Page 99 - Introduction to Paleobiology and The Fossil Record

86 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Approximate
depth (m)
pelagic environments
supralittoral
high tide
neritic oceanic
low tide
epipelagic 100

200
mesopelagic
1000
bathypelagic

4000
benthic environments
5000
a

littoral sublittoral bathyal abyssal hadal

11,000

Figure 4.7 Review of modern marine environments and their depth ranges, together with the
approximate positions of the main benthic zones. (Based on Ager 1963.)

exposed wave-battered platform around the column. Within the benthos – the beasts that
Scottish island of Oronsay. Unfortunately, live in or on the seabed – mobile nektoben-
few rocky coasts have been recorded from the thos scuttle across the seafloor and the fi xed

geological record; where they occur, often or sessile benthos are fixed by a variety of

associated with paleo-islands, there are excit- structures. Infaunal organisms live beneath
ing and unusual biotas and sediments (Johnson the sediment–water interface, while epifauna
& Baarli 1999). live above it.
The majority of fossil animals have been Members of most communities are involved
found in marine sediments, occupying a wide in some form of competition for food, light
range of depths and conditions. The distribu- and space resources. For example, the strati-

tion of the marine benthos is controlled prin- fication of tropical rain forests refl ects com-
cipally by depth of water, oxygenation and petition in the upper canopy for light, while
temperature. The main depth zones and vegetation adapted for damp, darker condi-
pelagic environments are illustrated on Fig. tions is developed at lower levels. Similar
4.7. In addition, the photic zone is the depth stratifi cation or tiering is a feature of most
of water penetrated by light; this can vary marine communities, becoming higher and
according to water purity and salinity but in more sophisticated through geological time
optimum conditions it can extend down to (Fig. 4.9), rather like the skyscrapers in Man-
about 100 m. Terrestrial environments are hattan seeking to optimize space on a densely
mainly governed by humidity and tempera- populated island. Low-level tiers were typi-
ture, and organisms inhabit a wide range of cally occupied by brachiopods and corals
continental environments, ranging from the during the Paleozoic, while the higher tiers
Arctic tundras to the lush forests of the were occupied by crinoids. The Mesozoic and
tropics. Cenozoic faunas, however, are more mollus-
Marine environments host a variety of life- kan-based with the lower tiers occupied by
styles (Fig. 4.8). The upper surface waters are epifaunal bivalves and brachiopods and
rich in fl oating plankton, and nektonic organ- the upper tiers occupied by bryozoans and
isms swim at various levels in the water crinoids.