GLOSSARY 569
rhizome “root”; an underground stem.
rostrum “beak”; the snout or anteriormost part
of the head (adj. rostral).
ruga (pl. rugae) “roughness”; irregular small
projections (adj. rugose).
ruminant a mammal that digests its food in
several stages (e.g. a camel or a cow).
saccus (pl. sacci) “bag”; empty structure on the
side of some pollen grains (adj. saccate).
Scala naturae the “chain of being”, a sequence
of organisms, from simple to complex, once
interpreted as evidence for unidirectional
evolution.
scandent “climbing”.
scavenging feeding on organisms that are already
dead.
schizochroal eye an eye with reduced numbers of
large spaced lenses, seen in trilobites (cf. holo-
chroal eye).
sclerite a “hard” skeletal plate.
scleroprotein the tough proteinaceous material
that makes up the periderm of graptolites.
sclerotized “hardened”.
selectivity discrimination among species, espe-
cially for survival during an extinction event,
based on ecological characteristics.
selenizone “moon zone”; infi lled track of the
apertural slit of a gastropod.
semirelief (of a trace fossil) seen on the surface
of a bed (cf. full relief).
sepal one of the outermost parts of a fl ower,
lying outside the petals.
septum (pl. septa) “fence”; a dividing wall within
the skeleton of various animals (adj. septate).
sequence stratigraphy the sedimentary sequences
into major packets corresponding to times of
transgression, regression and non-deposition.
sere a plant or epifaunal community that is one
of a succession of unstable assemblages on the
way to the establishment of a climax
community.
sessile “sitting”; organisms that live on the
seabed, and which do not move.
seta (pl. setae) “bristle”; a stiff hair.
sexual dimorphism “two forms”; differences in
the morphology of males and females of a
species.
sexual selection selection of traits for improving
the chances of mating.
sicula the small cone that is the fi rst part of a
graptolite rhabdosome to form.
siderite a form of iron carbonate (FeCO
3
) that
occurs commonly in concretions around fossils.
Signor–Lipps effect the backwards smearing of
fossil occurrences; the observation that the last
fossil observed was almost certainly not the last
representative of a taxon.
sinistral “left-handed” (cf. dextral).
siphon an extendable tube in a mollusk, used for
sucking in water with food particles and for
expelling fi ltered water.
siphuncle connecting strand of soft tissue that
extends through the chambers of a cephalopod
shell.
skeleton supporting structure in an organism,
usually involving some mineralized tissues; may
be internal (endoskeleton) or external
(exoskeleton).
solitary an organism, usually a coral, that lives
in isolation (cf. colonial).
somite “body”; a body segment in an
arthropod.
sparry calcite calcite (CaCO
3
) that occurs as
large crystals (cf. micrite).
speciation the process of formation of a new
species, either by splitting (branching) or by
lineage evolution, from a pre-existing species.
species a group of organisms, or populations,
that includes all the individuals that normally
interbreed, and which can produce viable off-
spring; typically the smallest unit in the hierar-
chy of a classifi cation of organisms.
species selection selection at the level of species.
sphincter an opening that may be closed by mus-
cular activity.
spicule “small ear of corn”; a tiny needle-like
calcareous or siliceous structure that forms part
of the skeleton of a sponge.
spiracle “to breathe”; an opening near the mouth
in a blastozoan; the breathing hole behind the
head in a shark.
spiralian animal with an initial sequence of cell
division that follows a spiral track (cf. radialian);
higher taxon (mollusk–annelid–brachiopods
plus most fl atworm–rotifers (platyzoans)).
spongin a horny organic material that forms
around the skeleton of many sponges.
spontaneous generation the idea that life could
arise suddenly from non-life.
sporangium (pl. sporangia) spore-bearing struc-
ture in a land plant.
sporophyte “spore plant”; in plants that show
alternation of generations, the stage that pro-
duces spores and that engages in asexual repro-
duction (cf. gametophyte).
spreite (pl. spreiten) “trace”; indications of
former positions of a burrow.