxa into allowing biodiversity to continue to expand
deep water, or can innovative organisms arise way beyond the plateau of the Paleozoic
at any depth and those in shallower-water fauna (see p. 541).
environments are just more resistant to extinc- Unlike biodiversity change, where we have
tion and can readily migrate into deeper water numbers of taxa to count and monitor, eco-
(Jablonski & Bottjer 1990)? Perhaps it was a logical change is much more diffi cult to describe
combination of both. and quantify. Since some changes are much
