Facial suture (trilobite anatomy)
Encyclopedia
Facial sutures are sutures in the cephalon
(head) of most trilobite
s, along which the exoskeleton
splits during molting
.
(or molting).
Primitive trilobites from the Early Cambrian
(like Fallotaspis
, Eofallotaspis, Schmidtiellus, and Profallotaspis
) lacked facial sutures. They are all classified under the suborder Olenellina
. 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.
, 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 three main types according to where the sutures end relative to the gena
l angle (the edges where the side and rear margins of the cephalon converge).
In some trilobites the sutures may be difficult to see as they run along the margins of the cephalon. This is considered a fourth type of suture known as Marginal (or Hypoparian sutures). In this type, the sutures run mostly or wholly along the margin of the cephalon. They are not considered to be a formal phylogenetic grouping like the previous groups as they have independently developed in several groups of trilobites. Other types of sutures became more marginal when trilobite groups acquire secondary blindnesss (a “devolution” of visual organs in some trilobites). The marginal sutures exhibited by the harpetids
and trinucleioids
, for example, are derived from opisthoparian sutures. Example genera from both groups are Harpes and Cryptolithus, both of which are blind.
There are also two types of sutures in the dorsal surface connected to the compound eye. They are:
that extends into the ventral side of the trilobite). The following are the types of ventral sutures.
Cephalon (arthropod head)
The cephalon is the head section of an arthropod. It is a tagma, i.e., a specialized grouping of arthropod segmentsThe word cephalon derives from the Greek κεφαλή , meaning "head". It is rather used in Trilobite terminology. In insects, head is a preferred term...
(head) of most trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. 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 era before...
s, along which the exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
splits during molting
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
.
Description
Facial or Cephalic sutures are the natural fracture lines in the cephalon of trilobites. Their function is to assist the trilobite in shedding its old exoskeleton during ecdysisEcdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
(or molting).
Primitive trilobites from the Early Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
(like Fallotaspis
Fallotaspis
Fallotaspis is a Cambrian trilobite genus found in the United States and Morocco.-Sources:***...
, Eofallotaspis, Schmidtiellus, and Profallotaspis
Profallotaspis
Profallotaspis is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the late Atdabanian stage, which lasted from 530 to 524 million years ago during the early part of the Cambrian Period....
) lacked facial sutures. They are all classified under the suborder Olenellina
Olenellina
Olenellina is a suborder of the order Redlichiida of Trilobites. The suborder contains two superfamilies: Olenelloidea , and Fallotaspidoidea . The Olenillina are some of the most primitive trilobites...
. 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
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
and phylogeny of trilobites.
Dorsal sutures
The dorsal surface of the trilobite cephalon (the frontmost tagmaTagma
A tagma may refer to:* Tagma , a grouping of segments, usually in arthropod anatomy* tagma , a subdivision of the Byzantine army...
, 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
Glabella
The glabella, in humans, is the space between the eyebrows and above the nose. It is slightly elevated, and joins the two superciliary ridges.-Etymology:The term is derived from the Latin glabellus, meaning smooth, as this area is usually hairless....
(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 three main types according to where the sutures end relative to the gena
GENA
GENA stands for General Event Notification Architecture.GENA Base defines an HTTP notification architecture that transmits notifications between HTTP resources. An HTTP resource could be any object which might need to send or receive a notification, for example a distribution list, buddy list,...
l angle (the edges where the side and rear margins of the cephalon converge).
- Proparian - The facial suture ends ahead of the genal angle, along the lateral margin. Example genera showing this type of suture include DalmanitesDalmanitesDalmanites is a genus of trilobite in the order Phacopida.The trilobites of this genus have slightly convex exoskeletons with an average diameter of 4–7 cm . The cephalon is semicircular or parabolic. The glabella is often pear-shaped, and tapers outward toward the front...
of PhacopinaPhacopinaPhacopina is a suborder including many trilobites.-External links:* - various photos, some of Phacopina...
(PhacopidaPhacopidaPhacopida is an order of trilobite that lived from the Ordovician to the Devonian. It is made up of a morphologically diverse group of related suborders....
) and Ekwipagetia of Eodiscina (AgnostidaAgnostidaAgnostida is an order of arthropod which first developed near the end of the Early Cambrian period and thrived during the Middle Cambrian. They are present in the lower Cambrian fossil record along with trilobites from the Redlichiida, Corynexochida, and Ptychopariida orders...
). - Gonatoparian - The facial suture ends at the tip of the genal angle. It is also known as Marginal or Hypoparian. Example genera showing this type of suture include CalymeneCalymeneCalymene is a genus of trilobites in the order Phacopida that were found throughout North America, North Africa, and Europe in primarily Silurian outcrops. Calymene are easily confused with Gravicalymene and Flexicalymene, also part of the Calymenidae family...
and Trimerus of CalymeninaCalymeninaCalymenina is a suborder of the trilobite order Phacopida....
(PhacopidaPhacopidaPhacopida is an order of trilobite that lived from the Ordovician to the Devonian. It is made up of a morphologically diverse group of related suborders....
). - Opisthoparian - The facial suture ends at the posterior margin of the cephalon. Example genera showing this type of suture include Peltura of Olenina (PtychopariidaPtychopariidaPtychopariida is a large, heterogeneous order of trilobite containing some of the most primitive species known. Many date to the Early Cambrian Period, but the order was extant through the Late Ordovician...
) and BumastusBumastusBumastus is an extinct genus of trilobites which existed from the Early Ordovician period to the Late Silurian period. They were relatively large trilobites, reaching a length of . They were distinctive for their highly globular, smooth-surfaced exoskeleton...
of Illaenina (CorynexochidaCorynexochidaCorynexochida is an order of trilobite that lived from the Lower Cambrian to the Middle Devonian. Like many of the other trilobite orders, Corynexochida contains many species with widespread characteristics....
). This is the most common type of facial suture.
In some trilobites the sutures may be difficult to see as they run along the margins of the cephalon. This is considered a fourth type of suture known as Marginal (or Hypoparian sutures). In this type, the sutures run mostly or wholly along the margin of the cephalon. They are not considered to be a formal phylogenetic grouping like the previous groups as they have independently developed in several groups of trilobites. Other types of sutures became more marginal when trilobite groups acquire secondary blindnesss (a “devolution” of visual organs in some trilobites). The marginal sutures exhibited by the harpetids
Harpetida
Harpetida is one of the nine orders of the extinct arthropod class Trilobita. They lived from the Upper Cambrian to the Late Devonian period....
and trinucleioids
Asaphida
Asaphida is a large, morphologically diverse order of trilobite found in strata dated from the Middle Cambrian to the Silurian. The order contains six superfamilies , but no suborders...
, for example, are derived from opisthoparian sutures. Example genera from both groups are Harpes and Cryptolithus, both of which are blind.
There are also two types of sutures in the dorsal surface connected to the compound eye. They are:
- Ocular suture - are sutures surrounding the edges of the compound eye. Trilobites with these sutures lose the entrie surface of the eyes when molting. It is common among Cambrian trilobites.
- Palpebral suture - 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 parts of the exoskeletonExoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
that extends into the ventral side of the trilobite). 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 hpostomes where the hypostome is fused to the doublure.
- Median suture - exhibited by asaphidAsaphidaAsaphida is a large, morphologically diverse order of trilobite found in strata dated from the Middle Cambrian to the Silurian. The order contains six superfamilies , but no suborders...
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.