Page 105 - Introduction to Paleobiology and The Fossil Record
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92 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
dry land
freshwater
supratidal zone
intertidal zone calcisponges hermatypic corals
continental slope shelf demosponges bryozoans brachiopods cirripedes limulids
200 m annelids with calcareous tubes echinoderms chitons and scaphopods cephalopods bivalves gastropods ostracodes other crustacea
2000 m hexactinellids ahermatypic corals
abyssal
Figure 4.14 Distribution of living organisms across a depth gradient. (From Brenchley & Harper
1998.)
Water temperature in the oceans decreases ish waters have mainly low-diversity assem-
steadily to the base of the thermocline, the blages with bivalves, crustaceans, ostracodes
layer within a body of water where the tem- and small benthic forams, whereas hypersa-
perature changes rapidly, at around 1000 m line assemblages are of very low diversity
depth, where it reaches about 6°C. Tempera- with just a few bivalves, gastropods and
tures on the ocean floor rarely exceed more ostracodes.
than 2°C. Temperature also changes with lati- Depth is one of the most often quoted con-
tude and obviously affects the broad geo- trols on the distribution of marine organisms
graphic distribution of organisms; those from (Fig. 4.14). Although the direct affects of
the poles are generally quite different from depth are related to hydrostatic pressure,
those from the tropics. many other factors, both chemical and physi-
Salinity, too, controls the distribution of cal, are related to depth; for example, in
organisms. Most marine animals are isotonic general terms, the grain size of sediment and
(“same salinity”) with seawater and live water temperature decreases with depth.
within narrow (stenohaline) rather than wide Although hydrostatic pressure does not
(euryhaline) ranges of salinity, commonly usually distort the shells and soft tissues of
with 30–40‰ dissolved salts in seawater. In organisms it can dramatically affect organ-
broad terms normal marine water is charac- isms with pockets of gas in their bodies, such
terized by stenohaline groups such as the as fishes and nautiloids. Apart from the effects
ammonites, belemnites, brachiopods, corals, of hydrostatic pressure, depth can also control
echinoderms and large benthic forams. Brack- the solubility of calcium carbonate; cold water
