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Morphological features

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.

Main features of the cephalon

Asaphoid protaspis

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).

Glabellar tubercle

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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