Co

Figure 9.8 Stratigraphie ranges of the main foraminiferan groups. (Based on various sources.

spine pore cortical shell cortical shell spine pore

medullary shells bar

Spumellaria

Figure 9.9 Descriptive morphology of the radiolarians.

medullary shells bar

mDoom tmoaa

. joint chambered lattice shell tripod

Spumellaria

Figure 9.9 Descriptive morphology of the radiolarians.

Nassellaria

Acritarchs

The acritarchs are a mixed bag of entirely fossil, hollow, organic-walled microfossils that are impossible to classify. The acritarchs are probably polyphyletic; they include a wide range of forms, probably representing the cyst stages or resting phases in the life cycles of various groups of planktonic algae. Funda mental work on the group by Alfred Eisenack (1891-1982) initially suggested that these tiny fossils were the eggs of planktonic invertebrates; however, later he considered the group to be fossil members of the phytoplankton, plants rather than animals. William Evitt of Stanford University, in establishing the scope of the group in the early 1960s, noted that his term "acritarch" (meaning "uncertain origin")

Box 9.5 Classification of the radiolarians

The classification of the radiolarians is currently in a state of flux. Six orders are recognized (De Wever et al. 2001).

Order ARCHAEOSPICULARIA

• Mid Cambrian to Silurian Order ALBAILLELLARIA

• Late Ordovician to Late Silurian or ?Devonian Order LATENTIFISTULARIA

• Early Carboniferous (or earlier?) to Permian Order SPUMELLARIA

• Paleozoic (precise age uncertain) to Recent Order ENTACTINARIA

• (?Cambrian), Ordovician to Recent Order NASSELLARIA

• (?Late Paleozoic), Triassic to Recent

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