Basic Body Plan
description of trilobites and identifying their taxonomic postion
among some 5000 distinct genera requires complex terminologies.
The figure to the right depicts the fundamental features of
the trilobite body plan as viewed dorsally from above the exoskeleton
(without legs or antenna). In the axial direction, the The
trilobite body has three major axial components, or tagmata:
1 – Cephalon
2 – Thorax
3 – Pygidium
trilobites are actually named for the three lobes in the longitudinal
4 – Right
5 – Axial lobe
6 – Left pleural lobe
Not all workers agree on whether there are truly three tagmata in trilobita,
with some arguing that the pygidium was a not a separate tagma, but a
frozen growth zone (Hughes,
2003). Exoskeleton convexity (exhibited as vertical height) shows great
diversity among trilobite (Treatise, p250), generally increasing through
particularly among trilobites with larger pygidiums showing effacement.
Dorsal Cephalon Morphology
head or cephalon of the trilobite is a morphologically complex
structure, Center most, the glabella is normally domed, forming
a cover for the crop, or stomach. The fixigena is
a fixed shell anterior to (in front of the glabella) called
a fixed cheek. It surrounds the glabella, but is inside of
the facial sutures.
In contrast, the librigena is
outside the facial sutures on either axial side and anterior
to the glabella. Librigena were often lost during molting (ecdysis),
and are hence called the free cheeks. The trilobite
cranidium is composed of the glabella together
with the fixigena. Many
trilobite fossils are found with their free cheeks missing.
morphological terminology defines many other smaller parts
of the cephalon
used in scientific descriptions, including:
1 – preocular
area; 2 – palpebral area; 3 – postocular area;
4 – posterolateral projection; 5 – occipital
ring; 6 – glabella; 7 – posterior area; 8 – lateral
border; 9 – librigenal area; 10 – preglabellar
Facial Suture Morphology
or cephalic sutures play an important role in trilobite taxonomy,
phylenogy and identification. The sutures are the natural fracture
lines in the cephalon of trilobites that separate when the
undergoes ecdysis, shedding the old exeskeleton, in order to
grow. Not all trilobites had facial sutures. Some
from Order Redlichida, Suborder Olenellina had no sutures and
are, in fact, believed to have predated
the evolution of sutures. Genera Fallotaspis, Nevadia
and Olenellus are
examples of early Olenellid trilobites lacking sutures that
went extinct near the end of the early
but were the ancestors of trilobite orders that followed.
Other later trilobites secondarily lost facial sutures.
Dorsal Suture Morphology
shown in the figure, there
are three main categories of facial suture types grouped according
to dorsal features: 1) proparian; 2)
gonatoparian, and 3) opisthoparian.
The dorsal surface of the trilobite exoskeleton cephalon
- the cranidium and the librigena (i.e., the librigina are also
known as the free cheeks). The cranidium can be further
divided into the glabella
lobe in the cephalon) and the fixigena ("fixed cheeks").
The facial sutures lie along the anterior edge, at the division
the cranidium and the librigena. Given this information, trilobite
facial sutures on the dorsal side can differentiated
according to where the sutures end relative
to the genal angle (the edges where the side and rear margins
of the cephalon converge in the figure). Broadly, five categories
are found among trilobita:
- Sutures Absent -
Facial sutures are lacking in the Suborder
Olenellina of Order
Redlichida. This is considered a primitive state occuring
in early Cambrian trilobites, though in later trilobite taxa,
they were lost secondarily during evolution, a condition
known as hypoparian or marginal.
2 - Proparian Sutures - The facial suture ends
in front of the genal angle, along the lateral margin. Examples
having proparian sutures are Dalmanites of Order
Phacopida and Suborder
Eodiscina of Order Agnostida.
3 - Gonatoparian Sutures - The facial suture ends at the tip
of the genal angle. Examples of trilobites
having gonatoparian sutures are Calymene and
of Suborder Calymenina
of Order Phacopida.
4 - Opisthoparian Sutures - The facial suture ends at the
posterior margin of the cephalon. Examples of trilobites showing opisthoparian
of Suborder Olenina in Order Ptychopariida
and Bumastus of Suborder Illaenina in Order Corynexochida.
The most common category of trilobite facial suture is opisthoparian.
5 - Hypoparian or marginal - The
facial dorsal sutures were secondarily lost during evolution. Several descendent
show the dorsal suture morphology changing, essentially moving to coincide
with the margins of the cephalon. Also, as the visual surface of the eye
is on a shrinking
the number of eye lenses tends to decrease, with the eventually of losing
the eyes. The secondary loss of dorsal sutures likely arose from the proparian
state, with examples found in some
Eodiscina from Order
Agnostida, all the Suborder
Agnostina trilobites, and
some Suborder Phacopina trilobites such as Ductina. The marginal sutures exhibited
by the harpetids and trinucleioids,
postulated to have been evolved from opisthoparian sutures (Clarkson, et. al,
2006]. Other blind trilobites have facial sutures.
Ventral Suture MorphologyReferring
to the figure left, the dorsal facial sutures extend downward
to the ventral side of the cephalon becoming the connective sutures dividing the doublure.
There four categories of ventral sutures.
- Connective Suturess
- These are sutures that continue from the facial sutures
past the front margin of the cephalon.
2 - Rostral Sutures -
These sutures connects the rostrum to the
cranidium. only seen in trilobites that actually have a rostrum.
3 - Hypostomal Sutures - These
sutures separate the hypostome from the doublure when the hypostome is
the attached type.
It is absent when the hypostome is the natant (i.e. free-floating) type. it
is also absent in some coterminant hypostomes where the hypostome is fused
4 - Median Sutures - These sutures
occur when instead of becoming connective
sutures, the two dorsal sutures
converge at a point
in front of the cephalon then divide straight down the center of the doublure. This
category of suture is seen in trilobites of Order Asaphida.
rostrum (also called the rostral plate) is also an important
trilobite diagnostic body part. It is the middle part
the cephalon and in front of the hypostome, and separated
from the rest of the doublure by the
suture. It is also where the hypostome usually attaches
in trilobites with conterminant or impendent condition hypostomes.
During trilobite molting,the rostrum
is used to anchor the front part of the trilobite as the cranidium
hypostome was the hard mouth part of the trilobite found on
the ventral underside of the cephalon, normally below
the glabella. Hypostomes are classified
in three categories (see figure where the doublure is shown in
black, the inner surface of the cephalon is light
gray, and the hypostome is red). The glabella is outlined
blue dashed lines) depending on whether they are permanently
attached to the rostrum or
not, and how they align to the front of dorsal tip
glabella. The three diagnostic hypostome types as illustrated in
the figure are (see Fortey, 1988, for detailed decription of
these hypostome conditions):
Natant - The hypostome is not
doublure and is
Conterminant - The hypostome is
attached to rostrum of doublure and aligned with front edge of glabella in
the same manner as the natant condition.
Impendent - The hypostome attached
to rostral plate but is not aligned to glabella. Note in the image that the
glabellar lobe is expanded in the forward direction all the way to the cephalic
margin in such a way that its forward part is underlain by the cephalic doublure.
In the impendent condition, the hypostome remains rigidly attached to the doublure
as in the conterminant condition, albeit, the positional relationship to
the frone of the glabella is lost.
thorax is the major tagma (as with insects) in
the middle between head and tail of the trilobite, i.e., cephalon
and pygidium. It comprises multiple articulated segments.
Most trilobites had between two and 16 thoracic
segments, though trilobites with up to about 100 are known.
Each segment consists of the central axial
lobe between outer pleurae
(plural lobes). Ventrally below each segment was a pair of
legs and gills. The pleurae
are phenotypic variants, sometimes smooth and sometimes bearing
thoracic spines. Leg muscles attached either to apodemes
(growth projections) on the ventral surface of the exoskeleton,
or directly to the exoskeleton. In trilobites diagnostic, it
can often be difficult to distinguish between thoracic and
were vulnerable creatures in dangerous seas. The exoskeleton
afforded some amount of dorsal protection,
but the ventral side had no such shield protecting the soft tissue.
It is hardly surprising that they developed the ability
to enroll, at least in part. The articulated thoracic segments,
allowed the trilobite to bend
up and down, and in some species (e.g., phacops) to fully roll
into a ball. Also see: early
trilobite protaspid and meraspid development.
The pygidium is rear most or posterior most tagma
in the trilobite body plan. It is made up of a number
of segments in a manner like the thorax and may have a telson
(tail spine) fused to it. Trilobites are described based on several
pygidial conditions based on size relative to the cephalon:
pygidium smaller than
Subisopygous - pygidium sub equal to, somewhat smaller than cephalon
Isopygous - pygidium
equal in size to cephalon (see
Macropygous - pygidium larger than cephalon
there are named conditions to characterize the pygidial segments
(lobes and Pleurae) in comparison to those of
condition - pygidium
morphology similar to thoracic segments.
pygidium morphology quite different from thoracic segments,
particularly spines and lobes.
pygidiums is generally considered an advanced characteristic,
and size increase is an apparent evolutionary trend across
the Cambrian and Ordovician, except among the agnostids. Pygidial
size variation is not uncommon even at the family level (Treatise,
p251). Also see: early
trilobite protaspid and meraspid development.