trilobites, their phenotypic variation (i.e., wide range
of morphological characteristics), taxonomy, reproduction,
development (growth), lifestyles, survival, evolution and so
forth involves somewhat complex terminology. Most of the terms
needed for a complete understanding are given in the trilobite
a start point, it is useful summarize some of the central terms.
Trilobites had a chitinous exoskeleton that
was divided side to side (longitudinally) into three
lobes (this is where the trilo in the name trilobite comes
from); these are the pleura: 1) right
pleural lobe, 2) axial lobe, 3) and left
pleural lobe. In the axial direction (head to tail), they also
had : 1) the head
(or cephalon), a thorax having
a few to a large number of transverse segments, the rearmost
tagma, the pygidium. All
of these tagmata are prominently mentioned when describing
and differentiating trilobites across their phylogeny (orders,
families, genera and species).
|Terms used to describe trilobites and their
- 18S ribosomal RNA: genes coding for 18S rRNA
are referred to as 18S rDNA. Sequence data from these
genes is widely used in molecular analysis to reconstruct
the phylogenetic or evolutionary history of organisms.
These highly conserved sequences evolve slowly, making
it usable to construct ancient lineage divergences
eyes with compound lenses, each with separate
each lens separated from others by shallow
scleral walls; the corneal membrane ends
at lens edge. also see
holochroal eyes, and schizochroal
ingrowth to the exoskeleton to
which muscles attached.
or axial lobe:
central lobe running the length of the trilobite
body (Also see: Trilobite
Basic Body Plan)
the appendages of arthropods may be either
biramous or uniramous. A biramous limb, as the
prefix suggests, branches into two, and each
branch consists of
segments attached end to end. Also see uniramous
the frontmost trilobite part, i.e., the head,
a complex structure.
- chitin: (C8H13O5)n
is a long-chain polymeric polysaccharides of beta-glucose
that forms a hard, semitransparent material and found
as a component of many arthropod exeskeletons.
a monophyletic group an ancestor and all of
an approach to biological classification (phylogenetics)
where organisms are grouped together based on
whether or not they share morphological traits
(particularly unique characteristics) derived
from the group's last common ancestor, and that
are not present in more distant ancestors. The
process results in an evolutionary taxonomy that
shows ancestral progenitors and descendants in
a tree-like structure.
close interlocking of opposing surface during
a condition where the hypostome is
attached to the anterior doublure with a suture.
hypostome conditions are impendent and natant.
(Also see trilobite
hypostome and Conterminant and
the glabella together
with the fixigena.
referring to the top or back surface, as opposed
shell continuation under the trilobite, typically
as a ventral cephalic and/or pygidial rim.
the moulting of the
in many invertebrates. This process of moulting
is the defining feature
Ecdysozoa, which includes Class Trilobita.
furrows and lobes have lost detail or entirely
disappeared yielded a smoother exoskeleton surface,
which putatively facilitated burrowing to hide
from predators or prey.
loss of detail.
- exoskeleton: an external skeleton that supports
and protects an animal's body. Common in invertebrate
animals, such as insects and crustaceans
such as crabs and lobsters. They are often called
shells in larger animals.
the bending of the trilobite body, especially
to enclose vulnerable ventral organs in a protective
wrapper formed by the dorsal exoskeleton of
pygidium and thoracic segments. (Also see trilobite
adjacent flat surfaces, normally where two adjoining
body parts contact and can articulate (i.e, where
they are hinged); e.g., the articulating facets on
thoraxic pleural segments.
the fixed cheek cephalic shell
surrounding glabella inside
of facial sutures. Also
see gena and librigena.
cheeks: essentially the librigena lost
by a trilobite during ecdysis.
- fulcrate referring
to thoraxic segments with a geniculation (bend) separating
a horizontal inner portion from an inclined outer
- furrow: a groove, often defining adjacent
convex features of the exoskeleton surface, e.g.,
between lobes of the glabella
the shell of the cephalon surrounding the glabella that is normally divided by sutures into the fixigena
and the librigena.
angle: the angle
formed by the meeting of the lateral (side) and
posterior (rear) margins of the cephalon; the angle
is important in trilobite taxonomy.
spine: a spine
anywhere on the gena (i.e., cheek) of the cephalon,
typically pointing at the genal angle.
the middle (axial) portion of cephalon that
is typically convex or lobed. The glabella
forms a dome over the top of the crop (i.e.,
the trilobite's stomach)
a condition where the facial sutures end at the
of the genal angle.
granules or granule-like features on the exoskeleton.
the final trilobite development stage following
the meraspis stage;
the trilobite successively adds thoraxic segments
until the number is reached correcponding to
that of adults The cephalic and
pygidial features of
the adult are also attained. Also see trilobite
development and ontogeny.
a type of trilobite eye having all the compound lenses
directly in contact, and all covered by a single,
continuous corneal surface. Holochroal eyes were
usually high in number (15,000 and more), and have
been found in all nine of the trilobite orders. A
hexagonal shape enabled tight packing (much like
a graphite crystal). These eyes had no sclera. -
also see abathochroal, schizochroal, the other types
of trilobite eyes.
a condition of growth nominal size.
adjective meaning related to the hypostome.
a portion of the exoskeleton underneath the cephalon,
thought to be a mouthpart in trilobites. Typically,
the anterior edge of the hypostome is directly under
the anterior edge of the glabella.
a type of conterminent hypostomal attachment where
the doublure underlies the anterior glabellar edge,
such that the anterior hypostomal border does not
match the anterior glabellar border. Contrast with conterminant and
a condition where the trilobite's cephalon and
pygidium have similar size and possible symmetry.
Contrast with micropygous and macropygous.
a place of extraordinary fossil preservation, often
including mineralization of soft tissues.
the so-called free cheek of the cephalic shell
on either side of the glabella outside
of the facial sutures. They were often lost
during ecdysis (moulting) or after death. see
- macropygous: a
trait of the pygidium being larger than the
cephalon - contrast with isopygous and micropygous.
the trilobite developmental stage following
the protaspis stage
where the trilobite has more than one thoraxic segment,
but fewer than the adult number of an adult.
Aso see protaspis and holaspis stage.
Also see trilobite
development and ontogeny.
- micropygous: a
morphological condition where the pygidium much smaller
than cephalon. Also see isopygous and
- monophyletic: a monophyletic taxon
forms a clade, meaning
it comprises an ancestral species and all of its
descendants. The term is synonymous with the uncommon
term holophyly. Monophyletic groups are typically
characterized by shared derived characteristics
where hypostome is free and not attached to
the cephalic doublure.
(Also see trilobite
hypostome and natant and hypostome
related to the to the trilobite occiput.
spine: spine located at the occiput.
the posterior portion of trilobite cephalon, where
the axial portion is considered part of the glabella , i.e., the occipital ring.
- ommatidia: units of the compound
eyes of arthropods like insects, crustaceans and
trilobites. An ommatidium
contains a cluster of photoreceptor cells surrounded
by support cells and pigment cells, the outer part
of which is covered with a transparent
the facial sutures terminating along the rear margin
of the trilobite cephalon adaxial to the genal angle.
Also see gonatoparian and proparian.
the posterior, narrower portion of the trilobtes
thorax when it is divided into wider, anterior prothorax and opisthothorax.
that portion of trilobite fixigena associated with
along the edge of the rostrum.
the study of evolutionary relationships (progenitors
and descendents) among groups of organisms.
The relationships are hypothesized through morphological
data for fossils,
and with molecular phylogenetics
using nucleotide sequences encoding genes or amino
acid sequences encoding proteins for extant organisms.
Sequencing of extant organisms can also assist
classification at a basal level from which the
progenitors, descendents and evolutionary branching
of long extinct organisms can be inferred. Trilobite
phylogenetics is largely based on cladistics.
(plural, pleurae) lateral (side) part of thoraxic
segment or lateral (non axial) part of pygidium.
(Also see trilobite
thorax and thoracic
axial and pleural lobes)
referring to pleurae.
extending from the end of the axis.
field: portion of cephalon in
glabella. (Also see: Trilobite
Dorsal Cephalon Morphology)
- proboscis: an elongated appendage
from the head of an animal that in vertebrates
nomally is a to tubular mouthpart used for feeding
facial sutures ending forward of the genal angle.
- protaspis: (plural
= protaspides) earliest developmental stage of
a trilobite lacking segments. Also see meraspis and holaspis.
These developmental forms have the smallest known
exoskeletons, tending to be spherical to sub-hemispherical
and lacking transeverse articulation. Also see
development and ontogeny.
- Protostomia: a clade of animals that with
deuterostomes and a few larger phyla comprise
the Bilateria, animals with bilateral
symmetry and three germ layers.
The major distinctions between deuterostomes and
protostomes are found in embryonic development.
anterior and wider portion of thorax when it is divided
into prothorax and narrower, posterior opisthothorax,
as seen in some Redlichiida.
pertaining to the pygidium.
hindmost segment; tail. See pygidium
pear-shaped, typically referring to the glabella
plate: (rostrum) middle portion of the
ventral cephalic doublure, typically where the
eyes: with compound lenses, each with
separate cornea, each lens separated from others
by deep scleral walls; corneal membrane extends
downward into sclera. see abathochroal, holochroal.
a Latin word meaning "in the sense
of". It is often seen in the context of a
paleontologists view, nomally with the appropriate
- seta (plural
setae): a stiff hair, bristle, or bristle-like
process or part of an organism often used to
cling to surfaces
or gain traction.
- spiniform: Shaped like a spine
bearing a spine or spines.
pygidium large, but not quite as large as cephalon.
nearly parallel in arrangement, slightly diverging
linear break in exoskeleton, typically along
which parts separated during molting.
(or synapomorphic trait or character state) A
cladistics term where
a trait is shared (a symmorphy)
by two or more taxa from which it can be inferred
to have been a trait present in their shared most
recent common ancestor.
- tagma (tagmata
plural): the grouping of multiple segments or
metameres into a coherently functional morphological
unit. The head, thorax,
and abdomen of insects is an example. For trilobites:
cephalon, thorax and pygidium.
tail spine fused to posterior of pygidium.
(plural terga) the dorsal portion of an arthropod
segment other than the head. The anterior edge is
called the base and posterior edge is called the
apex or margin. A tergum can be divided into several
plates, called tergites. In a thoracic segment, the
tergum may be divided into an anterior notum and
a posterior scutellum. In crustaceans, it is known
as a pleuron when it overhangs the insertion of the
limb on each side as a free plate. The ventral portion
of a segment is called the sternum.
middle, segmented body portion between cephalon and
pygidium. aloso see
pertaining to the thorax.
- tubercle: a
nodule, nominally a small and roundish outgrowth
on the trilobite exoskeleton.
bearing tubercles or similar sculpturing on the outer
the appendages of arthropods may be either biramous or
uniramous. A uniramous limb comprises a single
series of segments attached end-to-end. For
a time, possession of uniramous limbs was believed
a shared, derived character, so uniramous arthropods
were grouped into a taxon called Uniramia.
It is now believed that several groups of arthropods
evolved uniramous limbs from ancestors with
biramous limbs, so this taxon is no longer
used. Also see biramous
referring to the underside, as opposed to dorsal.
a groove along the anterior and/or lateral cephalic doublure to
accommodate the edge of the pygidium and
thoracic pleurae during
enrollment (e.g., a trait of some Phacopida).
- Ventral nerve cord: a nervous system of some
invertebrate phyla, notably nematodes, annelids
and the arthropods comprising a cerebral
ganglia anteriorly with the nerve cords running
down the organisms's ventral plane.