Trilobite Morphology

Trilobite Basic Body Plan

Scientific 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

However, trilobites are actually named for the three lobes in the longitudinal direction.:

4 – Right pleural lobe
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 the Cambrian and Ordovician, and particularly among trilobites with larger pygidiums showing effacement.

Trilobite Dorsal Cephalon Morphology

The 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.

Other trilobite 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 area

Trilobite Facial Suture Morphology

Facial 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 trilobite undergoes ecdysis, shedding the old exeskeleton, in order to grow. Not all trilobites had facial sutures. Some early

trilobites 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 Cambrian, but were the ancestors of trilobite orders that followed. Other later trilobites secondarily lost facial sutures.

Trilobite Dorsal Suture Morphology

As 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 can be divided into two regions - 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 (the central lobe in the cephalon) and the fixigena ("fixed cheeks"). The facial sutures lie along the anterior edge, at the division between 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:

1 - 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 of trilobites 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 Trimerus 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 sutures include Peltura 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 species 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 free cheek (the librigena), 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, however, postulated to have been evolved from opisthoparian sutures (Clarkson, et. al, 2006]. Other blind trilobites have facial sutures.

Trilobite 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.

1 - 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 front part of the dorsal 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 to the doublure.
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

The rostrum (also called the rostral plate) is also an important trilobite diagnostic body part. It is the middle part of the caphalic doublure and is located at the front part of the cephalon and in front of the hypostome, and separated from the rest of the doublure by the rostral 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 separates from the librigena

Hypostome

The 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 in 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 on the 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 attached to doublure and is aligned with front edge of glabella.
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.

Trilobite Thorax

The 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 pygidial segments.

Trilobites 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.

Trilobite Pygidium

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:

Micropygous - pygidium smaller than cephalon (see Redlichiid example)
Subisopygous - pygidium sub equal to, somewhat smaller than cephalon
Isopygous - pygidium equal in size to cephalon (see Agnostid example)
Macropygous - pygidium larger than cephalon

Additionally, there are named conditions to characterize the pygidial segments (lobes and Pleurae) in comparison to those of the thorax:

Homonomous condition - pygidium morphology similar to thoracic segments.

Heteronomous condition - pygidium morphology quite different from thoracic segments, particularly spines and lobes.

Large 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.

Fallotaspis typical Early Cambrian Order Redlichiida Suborder Olenellina Superfamily Fallotaspidoidea Family Fallotaspididae Zagora, Morocco	Eldregeops (Phacops) rana Middle Devonian Order Phacopida Family Phacopidae Windom Shale, New York	Flexicalymene meeki Late Ordovician Order Phacopida Family Calymenidae Richmond Formation Mount Orab, Ohio	Selenopeltis Ordovician Order Lichida Family Odontopleuridae Mount Boutshafrin, Alnif, Morocco
One of the earliest occurring trilobites from the Lower Cambrian of Morocco	Phacops trilobites could fully enroll with the pleurae and doublure forming a closed seal	Note how each thoracic segment comprises an axial lobe in between pleural lobes.	Even trilobites with long pleural spines such as Selenopeltis could enroll leaving spines jutting out.
Paralejurus hamlagdadicus Hypostome	Illaenus tauricornis
Middle Devonian Order Corynexochida Family Styginidae Laatchana, Morocco	Ordovician Order Corynexochida Family Illaenidae Wolchow River, Russia
The hypostome anchor for the mouthparts is present	Corynexochid trilobite exhibiting extreme effacement of cephalon and pygidium