Page 234 - Introduction to Paleobiology and The Fossil Record
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PROTISTS 221
Netromorphs
• Elongate, commonly fusiform morphs with poles variably developed as processes or spines
Diacromorphs
• Spherical to ellipsoidal, with ornament restricted to around the poles
Prismatomorphs
• Polygonal or prismatic, with edges commonly extended as fl anges
Oomorphs
• Egg-shaped forms, one end smooth and the other highly ornamented
tions, particularly during the Ordovician and
Silurian.
The acritarchs are some of the oldest docu-
mented fossils with a history of over 3000 myr,
although the group was not common until
some 1 Ga, when the fi rst major diversifi ca-
tion of the group, predating the Ediacara
biota (p. 242), was marked by large sphero-
Multiplicisphaeridium
morphs, acanthomorphs and polygono-
morphs. During the important Early Cambrian
radiation of the group, spinose morphs such
as Baltisphaeridium and Micrhystridium,
together with the crested Cymatiosphaera,
Baiomeniscus appeared. Significantly these armored vesicles
Leiofusa
evolved during the expansion of marine pred-
ators: Was this a form of arms race or merely
Villosacapsula a coincidence? By the Late Cambrian to Early
Ordovician, acritarch palynofacies (pollen
Figure 9.13 Some acritarch morphotypes: and spore assemblages) were dominated by
Multiplicisphaeridium (×800), Baiomeniscus three main groupings: the Acanthodiacro-
(×200), Leiofusa (×400) and Villosacapsula dium, Cymatiogalea and Leiofusa groups
(×400).
(Box 9.8). The acritarchs declined during the
Devonian, and are rare in Carboniferous-
Triassic rocks. Nevertheless the group staged
Evolution and geological history a weak recovery during the Jurassic and con-
tinued through the Cretaceous and Tertiary.
Acritarchs had a wide geographic range,
apparently mainly controlled by latitude; the
entire group ranged from the poles through Dinofl agellates
the tropics. The wide distribution of the group The dinofl agellates, or “whirling whips”,
is similar to that of the dinofl agellates and comprise a group of microscopic algae with
strongly suggests that acritarchs were also organic-walled cysts. The life history of these
members of the phytoplankton. Biogeographic organisms thus oscillates between a motile
provinces have been established for the Ordo- (swimming) and a cyst (resting) stage; the
vician, Silurian and Devonian periods and cysts usually range in size from 40 to 150 μm.
have helped reconstruct ancient climate belts The motile phase is either flexible and unar-
and oceanic currents. Acritarchs have also mored, or rigid and armored with a network
been of considerable value in regional correla- of plates, the theca; the arrangement of the
