Glossary of Trilobite Terms
last revised 01 January 2008 by S. M. Gon III

Highlighted terms below are linked to related images which you can also access directly here:
1.  Trilobite dorsal morphology (the majority of trilobite features are dorsal)
2.  Trilobite ventral morphology (e.g., doublure, rostral plate, hypostome)
3.  Major trilobite features (e.g., cephalon, cranidium, palpebral area)
4.  Pygidium/cephalon size relationships (e.g., micropygous, isopygous, macropygous).
5.  Facial suture types (e.g., proparian, gonatoparian, opisthoparian).
6.  Hypostome conditions (e.g., natant, conterminant, impendent).
7.  Trilobite enrollment (e.g., vincular furrows, coaptation).
8.  Trilobite ontogeny (e.g., protaspis, meraspis, holaspis, anamorphic, epimorphic).
9.  Trilobite eyes (e.g., abathochroal, holochroal, schizochroal).
10.Trilobite prosopon (e.g., granulate, tuberculate, terrace ridges)

abathochroal eyes with compound lenses, each with separate cornea, each lens separated from others by shallow scleral walls; corneal membrane ends at lens edge. see holochroal, schizochroal.
abaxial away from the axis (center line)
adaxial toward the axis (center line).
anamorphic a mode of development in which each molt results in addition of one or few segments. see epimorphic, hemianamorphic.
articulation separation of two rigid tergites (rigid dorsal units) by a flexible joint.
articulating facets see facets.
axial pertaining to the axis.
axis central lobe running the length of the trilobite body; the glabella is the main expression of the cephalic axis.
cephalic pertaining to the cephalon.
cephalon frontmost trilobite tagma (functional set of segments); head.
cheek see gena.
coaptation close interlocking of opposing surface during enrollment
conterminant hypostome attached to anterior doublure via suture. see impendent, natant.
dorsal referring to the top or back surface, as opposed to ventral (underside).
doublure shell continuation under the trilobite, typically as a ventral cephalic, thoracic, and/or pygidial rim.
effacement loss of detail; effacement is an evolutionary trend in trilobites.
enrollment bending of the body to enclose vulnerable ventral organs in a protective clam-like or sphereoid capsule formed by the cephalon, pygidium and thoracic segments. 
epimorphic a mode of development in which molts are not accompanied by segment addition. see anamorphic, hemianamorphic.
facets flat surfaces, typically where two adjoining body parts meet and articulate (move); e.g., articulating facets on thoracic pleural segments.
fixigena "fixed cheek" cephalic shell surrounding glabella inside of facial sutures. see gena, librigena.
fulcrate referring to thoracic segments with a geniculation (bend) separating a horizontal adaxial portion from an inclined abaxial portion.
furrow linear groove, often defining adjacent convex features, such as lobes of the glabella
gena (pl., genae) shell of the cephalon surrounding the glabella, (also called cheeks) usually divided by sutures into fixigena (fixed cheek) and librigena (free cheek).
genal angle angle formed by the meeting of the lateral (side) and posterior (rear) margins of the cephalon; sometimes expressed as a genal spine. 
genal spine a spine anywhere on the gena (cheek) of the cephalon, typically at the genal angle, but see metagenal spine, progenal spine.
glabella middle (axial) portion of cephalon, typically convex and lobed.
gonatoparian facial sutures ending at the tip of the genal angle.
granulate bearing granules or similar sculpturing (prosopon) on the outer exoskeleton.
hemianamorphic typical mode of development in trilobites, in which early stages (protaspis, meraspis) show anamorphic development, while later development (holaspis) is epimorphic.
holaspis final developmental stages beyond meraspis, attaining a certain number of thoracic segments considered typical of "adult" individuals, and with cephalic and pygidial characters of adult.
holochroal eyes with adjacent compound lenses directly in contact, all covered by a single continuous smooth corneal surface. see abathochroal, schizochroal.
hypertrophy grown well beyond normal size.
hypostomal pertaining to the hypostome.
hypostome a piece of exoskeleton on the venter (underside) of the cephalon, thought of as a mouthpart. Typically the anterior edge of the hypostome lies directly under the anterior edge of the glabella.
impendent type of conterminent hypostomal attachment, but doublure underlies anterior glabellar edge, so anterior hypostomal border does not match anterior glabellar border. see conterminant, natant.
isopygous cephalon and pygidium similar in size. see micropygous, macropygous.
konservat-lagerstatten special category of fossil deposits (fossil-lagerstätten) where soft tissue features have been conserved particularly well (as in the Burgess Shales). see lagerstätten.
lagerstätten geological deposit, especially fossil-lagerstätten (fossil beds), konzentrat-lagerstätten (concentrations of fossil parts) and konservat-lagerstätten (fossil beds with remarkably conserved soft body detail - see above)
librigena "free cheek" cephalic shell to the right and left of the glabella outside of facial sutures, often lost during molting or after death. see fixigena, gena.
macropygous pygidium larger than cephalon. see isopygous, micropygous.
meraspis developmental stage following protaspis, bearing more than one segment, but with fewer than adult number of thoracic segments. see protaspis, holaspis.
metagenal spine genal spine originating along the posterior margin of the cephalon, adaxial of the genal angle, called metafixigenal if it springs from the fixigena, or metalibrigenal if from the librigena.
micropygous pygidium much smaller than cephalon. see isopygous, macropygous.
natant hypostome free, not attached to the cephalic doublure.
occipital pertaining to the occiput.
occiput posterior portion of cephalon, the axial portion is considered part of the glabella (occipital ring).
opisthoparian facial sutures ending along the rear margin of the cephalon adaxial to the genal angle. see gonatoparian, proparian.
opisthothorax posterior, narrower portion of thorax when it is divided into wider, anterior prothorax and opisthothorax.
palpebral portion of fixigena associated with the eye.
perrostral along the edge of the rostrum.
pleura (pl., pleurae) lateral (side) part of thoracic segment or lateral (non axial) part of pygidium.
pleural referring to pleurae.
postaxial extending from the end of the axis. (e.g., a postaxial spine.)
preglabellar field portion of cephalon in front of glabella.
progenal spine genal spine originating anywhere along the margin of the cephalon, anterior of the genal angle, called profixigenal if it springs from the fixigena, or prolibrigenal if from the librigena.
proparian facial sutures ending forward of the genal angle.
prosopon exoskeletal sculpturing, including pits, ridges, furrows, granules, tubercles, and small spines.
protaspis developmental stage lacking articulated segments. see meraspis, holaspis.
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 body section (tagma) composed of fused segments; tail, separated from thoracic segments by an articulation.
pyriform pear-shaped, typically referring to the glabella
rostral plate (rostrum) middle portion of the ventral cephalic doublure, typically where the hypostome attaches in conterminant or impendent trilobites.
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.
spinose bearing a spine or spines.
sternite ventral exoskeleton unit, not calcified in trilobites, separated from other such units by sutures or unsclerotized membrane, typically axial.
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 separate during molting.
tagma (pl. tagmata) grouping of segments that form a functional unit. cephalon, thorax, & pygidium are trilobite tagmata.
tergite a rigid dorsal exoskeletal unit, calcified in trilobites, composed of either single segments (e.g., individual thoracic segments) or fused segments (e.g., the pygidium) separated from other such units by articulations (e.g., thoracic tergites) or sutures (e.g., cephalic genae).
terrace ridge
linear assymetrical ridges running subparallel to the margin on the outer portion of pleural region, border, doublure, and hypostomes of some trilobites; a type of prosopon.
thorax middle, segmented body portion between cephalon and pygidium. see prothorax, opisthothorax
thoracic pertaining to the thorax.
tuberculate bearing tubercles or similar sculpturing on the outer exoskeleton. See prosopon.
ventral referring to the underside. 
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 feature of some Phacopida).

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