Glossary of Trilobite Terminology


Proetid TrilobiteUnderstanding 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 glossary below.

As 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 shield (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 diversity:
  • 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 or splitting.
  • abathochroal eyes: eyes with compound lenses, each with separate cornea, each lens separated from others by shallow scleral walls; the corneal membrane ends at lens edge. also see holochroal eyes, and schizochroal eyes.
  • apodemes: ingrowth to the exoskeleton to which muscles attached.
  • axis or axial lobe: central lobe running the length of the trilobite body (Also see: Trilobite Basic Body Plan)
  • biramous: 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 a series of segments attached end to end. Also see uniramous legs.
  • cephalon: 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.
  • clade: a monophyletic group an ancestor and all of its descendants.
  • cladistics: 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.
  • coaptation: close interlocking of opposing surface during enrollment.
  • conterminant: a condition where the hypostome is attached to the anterior doublure with a suture. Other trilobite hypostome conditions are impendent and natant. (Also see trilobite hypostome and Conterminant and hypostome image)
  • cranidium: the glabella together with the fixigena.
  • dorsal: referring to the top or back surface, as opposed to ventral.
  • doublure: shell continuation under the trilobite, typically as a ventral cephalic and/or pygidial rim.
  • ecdysis: the moulting of the cuticula (exoskeleton) in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, which includes Class Trilobita.
  • effaced: furrows and lobes have lost detail or entirely disappeared yielded a smoother exoskeleton surface, which putatively facilitated burrowing to hide from predators or prey.
  • effacement: 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.
  • enrollment: the bending of the trilobite body, especially to enclose vulnerable ventral organs in a protective wrapper formed by the dorsal exoskeleton of the cephalon, pygidium and thoracic segments. (Also see trilobite thorax)
  • facets: 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.
  • fixigena: the fixed cheek cephalic shell surrounding glabella inside of facial sutures. Also see gena and librigena.
  • free 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 portion.
  • furrow: a groove, often defining adjacent convex features of the exoskeleton surface, e.g., between lobes of the glabella
  • gena: the shell of the cephalon surrounding the glabella that is normally divided by sutures into the fixigena and the librigena.
  • genal 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.
  • genal spine: a spine anywhere on the gena (i.e., cheek) of the cephalon, typically pointing at the genal angle.
  • glabella: 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)
  • gonatoparian: a condition where the facial sutures end at the tip of the genal angle.
  • granulate: granules or granule-like features on the exoskeleton.
  • holaspis: 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.
  • holochroal: 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.
  • hypertrophy: a condition of growth nominal size.
  • hypostomal: adjective meaning related to the hypostome.
  • 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.
  • impendent: 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 natant.
  • isopygous: a condition where the trilobite's cephalon and pygidium have similar size and possible symmetry. Contrast with micropygous and macropygous.
  • konservat-lagerstatte: a place of extraordinary fossil preservation, often including mineralization of soft tissues.
  • librigena: 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 fixigena, gena.
  • macropygous: a trait of the pygidium being larger than the cephalon - contrast with isopygous and micropygous.
  • meraspis: 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 macropygous.
  • monophyletic: a monophyletic taxon forms a clade, meaning that 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 (synapomorphies).
  • natant: hypostome condition where hypostome is free and not attached to the cephalic doublure. (Also see trilobite hypostome and natant and hypostome image)
  • occipital: related to the to the trilobite occiput.
  • occipital spine: spine located at the occiput.
  • 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 cornea.
  • opisthoparian: the facial sutures terminating along the rear margin of the trilobite cephalon adaxial to the genal angle. Also see gonatoparian and proparian.
  • opisthothorax: the posterior, narrower portion of the trilobtes thorax when it is divided into wider, anterior prothorax and opisthothorax.
  • palpebral: that portion of trilobite fixigena associated with the eye.
  • perrostral: along the edge of the rostrum.
  • phylogenetics: 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.
  • pleura: (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)
  • pleural: referring to pleurae.
  • postaxial: extending from the end of the axis.
  • preglabellar field: portion of cephalon in front of 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 and sucking.
  • proparian: 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 in shape, and lacking transeverse articulation. Also see trilobite 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.
  • prothorax: anterior and wider portion of thorax when it is divided into prothorax and narrower, posterior opisthothorax, as seen in some Redlichiida.
  • pygidial: pertaining to the pygidium.
  • pygidium: hindmost segment; tail. See pygidium morophology.
  • pyriform: pear-shaped, typically referring to the glabella
  • rostral plate: (rostrum) middle portion of the ventral cephalic doublure, typically where the hypostome attaches.
  • schizochroal 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.
  • Sensu: a Latin word meaning "in the sense of". It is often seen in the context of a paleontologists view, nomally with the appropriate literature citation.
  • 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
  • spinose: bearing a spine or spines.
  • subisopygous: pygidium large, but not quite as large as cephalon.
  • subparallel: nearly parallel in arrangement, slightly diverging or converging.
  • suture: linear break in exoskeleton, typically along which parts separated during molting.
  • synapomorphy: (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.
  • telson: tail spine fused to posterior of pygidium.
  • tergum: (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.
  • thorax: middle, segmented body portion between cephalon and pygidium. aloso see prothorax, opisthothorax
  • thoracic: pertaining to the thorax.
  • tubercle: a nodule, nominally a small and roundish outgrowth on the trilobite exoskeleton.
  • tuberculate: bearing tubercles or similar sculpturing on the outer exoskeleton.
  • uniramous: 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 to be a shared, derived character, so uniramous arthropods were grouped into a taxon called Uniramia. It is now believed that several groups of arthropods independently evolved uniramous limbs from ancestors with biramous limbs, so this taxon is no longer used. Also see biramous legs.
  • ventral: referring to the underside, as opposed to dorsal.
  • vincular furrow: 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.