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PROTISTS 211
spectrum of shapes, from simple spherical during the Late Carboniferous and Permian.
compartments through tubular to clavate Despite a high diversity during the Late
forms. Moreover, the shape and position of Permian, they became extinct at the end of the
the aperture may vary. Surface ornament may Paleozoic, and the Endothyridae and the
include ribs and spines or be merely punctate Miliolinidae were very much reduced in
or rugose. Foraminifera are classifi ed diversity.
according to test type and ornamentation Although Triassic assemblages were gener-
(Box 9.2). ally impoverished, the stage was set for a con-
siderable radiation during the Jurassic. Two
Life modes hyaline groups, the benthic Nodosariidae and
planktonic Globigerinidae, diversifi ed, while
The foraminiferans have adopted two main the agglutinates, Lituolitidae and Orbitolini-
life modes, benthic and planktonic. The dae, continued. The planktonic foraminifer-
majority are benthic, epifaunal organisms; ans diversified in the Cretaceous, culminating
they are either attached or cling to the sub- in the near extinction of the group during the
strate or crawl slowly over the seabed by Cretaceous–Tertiary (KT) mass extinction.
extending their protoplasmic pseudopodia. Two further periods of diversifi cation took
Infaunal types live within the top 15 cm of place during the Paleocene-Eocene and the
sediment. Most benthic forms have a restricted Miocene.
geographic range. Planktonic foraminiferans
are most diverse in tropical, equatorial regions
and may be extremely abundant in fertile Radiolaria
areas of the oceans, particularly where upwell- The radiolarians are marine, unicellular,
ing occurs. planktonic protists with delicate skeletons
The functional morphology of these groups usually composed of a framework of opaline
can now be modeled mathematically (Box silica (Fig. 9.9). Their name is derived from
9.3) and potentially can be related to different the radial symmetry, commonly marked by
life modes in the group. Moreover their rela- radial skeletal spines, characteristic of many
tionships to different environments, past and forms. Many others, however, lack radial
present, are well established (Box 9.4). symmetry. Most radiolarians feed on bacteria
and phytoplankton, but also on copepods and
Evolution and geological history crustacean larvae and occupy levels in the
water column from the surface to the abyssal
The earliest foraminiferans are known from depths, although most live in the photic zone
the Lower Cambrian, represented by simple commonly associated with symbiotic algae.
agglutinated tubes assigned to Bathysiphon, a The radiolarian ectoplasm covers the test and
living benthic genus (Fig. 9.8). More diverse holds symbiotic zooxanthellae, microorgan-
agglutinated forms appeared during the Ordo- isms enclosed within the cell mass, and perfo-
vician while microgranular tests evolved rations, providing some nourishment. The
during the Silurian; however, it was not until radiolarian endoplasm (surrounded by the
the Devonian that multichambered tests prob- capsular membrane) contains the nucleus and
ably developed. Nevertheless, Carboniferous other inclusions. The group has two types of
assemblages have a variety of uniserial, bise- pseudopodia: the axopodia are rigid and not
rial, triserial and trochospiral agglutinated ramified, whereas the filipodia are thin, rami-
tests. Around the Devonian–Carboniferous fied extensions of the ectoplasm.
boundary the fi rst partitioned tests displaying
multilocular growth modes (the addition of Morphology and classifi cation
new chambers in series) appeared. Two fami-
lies, the Endothyridae and Fusulinidae, domi- The radiolarian skeleton or test consists of
nated Carboniferous assemblages and the isolated or networked spicules, composed of
porcellaneous Miliolinidae achieved impor- opaline silica and forming sponge-like struc-
tance in the Permian. The Fusulinidae were tures or trabeculae. Three of the main
generally large, specialized foraminiferans, groups are recognized (Box 9.5) on the basis
adapted to carbonate and reef-type facies of skeletal structure and arrangement of
