All trinucleoids have asaphoid protaspids (where known) and fused cheeks in holaspids (adult growth stages) but this is an advanced feature. Chatterton et al. (1994) described specimens with functional sutures (that is, cheeks that were not fused) and a small rostral plate
in the last protaspid stages. An important part of the Trinucleoidea is
the Family Trinucleidae. This is a very distinctive family for two
reasons - first they have a characteristic pitted cephalic fringe and
secondly they are blind (as are all trinucleoids).
fringe is an expandeed area around the margin of the cephalon with
several arcs of perforations. There are two lamellae or layers to the
fringe, separated by a suture. Each lamella is indented by these pits
which are sealed above and below the suture by a disc (Clarkson 1998).
What the fringe was for is not entirely clear. Possibly it was at least
partly for detecting current so the animal could orientate itself in
its burrow to avoid being tipped over by the current (Clarkson 1998).
The earliest trinucleid, Orometopus,
had eyes and facial sutures but no pitted fringe, a situation which was
reversed during trinucleid evolution. It has been suggested that the
fringe took over from the eyes as the main sensory organ due to the
animals living in deep water where there was little if any light
(Whittington in Kaesler 1997).
Trinucleus fimbriatus, Family Trinucleidae (photo by author)
Author: Laurence Dale
Last updated: 18/11/06
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