Ventral median suture
The main features of the trilobite cephalon include the bulbous, axial
structure known as the glabella and the lateral areas, the cheeks. The
cheek regions may be divided by facial sutures into free cheeks and fixed cheeks. Around its margin, the trilobite exoskeleton folds
under the ventral side of the animal in a region called the doublure.
The ventral median suture is the place where the doublures of the free
cheeks join and an important defining feature of the Asaphida. However
it may be lost in some groups where the free cheeks have fused such as
Trinucleacea and Nileidae (Fortey and Chatterton 1988).
The suture may have appeared either by a single mutation or by reduction and loss of an intervening rostral plate. Fortey and Chatterton (1988) favoured the former option as no rostral plate was then known from any asaphoid protaspis (see below). However Chatterton et al. (1994) described trinucleoid larvae with a small triangular rostral plate.
This is a recurrent form of early growth stage among several
families of the Asaphida - Asaphidae, Nileidae, Remopleurididae and
Trinucleidae. They are rounded in outline with a doublure that is
enrolled rather than inturned. They have spines on the fused
cranidium/protopygidium as well as on the hypostome. During their
growth dorsal furrows may become more distinct and the largest asaphoid
protaspids are large by trilobite larval standards, exceeding 1mm in
length (Fortey and Chatterton 1988).
This is a feature of the more adavanced members of the Asaphida (Fortey and Chatterton 1988). It was decribed in Nileus armadillo by Fortey and Clarkson (1976)
as being a thinning of the exoskeleton on the inside, the outer surface
being completely smooth. They found that its location would complement
the visual fields of the animal's eyes and so a visual function was
suggested, although this tubercle was surely less effective than the
eyes, perhaps only being light/dark sensitive.
However there are few other examples of tubercles with similar structure. For instance Ogygiocaris, an asaphid, has a node on the extenal surface, which bears a number of symmetrically placed pits.
Author: Laurence Dale
Last updated: 18/11/06
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