GLOSSARY 561
biomass of primary producers, and smaller and
smaller biomasses of successive consumers.
food web the complex feeding interactions
among members of a community.
foramen (pl. foramina) “pierce”; a small
opening.
foramen magnum “big opening”; the large
opening at the back or base of the skull through
which the spinal cord passes.
formation (in stratigraphy) a rock unit that may
be identifi ed and mapped in a regional context;
subdivided into members, and combined with
other formations into a group.
fossil “dug up”; the remains of a plant or animal
that died in the distant past.
fragmentation breaking of a shell or skeleton
into small pieces.
framework reef a reef whose basic structure is
formed entirely from organic skeletons (corals,
archaeocyathans, bryozoans, crinoids, etc.) (cf.
reef).
free cheek the lateral portion of the cephalon of
a trilobite which is divided from the central
portion by a facial suture, and which separates
during molting (cf. facial suture).
fringing reef a reef that lies on the margins of a
landmass, with no intervening lagoon (cf. atoll,
barrier reef).
frustule “a bit”; the skeleton of a diatom.
fugichnion (pl. fugichnia) “escape trace”.
full relief (of a trace fossil) seen in three dimen-
sions (cf. semirelief).
furca (pl. furcae) “fork”; a backwards-pointing
fl exible spine in an arthropod.
fusellar tissue the bandage-like tissues compos-
ing the periderm of graptolites.
fusiform “spindle-shaped”.
gamete “marriage”; a sex cell, such as an egg or
sperm.
gametophyte “marriage plant”; in plants that
show alternation of generations, the stage that
produces gametes and which engages in sexual
reproduction (cf. sporophyte).
gas hydrates deposits of methane locked in an ice
lattice, found either in deep oceans or in perma-
frost regions.
genal spine “cheek”; the pointed spine at the pos-
terior lateral margin of the trilobite cephalon.
gene an identifi able sequence within a chromo-
some that codes for a particular feature of an
organism.
gene fl ow the movement of genes through a pop-
ulation by interbreeding.
gene pool the sum total of the genotypes of all
individual organisms in a defi ned population.
geniculation “little knee”; bent at right angles.
genotype the sum of the features of an organism,
or population, contained in the genes.
genus (pl. genera) the category in classifi cation
above the species.
geochronometry measurement of geological time
using absolute methods such as radiometric
dating.
geographic range the complete area within which
a species, or other taxon, lives.
germinal aperture opening for the passage of
gametes in a spore or pollen grain.
gill bars bars of cartilage or bone that support
the gill slits.
gill slits gill openings behind the head, found in
chordates.
glabella (pl. glabellae) “bald”; the raised middle
portion of a trilobite cephalon.
golden spike
(in stratigraphy) a point in a
rock section, equivalent to an instant in
geological time, that marks the internationally
accepted base of a stratigraphic division
(e.g. member, formation, system/period or
epoch).
gonad “generation”; the organ that produces sex
cells; the ovary or testis.
Gondwanaland ancient supercontinent com-
posed of South America, Africa, India, Australia
and Antarctica.
gradualism see phyletic gradualism.
gradualistic steady change (cf. catastrophe).
greenhouse gas a gas, such as methane or
carbon dioxide, that promotes heating of the
atmosphere.
group (in stratigraphy) a number of formations
occurring in sequence that share some broad-
scale features.
guard the bullet-shaped, solid, terminal part
of the belemnite shell (cf. phragomcone,
pro-ostracum).
habitat the environmental setting within which a
species, or a community, lives.
haploid (in cell biology) a half complement of
chromosomes, as found in the sex cells (cf.
diploid).
haptonema “fasten-thread”; a fl agellum-like
structure in coccolithophores.
hardground a sea or lake fl oor composed of con-
solidated calcareous sediment (cf. fi rmground).
herbaceous low-growing, bushy.
herbivore “plant-eater”.