259
rare earth elements
potential. The related acantharians have skeletons composed of strontium
sulfate, never preserved in sediments. The Heliozoa also have a radial symmetry, but lack a preservable skeleton. Particularly deep-sea sediments may
be rich in radiolarian skeletons. Radiolaria have existed since the Cambrian
and probably even earlier. Some seem to extend throughout the whole postPrecambrian; only few genera have a limited vertical range. This diminishes
their use in stratigraphy, but locally they have proved useful in correlation of
deposits or deep-sea cores, particularly when other fossils are lacking.
radiometric dating ~carbon, radioactive, ~radioisotopes and ~absolute age.
radionudides, natural two groups of natural radioisotopes exist, viz. cosmogenic
nuclides and primordial nuclides. Cosmogenic nuclides are constantly produced in the stratosphere, where primordial radioisotopes are the remainder of long-lived nuclides from the formation of the earth, such as potassium (40K) and the isotopes of the 238U, 232Th and families. Daughters of the
uranium and thorium families are useful tools for oceanographic studies
such as age determinations of sediment layers (210Pb, 230Th/232Th), growth of
corals (234U{238U) and transport of terrestrial material to the sea (234Thl
232Th).
radio-tags ~tagging.
radio tracking ~telemetry.
radula "tongue" or horny strip with, up to, thousands of teeth on the surface in
the mouth of Mollusca. In Gastropoda especially, a highly developed feeding
organ used as a grater, rasp, cutter, grasper, etc.
range ~biohorizon.
Raphidophyta (syn. Chloromonadida, Chloromonadophyceae) phylum of the
kingdom Protoctista. A small group (about ten species) of flagellate unicellular algae, which have green chromatophores with the pigments chlorophyll-a
and -c. Most live planktonic in fresh- or seawater. Formerly classified as an
order (Chloromonadida) of the class Phytomastigina (subphylum Mastigophora, phylum Sarcomastigophora, ~Protozoa).
raptorial feeding (syn. predatory feeding) feeding lilee a "raptor" (robber) used,
e.g., for birds of prey; in fish and plankton raptorial feeding stands for taking prey one by one, as opposed to filter feeding (~filter feeders).
rare earth elements also lanthanides. Elements lanthanium (57La) through luretium (71Lu). The outer electron shells of the commonly occurring trivalent
rare earth elements
potential. The related acantharians have skeletons composed of strontium
sulfate, never preserved in sediments. The Heliozoa also have a radial symmetry, but lack a preservable skeleton. Particularly deep-sea sediments may
be rich in radiolarian skeletons. Radiolaria have existed since the Cambrian
and probably even earlier. Some seem to extend throughout the whole postPrecambrian; only few genera have a limited vertical range. This diminishes
their use in stratigraphy, but locally they have proved useful in correlation of
deposits or deep-sea cores, particularly when other fossils are lacking.
radiometric dating ~carbon, radioactive, ~radioisotopes and ~absolute age.
radionudides, natural two groups of natural radioisotopes exist, viz. cosmogenic
nuclides and primordial nuclides. Cosmogenic nuclides are constantly produced in the stratosphere, where primordial radioisotopes are the remainder of long-lived nuclides from the formation of the earth, such as potassium (40K) and the isotopes of the 238U, 232Th and families. Daughters of the
uranium and thorium families are useful tools for oceanographic studies
such as age determinations of sediment layers (210Pb, 230Th/232Th), growth of
corals (234U{238U) and transport of terrestrial material to the sea (234Thl
232Th).
radio-tags ~tagging.
radio tracking ~telemetry.
radula "tongue" or horny strip with, up to, thousands of teeth on the surface in
the mouth of Mollusca. In Gastropoda especially, a highly developed feeding
organ used as a grater, rasp, cutter, grasper, etc.
range ~biohorizon.
Raphidophyta (syn. Chloromonadida, Chloromonadophyceae) phylum of the
kingdom Protoctista. A small group (about ten species) of flagellate unicellular algae, which have green chromatophores with the pigments chlorophyll-a
and -c. Most live planktonic in fresh- or seawater. Formerly classified as an
order (Chloromonadida) of the class Phytomastigina (subphylum Mastigophora, phylum Sarcomastigophora, ~Protozoa).
raptorial feeding (syn. predatory feeding) feeding lilee a "raptor" (robber) used,
e.g., for birds of prey; in fish and plankton raptorial feeding stands for taking prey one by one, as opposed to filter feeding (~filter feeders).
rare earth elements also lanthanides. Elements lanthanium (57La) through luretium (71Lu). The outer electron shells of the commonly occurring trivalent