Page 227 - Introduction to Paleobiology and The Fossil Record

214 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

perforations: the nassellarians (Fig. 9.10) and phaera, although the nassellarians had
entactinarians develop a lattice from bar-like appeared; they continued as the major group
spicules, each end having a bundle of spicules. through the Jurassic, Cretaceous and Early
The initial nassellarian spicule is enclosed in Tertiary. Late Tertiary forms evolved thinner
the cephalis, and the skeleton develops further skeletons, perhaps because of increased
by the addition of segments following axial competition with the diatoms for mineral
symmetry. By contrast, the initial entactinar- resources.
ian spicule is enclosed in a latticed or spongy Radiolarian oozes cover about 2.5% of the

test with radial symmetry based on a spherical ocean floors, accumulating at rates of 4–5 mm
body plan; this is similar to those of the per 1000 years. Radiolarians are useful in
spumellarians (Fig. 9.10), which however paleo-oceanographic investigations, and they
have a microsphere (instead of a spicule) are particularly useful in dating the formation
internally. of deep-water sediments accumulating beneath
the carbonate compensation depth (CCD),
Evolution and geological history where carbonate-shelled organisms such as
foraminiferans cannot survive. Radiolarian
Although some records suggest an origin in cherts and radiolarites commonly occur in
the Mid Cambrian or earlier, the radiolarians oceanic facies preserved in mountain belts
became common in the Ordovician, and they and are commonly associated with ophiolites,
are often found in deep-sea cherts associated sections of the ancient ocean crust and upper
with major subduction zones. The albaillellar- mantle that have been uplifted (see p. 48),
ians together with the entactinarians were the so they are very important in deciphering
dominant forms, although after the Devonian, the origins and destruction of ancient ocean
spumellarians with sponge-like tests were systems such as Tethys.
more prominent (Fig. 9.10). But the beauty of the radiolarian skeleton
Spumellarians remained important during has also assured the group’s place in the
the Triassic, with genera such as Capnuchos- history of art (Box 9.6).

Box 9.4 Forams and environments

The ratio of agglutinated : hyaline : porcellaneous foram tests has been used extensively to differenti-
ate among a range of modern environments. Ternary plots of the relative frequencies of test type

distinguish fields for hypersaline and marine lagoons, estuaries and open shelf seas (Fig. 9.7). Fossil
faunas may be plotted on these templates, and these allow paleontologists to estimate the salinity
of ancient environments.
The ratio of infaunal : epifaunal benthic foraminiferans has also been widely used to determine

the relative content of dissolved oxygen and/or organic carbon on the seafloor. Epifaunal and infaunal
foraminiferans can be distinguished by their test morphologies, where epifaunal forms occur mainly
in aerobic conditions with low amounts of organic carbon, and infaunal forms occur in more oxygen-
deficient conditions with higher organic carbon content.

Measures of the ratio of benthic : planktonic foraminiferans are also useful in environmental
studies. In general terms, the percentage of benthic taxa declines rapidly below depths of about
500 m in modern seas and oceans. Data from living assemblages have been used to interpret paleoen-
vironments with diverse fossil foraminiferan faunas. For example, microfossil analysis of the upper
part of the Late Cretaceous chalk of the Anglo-Paris basin has suggested water depths of between
600 and 800 m during the Turonian on the basis of the high proportions of planktonic foraminifer-
ans; however, by the Campanian, water depths of about 100 m are suggested by the rich benthic
fauna.