Page 242 - Introduction to Paleobiology and The Fossil Record
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PROTISTS 229
epivalve + epicingulum = epitheca Evolution and geological history
epivalve Both diatom frustules and, more commonly,
endospores are preserved in the fossil record.
epicingulum A Late Jurassic assemblage from western
Siberia that includes Stephanopyxis may be
girdle
the oldest known diatom fl ora. The fi rst
hypocingulum
diverse fl oras appeared during the mid-Creta-
ceous with almost 10 families recorded from
Aptian rocks; the group further diversifi ed
hypovalve
after the Turonian. Nearly 100 genera of
hypovalve + hypocingulum = hypotheca centric diatoms are recorded from the Upper
epitheca + hypotheca = frustule Cretaceous. Some of the first pennate diatoms
appeared during the Paleogene, colonizing
Figure 9.19 Descriptive morphology of the freshwater environments for the first time; the
diatoms.
group reached an acme during the Miocene.
Remarkably, diatom frustules can accumu-
late as thick deposits of diatomite (sometimes
Diatoms up to 500 m thick), which is a very porous
sediment, often with 80% as spaces, and per-
Diatoms are unicellular autotrophs that are meable with a density of about 0.5 g cm .
−1
included among the chrysophyte algae; they These diatomites, also termed kieselguhrs and
are characterized by large green-brown chlo- tripolis, are widely used as purifi ers for fi lter-
roplasts. Both individuals and loosely inte- ing drinks, medicines and water. Over 2
grated colonies of diatoms occur in a range of million tons are extracted each year for com-
aquatic environments from saline to freshwa- mercial use. Modern sedimentation rates
ter and across a range of temperatures, being suggest that 4–5 mm of diatomaceous ooze is
particularly common in the Antarctic plank- deposited over 1000 years; such an ooze cur-
ton. Both benthic and planktonic life modes rently occupies over 10% of the ocean fl oor
occur, although within the plankton one group today. Major commercial deposits occur in
– the Centrales – prefer marine environments; the Miocene of the Ardèche, France and in the
the Pennales, on the other hand, are more Pliocene and Pleistocene of Cantal, France are
common in freshwater lakes (Box 9.11). some of the main suppliers, although other
deposits occur in Spain, Germany and Russia.
Morphology and classifi cation The Miocene Monterey Formation in Califor-
nia is particularly widespread, occurring in
The diatom cell is contained within a siliceous both onshore and offshore basins; this diato-
skeleton or frustule comprising two unequally- maceous mudstone is also the source and
sized valves or thecae (Fig. 9.19). The smaller reservoir rock for most of California’s
hypotheca fits into the larger epitheca; the petroleum.
valve plates and congula of both valves
interface with the congulum of the epitheca
covering that of the hypotheca to form a Chitinozoans
connective seal. Chitinozoans are most common in fi ne-
During reproductive fi ssion, both the parent grained sediments, usually those deposited in
valves are used as the epitheca by the off- anoxic environments, and are associated with
spring, which then constructs its own pelagic macrofauna such as graptolites and
hypotheca. This process occurs a number of nautiloids together with acritarchs. In some
times each day, progressively reducing the size lithologies, such as black slates, chitinozoans
of the fustule. A stage of sexual reproduction are the only fossils preserved. These associa-
kicks in to restore the growth momentum of tions, together with their widespread geo-
the individual. graphic range, suggest that chitinozoans were
Classification of the group is based on shell at least pelagic. The group has proved
morphology (Box 9.11). extremely useful for both regional and global
