Trilobite Anatomy

Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period () and they flourished throughout the lower Paleozoic before slipping into a long decline, when, during the Devonian, all trilobite orders except the Proetida died out. The last extant trilobites finally disappeared in the mass extinction at the end of the Permian about 252 million years ago. Trilobites were among the most successful of all early animals, existing in oceans for almost 270 million years, with over 22,000 species having been described.

By the time trilobites first appeared in the fossil record, they were already highly diversified and geographically dispersed. Because trilobites had wide diversity and an easily fossilized exoskeleton, they left an extensive fossil record. The study of their fossils has facilitated important contributions to biostratigraphy, paleontology, evolutionary biology, and plate tectonics. Trilobites are often placed within the arthropod subphylum Schizoramia within the superclass Arachnomorpha (equivalent to the Arachnata), although several alternative taxonomies are found in the literature. More recently they have been placed within the Artiopoda, which includes many organisms that are morphologically similar to trilobites, but are largely unmineralised.

Trilobites evolved into many ecological niches; some moved over the seabed as predators, scavengers, or filter feeders, and some swam, feeding on plankton. Some even crawled onto land. Most lifestyles expected of modern marine arthropods are seen in trilobites, with the possible exception of parasitism (where scientific debate continues). Some trilobites (particularly the family Olenidae) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food. The largest trilobites were more than long and may have weighed as much as .

Evolution

Trilobite relatives

Trilobites belong to the Artiopoda, a group of extinct arthropods generally morphologically similar to trilobites, but aside from trilobites lacking mineralised exoskeletons. Because of their lack of mineralised exoskeletons, non-trilobite artipodans are typically only found in exceptionally preserved deposits, mostly during the Cambrian period. The exact relationships of artiopods to other arthropods is uncertain. They have been considered closely related to chelicerates (which include horseshoe crabs and arachnids) as part of a clade called Arachnomorpha, while others consider them to be more closely related to Mandibulata (which contains insects, crustaceans and myriapods) as part of a clade called Antennulata.

Fossil record of early trilobites

The earliest trilobites known from the fossil record are redlichiids and ptychopariid bigotinids dated to around 520 million years ago. Contenders for the earliest trilobites include ''Profallotaspis jakutensis'' (Siberia), ''Fritzaspis'' spp. (western USA), ''Hupetina antiqua'' (Morocco) and ''Serrania gordaensis'' (Spain). All trilobites are thought to have originated in present-day Siberia, with subsequent distribution and radiation from this location.

All Olenellina lack facial sutures (see below), and this is thought to represent the original state. The earliest sutured trilobite found so far (''Lemdadella''), occurs almost at the same time as the earliest Olenellina, suggesting the trilobites origin lies before the start of the Atdabanian, but without leaving fossils. Other groups show secondary lost facial sutures, such as all Agnostina and some Phacopina. Another common feature of the Olenellina also suggests this suborder to be the ancestral trilobite stock: early protaspid stages have not been found, supposedly because these were not calcified, and this also is supposed to represent the original state. Earlier trilobites may be found and could shed more light on the origin of trilobites.

Three specimens of a trilobite from Morocco, ''Megistaspis hammondi'', dated 478 million years old contain fossilized soft parts.

Divergence and extinction

Trilobites saw incredible diversification over time. For such a long-lasting group of animals, it is no surprise that trilobite evolutionary history is marked by a number of extinction events where some groups perished and surviving groups diversified to fill ecological niches with comparable or unique adaptations. Generally, trilobites maintained high diversity levels throughout the Cambrian and Ordovician periods before entering a drawn-out decline in the Devonian, culminating in the final extinction of the last few survivors at the end of the Permian period.

Evolutionary trends

Principal evolutionary trends from primitive morphologies, such as exemplified by '' Eoredlichia'', include the origin of new types of eyes, improvement of enrollment and articulation mechanisms, increased size of pygidium (micropygy to isopygy), and development of extreme spinosity in certain groups. Changes also included narrowing of the thorax and increasing or decreasing numbers of thoracic segments. Specific changes to the cephalon are also noted; variable glabella size and shape, position of eyes and facial sutures, and hypostome specialization. Several morphologies appeared independently within different major taxa (e.g. eye reduction or miniaturization).

Effacement, the loss of surface detail in the cephalon, pygidium, or the thoracic furrows, is also a common evolutionary trend. Notable examples of this were the orders Agnostida and Asaphida, and the suborder Illaenina of the Corynexochida. Effacement is believed to be an indication of either a burrowing lifestyle or a pelagic one. Effacement poses a problem for taxonomists since the loss of details (particularly of the glabella) can make the determination of phylogenetic relationships difficult.

Pre-Cambrian

Phylogenetic biogeographic analysis of Early Cambrian Olenellidae and Redlichiidae suggests that a uniform trilobite fauna existed over Laurentia, Gondwana, and Siberia before the tectonic breakup of the supercontinent Pannotia between 600 and . Tectonic breakup of Pannotia then allowed for the diversification and radiation expressed later in the Cambrian as the distinctive Olenellid province (Laurentia, Siberia, and Baltica) and the separate Redlichid province (Australia, Antarctica, and China).
Breakup of Pannotia significantly antedates the first appearance of trilobites in the fossil record, supporting a long and cryptic development of trilobites extending perhaps as far back as or possibly further back in time.

Cambrian

Very shortly after trilobite fossils appeared in the lower Cambrian, they rapidly diversified into the major orders that typified the Cambrian—Redlichiida, Ptychopariida, Agnostida, and Corynexochida. The first major crisis in the trilobite fossil record occurred in the Middle Cambrian; surviving orders developed isopygius or macropygius bodies and developed thicker cuticles, allowing better defense against predators (see Thorax below). The end-Cambrian mass extinction event marked a major change in trilobite fauna; almost all Redlichiida (including the Olenelloidea) and most Late Cambrian stocks became extinct. A continuing decrease in Laurentian continental shelf area is recorded at the same time as the extinctions, suggesting major environmental upheaval.

Notable trilobite genera appearing in the Cambrian include:
*'' Abadiella'' (Lower Cambrian)
*'' Buenellus'' (Lower Cambrian)
*'' Judomia'' (Lower Cambrian)
*''Olenellus'' (Lower Cambrian)
*'' Ellipsocephalus'' (Middle Cambrian)
*''Elrathia'' (Middle Cambrian)
*''Paradoxides'' (Middle Cambrian)
*''Peronopsis'' (Middle Cambrian)
*'' Xiuqiella'' (Middle Cambrian)
*''Yiliangella'' (Middle Cambrian)
*''Yiliangellina'' (Middle Cambrian)
*''Olenus'' (Late Cambrian)

Ordovician

The Early Ordovician is marked by vigorous radiations of articulate brachiopods, bryozoans, bivalves, echinoderms, and graptolites, with many groups appearing in the fossil record for the first time. Although intra-species trilobite diversity seems to have peaked during the Cambrian, trilobites were still active participants in the Ordovician radiation event, with a new fauna taking over from the old Cambrian one. Phacopida and Trinucleioidea are characteristic forms, highly differentiated and diverse, most with uncertain ancestors. The Phacopida and other "new" clades almost certainly had Cambrian forebearers, but the fact that they have avoided detection is a strong indication that novel morphologies were developing very rapidly. Changes within the trilobite fauna during the Ordovician foreshadowed the mass extinction at the end of the Ordovician, allowing many families to continue into the Silurian with little disturbance.
Ordovician trilobites were successful at exploiting new environments, notably reefs. The Ordovician mass extinction did not leave the trilobites unscathed; some distinctive and previously successful forms such as the Telephinidae and Agnostida became extinct. The Ordovician marks the last great diversification period amongst the trilobites: very few entirely new patterns of organisation arose post-Ordovician. Later evolution in trilobites was largely a matter of variations upon the Ordovician themes. By the Ordovician mass extinction, vigorous trilobite radiation has stopped and gradual decline is foreshadowed.

Some of the genera of Trilobites appearing in the Ordovician include:

*'' Cyclopyge'' (Early to Late Ordovician)
*''Selenopeltis'' (Early to Late Ordovician)
*'' Parabolina'' (Early Ordovician)
*''Cheirurus'' (Middle Ordovician)
*''Eodalmanitina ''(Middle Ordovician)
*'' Trinucleus'' (Middle Ordovician)
*''Triarthrus'' (Late Ordovician)

Silurian and Devonian

Most Early Silurian families constitute a subgroup of the Late Ordovician fauna. Few, if any, of the dominant Early Ordovician fauna survived to the end of the Ordovician, yet 74% of the dominant Late Ordovician trilobite fauna survived the Ordovician. Late Ordovician survivors account for all post-Ordovician trilobite groups except the Harpetida.

Silurian and Devonian trilobite assemblages are superficially similar to Ordovician assemblages, dominated by Lichida and Phacopida (including the well-known Calymenina). A number of characteristic forms do not extend far into the Devonian and almost all the remainder were wiped out by a series of dramatic Middle and Late Devonian extinctions. Three orders and all but five families were exterminated by the combination of sea level changes and a break in the redox equilibrium (a meteorite impact has also been suggested as a cause). Only a single order, the Proetida, survived into the Carboniferous.

Genera of trilobites during the Silurian and Devonian periods include:

*''Dalmanites'' (Early to Late Silurian)
*''Calymene'' (Silurian)
*''Encrinurus'' (Silurian)
*'' Exallaspis'' (Middle to Late Silurian)
*''Paralejurus'' (Early Devonian)
*'' Lioharpes'' (Middle Devonian)
*''Phacops'' (Middle to Late Devonian)

Carboniferous and Permian

The Proetida survived for millions of years, continued through the Carboniferous period and lasted until the end of the Permian (when the vast majority of species on Earth were wiped out). It is unknown why the order Proetida alone survived the Devonian. The Proetida maintained relatively diverse faunas in both deep and shallow water shelf environments throughout the Carboniferous. For many millions of years the Proetida existed untroubled in their ecological niche. An analogy would be today's crinoids, which mostly exist as deep water species; in the Paleozoic era, vast 'forests' of crinoids lived in shallow near-shore environments.

Some of the genera of trilobites during the Carboniferous and Permian periods include:

*'' Archegonus ''(Early to Middle Carboniferous)
*'' Hesslerides'' (Middle Carboniferous)
*''Endops'' (Middle Permian)
*'' Triproetus'' (Late Carboniferous to Early Permian)
*'' Ditomopyge ''(Late Carboniferous to Late Permian)

Final extinction

Exactly why the trilobites became extinct is not clear; with repeated extinction events (often followed by apparent recovery) throughout the trilobite fossil record, a combination of causes is likely. After the extinction event at the end of the Devonian period, what trilobite diversity remained was bottlenecked into the order Proetida. Decreasing diversity
of genera limited to shallow-water shelf habitats coupled with a drastic lowering of sea level ( regression) meant that the final decline of trilobites happened shortly before the end of the Permian mass extinction event. With so many marine species involved in the Permian extinction, the end of nearly 300 million successful years for the trilobites would not have been unexpected at the time.

Trilobites have no known direct descendants. Their closest living relatives would be the chelicerates due to both being arachnomorphs. Though horseshoe crabs are often cited as their closest living relatives, they are no closer evolutionarily than other chelicerates.

Fossil distribution

Trilobites appear to have been primarily marine organisms, since the fossilized remains of trilobites are always found in rocks containing fossils of other salt-water animals such as brachiopods, crinoids, and corals. Some trackways suggest trilobites made at least temporary excursions unto land. Within the marine paleoenvironment, trilobites were found in a broad range from extremely shallow water to very deep water. Trilobites, like brachiopods, crinoids, and corals, are found on all modern continents, and occupied every ancient ocean from which Paleozoic fossils have been collected. The remnants of trilobites can range from the preserved body to pieces of the exoskeleton, which it shed in the process known as ecdysis. In addition, the tracks left behind by trilobites living on the sea floor are often preserved as trace fossils.

There are three main forms of trace fossils associated with trilobites: ''Rusophycus'', ''Cruziana'' and ''Diplichnites''—such trace fossils represent the preserved life activity of trilobites active upon the sea floor. ''Rusophycus'', the resting trace, are trilobite excavations involving little or no forward movement and ethological interpretations suggest resting, protection and hunting. ''Cruziana'', the feeding trace, are furrows through the sediment, which are believed to represent the movement of trilobites while deposit feeding. Many of the ''Diplichnites'' fossils are believed to be traces made by trilobites walking on the sediment surface. Care must be taken as similar trace fossils are recorded in freshwater and post-Paleozoic deposits, representing non-trilobite origins.

Trilobite fossils are found worldwide, with thousands of known species. Because they appeared quickly in geological time, and moulted like other arthropods, trilobites serve as excellent index fossils, enabling geologists to date the age of the rocks in which they are found. They were among the first fossils to attract widespread attention, and new species are being discovered every year.

In the United States, the best open-to-the-public collection of trilobites is located in Hamburg, New York. The shale quarry, informally known as Penn Dixie, stopped mining in the 1960s. The large amounts of trilobites were discovered in the 1970s by Dan Cooper. As a well-known rock collector, he incited scientific and public interest in the location. The fossils are dated to the Givetian (387.2 - 382.7 million years ago) when the Western New York Region was 30 degrees south of the equator and completely covered in water. The site was purchased from Vincent C. Bonerb by the Town of Hamburg with the cooperation of the Hamburg Natural History Society to protect the land from development. In 1994, the quarry became Penn Dixie Fossil Park & Nature Reserve when they received 501(c)3 status and was opened for visitation and collection of trilobite samples. The two most common found samples are ‘’Eldredgeops rana'' and ''Greenops''.

A famous location for trilobite fossils in the United Kingdom is Wren's Nest, Dudley, in the West Midlands, where ''Calymene blumenbachii'' is found in the Silurian Wenlock Group. This trilobite is featured on the town's coat of arms and was named the ''Dudley Bug'' or ''Dudley Locust'' by quarrymen who once worked the now abandoned limestone quarries. Llandrindod Wells, Powys, Wales, is another famous trilobite location. The well-known '' Elrathia kingi'' trilobite is found in abundance in the Cambrian Wheeler Shale of Utah.

Spectacularly preserved trilobite fossils, often showing soft body parts (legs, gills, antennae, etc.) have been found in British Columbia, Canada (the Cambrian Burgess Shale and similar localities); New York, U.S.A. (Ordovician Walcott–Rust quarry, near Russia, and Beecher's Trilobite Bed); China (Lower Cambrian Maotianshan Shales near Chengjiang); Germany (the Devonian Hunsrück Slates near Bundenbach) and, much more rarely, in trilobite-bearing strata in Utah (Wheeler Shale and other formations), Ontario, and Manuels River, Newfoundland and Labrador.

Sites in Morocco also yield very well-preserved trilobites, many buried in mudslides alive and so perfectly preserved. An industry has developed around their recovery, leading to controversies about practices in restoral. The variety of eye and upper body forms and fragile protuberances is best known from these samples preserved similarly to bodies in Pompeii.

The French palaeontologist Joachim Barrande (1799–1883) carried out his landmark study of trilobites in the Cambrian, Ordovician and Silurian of Bohemia, publishing the first volume of ''Système silurien du centre de la Bohême'' in 1852.

Importance

The study of Paleozoic trilobites in the Welsh-English borders by Niles Eldredge was fundamental in formulating and testing punctuated equilibrium as a mechanism of evolution. Reprinted in

Identification of the 'Atlantic' and 'Pacific' trilobite faunas in North America and Europe implied the closure of the Iapetus Ocean (producing the Iapetus suture), thus providing important supporting evidence for the theory of continental drift.

Trilobites have been important in estimating the rate of speciation during the period known as the Cambrian explosion because they are the most diverse group of metazoans known from the fossil record of the early Cambrian.

Trilobites are excellent stratigraphic markers of the Cambrian period: researchers who find trilobites with alimentary prosopon, and a micropygium, have found Early Cambrian strata. Most of the Cambrian stratigraphy is based on the use of trilobite marker fossils.

Trilobites are the state fossils of Ohio (''Isotelus''), Wisconsin (''Calymene celebra'') and Pennsylvania (''Phacops rana'').

Taxonomy

The 10 most commonly recognized trilobite orders are Agnostida, Redlichiida, Corynexochida, Lichida, Odontopleurida, Phacopida, Proetida, Asaphida, Harpetida and Ptychopariida. In 2020, an 11th order, Trinucleida, was proposed to be elevated out of the asaphid superfamily Trinucleioidea. Sometimes the Nektaspida are considered trilobites, but these lack a calcified exoskeleton and eyes. Some scholars have proposed that the order Agnostida is polyphyletic, with the suborder Agnostina representing non-trilobite arthropods unrelated to the suborder Eodiscina. Under this hypothesis, Eodiscina would be elevated to a new order, Eodiscida.

Over 20,000 species of trilobite have been described.

Despite their rich fossil record with thousands of described genera found throughout the world, the taxonomy and phylogeny of trilobites have many uncertainties. Except possibly for the members of the orders Phacopida and Lichida (which first appear during the early Ordovician), nine of the eleven trilobite orders appear prior to the end of the Cambrian. Most scientists believe that order Redlichiida, more specifically its suborder Redlichiina, contains a common ancestor of all other orders, with the possible exception of the Agnostina. While many potential phylogenies are found in the literature, most have suborder Redlichiina giving rise to orders Corynexochida and Ptychopariida during the Lower Cambrian, and the Lichida descending from either the Redlichiida or Corynexochida in the Middle Cambrian. Order Ptychopariida is the most problematic order for trilobite classification. In the 1959 Treatise on Invertebrate Paleontology, what are now members of orders Ptychopariida, Asaphida, Proetida and Harpetida were grouped together as order Ptychopariida; subclass Librostoma was erected in 1990 to encompass all of these orders, based on their shared ancestral character of a natant (unattached) hypostome. The most recently recognized of the nine trilobite orders, Harpetida, was erected in 2002. The progenitor of order Phacopida is unclear.

Relationship to other taxa

Once soft-part anatomy had been recovered, the trilobites were originally allied to the Crustacea. A relationship with the Chelicerata, in a clade termed Arachnomorpha (Arachnata), was in vogue for some time, but, a more recent analysis of Panarthropoda suggests the trilobites are one of two major branches of Artiopoda.

Morphology

When trilobites are found, only the exoskeleton is preserved (often in an incomplete state) in all but a handful of locations. A few locations (') preserve identifiable soft body parts (legs, gills, musculature & digestive tract) and enigmatic traces of other structures (e.g. fine details of eye structure) as well as the exoskeleton.

Trilobites range in length from minute (less than ) to very large (over ), with an average size range of . Supposedly the smallest species is '' Acanthopleurella stipulae'' with a maximum of . The world's largest-known trilobite specimen, assigned to '' Isotelus rex'' is in length. It was found in 1998 by Canadian scientists in Ordovician rocks on the shores of Hudson Bay. However, a partial specimen of the Ordovician trilobite '' Hungioides bohemicus'' found in 2009 in Arouca, Portugal is estimated to have measured when complete in length.

The exoskeleton is composed of calcite and calcium phosphate minerals in a lattice of chitin that covers the upper surface (dorsal) of the trilobite and curled round the lower edge to produce a small fringe called the "doublure".Three distinctive '' tagmata'' (sections) are present: cephalon (head); thorax (body) and pygidium (tail).

Terminology

As might be expected for a group of animals comprising genera, the morphology and description of trilobites can be complex. Despite morphological complexity and an unclear position within higher classifications, there are a number of characteristics which distinguish the trilobites from other arthropods: a generally sub-elliptical, dorsal, chitinous exoskeleton divided longitudinally into three distinct lobes (from which the group gets its name); having a distinct, relatively large head shield (cephalon) articulating axially with a thorax comprising articulated transverse segments, the hindmost of which are almost invariably fused to form a tail shield (pygidium). When describing differences between trilobite taxa, the presence, size, and shape of the cephalic features are often mentioned.

During moulting, the exoskeleton generally splits between the head and thorax, which is why so many trilobite fossils are missing one or the other. In most groups facial sutures on the cephalon helped facilitate moulting. Similar to lobsters and crabs, trilobites would have physically "grown" between the moult stage and the hardening of the new exoskeleton.

Cephalon

A trilobite's cephalon, or head section, is highly variable with a lot of morphological complexity. The glabella forms a dome underneath which sat the "crop" or "stomach". Generally the exoskeleton has few distinguishing ventral features, but the cephalon often preserves muscle attachment scars and occasionally the hypostome, a small rigid plate comparable to the ventral plate in other arthropods. A toothless mouth and stomach sat upon the hypostome with the mouth facing backward at the rear edge of the hypostome.

Hypostome morphology is highly variable; sometimes supported by an un-mineralised membrane (natant), sometimes fused onto the anterior doublure with an outline very similar to the glabella above (conterminant) or fused to the anterior doublure with an outline significantly different from the glabella (impendent). Many variations in shape and placement of the hypostome have been described. The size of the glabella and the lateral fringe of the cephalon, together with hypostome variation, have been linked to different lifestyles, diets and specific ecological niches.

The anterior and lateral fringe of the cephalon is greatly enlarged in the Harpetida, in other species a bulge in the pre-glabellar area is preserved that suggests a brood pouch. Highly complex compound eyes are another obvious feature of the cephalon.

Facial sutures

Facial or cephalic sutures are the natural fracture lines in the cephalon of trilobites. Their function was to assist the trilobite in shedding its old exoskeleton during ecdysis (or molting).

All species assigned to the suborder Olenellina, that became extinct at the very end of the Early Cambrian (like '' Fallotaspis'', ''Nevadia'', '' Judomia'', and ''Olenellus'') lacked facial sutures. They are believed to have never developed facial sutures, having pre-dated their evolution. Because of this (along with other primitive characteristics), they are thought to be the earliest ancestors of later trilobites.

Some other later trilobites also lost facial sutures secondarily. The type of sutures found in different species are used extensively in the taxonomy and phylogeny of trilobites.

=Dorsal sutures

=
The dorsal surface of the trilobite cephalon (the frontmost tagma, or the 'head') can be divided into two regions—the cranidium and the librigena ("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.

Trilobite facial sutures on the dorsal side can be roughly divided into five main types according to where the sutures end relative to the genal angle (the edges where the side and rear margins of the cephalon converge).
*Absent – Facial sutures are lacking in the Olenellina. This is considered a primitive state, and is always combined with the presence of eyes.
*Proparian – The facial suture ends in front of the genal angle, along the lateral margin. Example genera showing this type of suture include ''Dalmanites'' of Phacopina (Phacopida) and ''Ekwipagetia'' of Eodiscina ( Agnostida).
*Gonatoparian – The facial suture ends at the tip of the genal angle. Example genera showing this type of suture include ''Calymene'' and ''Trimerus'' of Calymenina (Phacopida).
*Opisthoparian – The facial suture ends at the posterior margin of the cephalon. Example genera showing this type of suture include ''Peltura'' of Olenina (Ptychopariida) and '' Bumastus'' of Illaenina (Corynexochida). This is the most common type of facial suture.
* Hypoparian or marginal – In some trilobites, dorsal sutures may be secondarily lost. Several exemplary time series of species show the "migration" of the dorsal suture until it coincides with the margins of the cephalon. As the visual surface of the eye is on the diminishing free cheek (or librigena), the number of lenses tends to go down, and eventually the eye disappears. The loss of dorsal sutures may arise from the proparian state, such as in some Eodiscina like '' Weymouthia'', all Agnostina, and some Phacopina such as ''Ductina''. The marginal sutures exhibited by the harpetids and trinucleioids, are derived from opisthoparian sutures. On the other hand, blindness is not always accompanied by the loss of facial sutures.

The primitive state of the dorsal sutures is proparian. Opisthoparian sutures have developed several times independently. There are no examples of proparian sutures developing in taxa with opisthoparian ancestry. Trilobites that exhibit opisthoparian sutures as adults commonly have proparian sutures as instars (known exceptions being '' Yunnanocephalus'' and '' Duyunaspis''). Hypoparian sutures have also arisen independently in several groups of trilobites.

The course of the facial sutures from the front of the visual surface varies at least as strongly as it does in the rear, but the lack of a clear reference point similar to the genal angle makes it difficult to categorize. One of the more pronounced states is that the front of the facial sutures do not cut the lateral or frontal border on its own, but coincide in front of the glabella, and cut the frontal border at the midline. This is, inter alia, the case in the Asaphida. Even more pronounced is the situation that the frontal branches of the facial sutures end in each other, resulting in yoked free cheeks. This is known in ''Triarthrus'', and in the Phacopidae, but in that family the facial sutures are not functional, as can be concluded from the fact that free cheeks are not found separated from the cranidium.

There are also two types of sutures in the dorsal surface connected to the compound eyes of trilobites. They are:
*Ocular sutures – are sutures surrounding the edges of the compound eye. Trilobites with these sutures lose the entire surface of the eyes when molting. It is common among Cambrian trilobites.
*Palpebral sutures – are sutures which form part of the dorsal facial suture running along the top edges of the compound eye.

=Ventral sutures

=

Dorsal facial sutures continue downward to the ventral side of the cephalon where they become the Connective sutures that divide the doublure. The following are the types of ventral sutures.

*Connective sutures – are the sutures that continue from the facial sutures past the front margin of the cephalon.
*Rostral suture – is only present when the trilobite possesses a rostrum (or rostral plate). It connects the rostrum to the front part of the dorsal cranidium.
*Hypostomal suture – separates the hypostome from the doublure when the hypostome is of the attached type. It is absent when the hypostome is free-floating (i.e. natant). it is also absent in some coterminant hypostomes where the hypostome is fused to the doublure.
*Median suture – exhibited by asaphid trilobites, they are formed 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.

Rostrum

The rostrum (or the rostral plate) is a distinct part of the doublure located at the front of the cephalon. It is separated from the rest of the doublure by the rostral suture.

During molting in trilobites like ''Paradoxides'', the rostrum is used to anchor the front part of the trilobite as the cranidium separates from the librigena. The opening created by the arching of the body provides an exit for the molting trilobite.

It is absent in some trilobites like ''Lachnostoma''.

Hypostome

The hypostome is the hard mouthpart of the trilobite found on the ventral side of the cephalon typically below the glabella. The hypostome can be classified into three types based on whether they are permanently attached to the rostrum or not and whether they are aligned to the anterior dorsal tip of the glabella.

* Natant – Hypostome not attached to doublure. Aligned with front edge of glabella.
* Conterminant – Hypostome attached to rostral plate of doublure. Aligned with front edge of glabella.
* Impendent – Hypostome attached to rostral plate but not aligned to glabella.

Thorax

The thorax is a series of articulated segments that lie between the cephalon and pygidium. The number of segments varies between 2 and 103 with most species in the 2 to 16 range.

Each segment consists of the central axial ring and the outer pleurae, which protected the limbs and gills. The pleurae are sometimes abbreviated or extended to form long spines. Apodemes are bulbous projections on the ventral surface of the exoskeleton to which most leg muscles attached, although some leg muscles attached directly to the exoskeleton. Determining a junction between thorax and pygidium can be difficult and many segment counts suffer from this problem.

Volvation

Trilobite fossils are often found "enrolled" (curled up) like modern pill bugs for protection; evidence suggests enrollment ("volvation") helped protect against the inherent weakness of the arthropod cuticle that was exploited by anomalocarid predators.

Some trilobites achieved a fully closed capsule (e.g. ''Phacops''), while others with long pleural spines (e.g. ''Selenopeltis'') left a gap at the sides or those with a small pygidium (e.g. ''Paradoxides'') left a gap between the cephalon and pygidium. In ''Phacops'', the pleurae overlap a smooth bevel (facet) allowing a close seal with the doublure. The doublure carries a Panderian notch or protuberance on each segment to prevent over rotation and achieve a good seal. Even in an agnostid, with only 2 articulating thoracic segments, the process of enrollment required a complex musculature to contract the exoskeleton and return to the flat condition.

Pygidium

The pygidium is formed from a number of segments and the telson fused together. Segments in the pygidium are similar to the thoracic segments (bearing biramous limbs) but are not articulated. Trilobites can be described based on the pygidium being micropygous (pygidium smaller than cephalon), subisopygous (pygidium sub equal to cephalon), isopygous (pygidium equal in size to cephalon), or macropygous (pygidium larger than cephalon).

Prosopon (surface sculpture)

Trilobite exoskeletons show a variety of small-scale structures collectively called prosopon. Prosopon does not include large scale extensions of the cuticle (e.g. hollow pleural spines) but to finer scale features, such as ribbing, domes, pustules, pitting, ridging and perforations. The exact purpose of the prosopon is not resolved but suggestions include structural strengthening, sensory pits or hairs, preventing predator attacks and maintaining aeration while enrolled. In one example, alimentary ridge networks (easily visible in Cambrian trilobites) might have been either digestive or respiratory tubes in the cephalon and other regions.

Spines

Some trilobites such as those of the order Lichida evolved elaborate spiny forms, from the Ordovician until the end of the Devonian period. Examples of these specimens have been found in the Hamar Laghdad Formation of Alnif in Morocco. There is a serious counterfeiting and fakery problem with much of the Moroccan material that is offered commercially. Spectacular spined trilobites have also been found in western Russia; Oklahoma, USA; and Ontario, Canada.

Some trilobites had horns on their heads similar to several modern beetles. Based on the size, location, and shape of the horns it has been suggested that these horns may have been used to combat for mates. Horns were widespread in the Raphiophoridae family (Asaphida).
Another function of these spines was protection from predators. When enrolled, trilobite spines offered additional protection.
This conclusion is likely to be applicable to other trilobites as well, such as in the Phacopid trilobite genus ''Walliserops,'' that developed spectacular tridents.

Soft body parts

Only 21 or so species are described from which soft body parts are preserved, so some features (e.g. the posterior antenniform cerci preserved only in '' Olenoides serratus'') remain difficult to assess in the wider picture.

Appendages

Trilobites had a single pair of preoral antennae and otherwise undifferentiated biramous limbs (2, 3 or 4 cephalic pairs, followed by one pair per thoracic segment and some pygidium pairs). Each endopodite (walking leg) had 6 or 7 segments, homologous to other early arthropods. Endopodites are attached to the coxa, which also bore a feather-like exopodite, or gill branch, which was used for respiration and, in some species, swimming. A 2021 study found that the upper limb branch of trilobites is a "well-developed gill" that oxygenates the hemolymph, comparable to the book gill in modern horseshoe crab ''Limulus''. In '' Olenoides'', the partially articulated junction with the body is distinct from the exopods of Chelicerata or Crustacea. The inside of the coxa (or gnathobase) carries spines, probably to process prey items. The last exopodite segment usually had claws or spines. Many examples of hairs on the legs suggest adaptations for feeding (as for the gnathobases) or sensory organs to help with walking.

Digestive tract

The toothless mouth of trilobites was situated on the rear edge of the hypostome (facing backward), in front of the legs attached to the cephalon. The mouth is linked by a small esophagus to the stomach that lay forward of the mouth, below the glabella. The "intestine" led backward from there to the pygidium. The "feeding limbs" attached to the cephalon are thought to have fed food into the mouth, possibly "slicing" the food on the hypostome and/or gnathobases first. Alternative lifestyles are suggested, with the cephalic legs used to disturb the sediment to make food available. A large glabella, (implying a large stomach), coupled with an impendent hypostome has been used as evidence of more complex food sources, i.e. possibly a carnivorous lifestyle.

Internal organs

While there is direct and implied evidence for the presence and location of the mouth, stomach and digestive tract (see above) the presence of heart, brain and liver are only implied (although "present" in many reconstructions) with little direct geological evidence.

Musculature

Although rarely preserved, long lateral muscles extended from the cephalon to midway down the pygidium, attaching to the axial rings allowing enrollment while separate muscles on the legs tucked them out of the way.

Sensory organs

Many trilobites had complex eyes; they also had a pair of antennae. Some trilobites were blind, probably living too deep in the sea for light to reach them. As such, they became secondarily blind in this branch of trilobite evolution. Other trilobites (e.g., ''Phacops rana'' and '' Erbenochile erbeni'') had large eyes that were for use in well lit, predator-filled waters.

Antennae

The pair of antennae suspected in most trilobites (and preserved in a few examples) were highly flexible to allow them to be retracted when the trilobite was enrolled. One species ('' Olenoides serratus'') preserves antenna-like cerci, which project from the rear of the trilobite.

Eyes

Even the earliest trilobites had complex, compound eyes with lenses made of calcite (a characteristic of all trilobite eyes), confirming that the eyes of arthropods and probably other animals could have developed before the Cambrian. Improving eyesight of both predator and prey in marine environments has been suggested as one of the evolutionary pressures furthering an apparent rapid development of new life forms during what is known as the Cambrian explosion.

Trilobite eyes were typically compound, with each lens being an elongated prism. The number of lenses in such an eye varied: some trilobites had only one, while some had thousands of lenses in a single eye. In compound eyes, the lenses were typically arranged hexagonally. The fossil record of trilobite eyes is complete enough that their evolution can be studied through time, which compensates to some extent for the lack of preservation of soft internal parts.

Lenses of trilobites' eyes were made of calcite ( calcium carbonate, CaCO₃). Pure forms of calcite are transparent, and some trilobites used crystallographically oriented, clear calcite crystals to form each lens of each eye. Rigid calcite lenses would have been unable to accommodate to a change of focus like the soft lens in a human eye would; in some trilobites, the calcite formed an internal doublet