The facial sutures of Asaphiscus run from the anterior of the cephalon to the eye, then to the back of the cephalon.
|What are facial
They are lines on the cephalon along which the parts of the cephalon separate when the trilobite molts (sheds its exoskeleton -- see animation and discussion below). They typically run from somewhere along the anterior edge of the cephalon, toward and around the edge of the eye, and continue from there to end at either the side or rear of the cephalon. They are vital for proper molting and growth of trilobites, and they provide us with another character with which to help classify different trilobite groups.
There are three main categories of facial suture types (proparian, gonatoparian, and opisthoparian), described below. If you want more detailed definitions of the terms related to facial sutures, they can be found in the on-line glossary of trilobite terms.
The facial sutures determine the boundary between the central cranidium (glabella and fixed cheeks) and the librigenae (free cheeks). Trilobite specimens often lack librigenae. They are lost during molting, and also disarticulate easily following the death of the trilobite.
Here are some examples of the three suture types:
|Ptychopariid sutures diverge slightly from the eyes and run more or less straight to the anterior border.||Cedariiform posterior sutures cross the lateral border, then swing backward and inward to meet the posterior margin.||Kainelliform sutures diverge widely until they cross the anterior border, then swing strongly inward to run along the anterior margin.||Nileiform sutures converge and meet before reaching the anterior margin, running parallel to the anterior border.||Asaphid sutures converge and meet at an apex at or near the anterior margin, then form a median suture|
|Dalminitidiform sutures are proparian, with anterior sutures that converge and meet, closely following the anterior border; also called phacopid pattern.||Burlingiiform sutures are proparian, with anterior and posterior sutures running subparallel to create roughly rectangular fixigenae||Harpid stures are purely marginal, running along the cephalic margins; the eyes are often vestigial and no longer involved with the sutures||Entomaspidiform sutures run mostly along the margin, but involve the eyes via two narrow strips.||Trinucleid sutures are also largely marginal, but may run parallel to the margin on the dorsal cephalic surface.|
images ©2003 by S. M. Gon III
|In Xystridura, a typical redlichioid trilobite, there is a large rostral plate (pink) that involves the anterior and lateral doublure. The hypostome is probably permanently attached to the doublure, so that when the sutures separate, it remains docked to the rostral plate.|
|In Aphelaspis, a typical ptychoparioid trilobite, there is a small rostral plate (pink), and the hypostome is natant, so is never attached to the narrow cephalic doublure, but "floats" beneath the front of the glabella (held in place by muscles and connective tissue to the glabellar apodemes).|
|In Lachnostoma, a typical asaphoid, the free cheeks split evenly along a median ventral suture. There is no rostral plate. There is a hypostomal suture, which allows the hypostome to separate as needed. Notice that the cephalic doublure is extremely wide.|
Molting (Ecdysis) and the Role of Sutures
||Trilobites, as arthropods, grew by shedding their old, rigid exoskeleton in a process called ecdysis, or molting. They underwent this process several times during their development. The animation below depicts a Paradoxides trilobite molting (left is the animal
in top view). In the animation, the facial
sutures (red) split,
opening the cephalon (separating the cranidium
from the librigenae and associated structures). This provides an exit for
the molting trilobite (purple) from its old exoskeleton (orange). The arching of the body plants the rear pleural spines securely
into the substrate and likewise anchors the rostral
plate at the anterior cephalic margin downward,
providing an exit ramp for the emerging newly-molted animal. This sequence
is consistent with fossils of Paradoxides in which the librigenae
and rostral-hypostomal plate are found inverted beneath the axial cranidial
shield. All it would take is slightly more forward
tilting to have the molted librigenal assemblage flip into such an inverted
This animated image ©2000 by S. M. Gon III
Adapted from Whittington, 1990 via the 1997 Treatise (p. 156-7).
|Fossil evidence of the molting sequence|
|In another example of the consequence of molting, the displaced cranidium
and librigenae of the Canadian trilobite Pseudogygites latimarginalis
in the photographs shown below is explained by the reconstruction of the
molting sequence shown under the photographs: The facial sutures separate,
allowing the molting trilobite (purple) an opening through which it emerges
from its old exoskeleton (white).