An Agnathan, Cephalaspis magnifica, from Spittal Quarry
The Agnatha or jawless fishes consist of two distinct groups: the Cephalspidomorphs or Monorhina and the Pteraspidomorphs or Diplorhina. The former includes both groups of living Agnathans, the Lampreys and Hagfishes as well as the diverse armoured Palaeozoic forms (Moy-Thomas & Miles, 1971).
Cephalaspis magnifica (Traquair,1893)
The outer layer of the shield is, unfortunately, missing over much of the surface, but the ornamentation is visible here and there. It consists of minute pustules which are larger and more prominent round the edges of the shield and the orbits. A faint oval can be seen which probably indicates the limit of the ventral mozaic of bones which contain the mouth and gill openings.
The Cephalaspids have mozaics of bones on the dorsal surface of the head shield which are referred to as the dorsal field located behind the orbits and two lateral fields located near the edge of the shield. The dorsal field shape is preserved in the specimen, but the lateral fields have not been preserved. The purpose of these fields is unknown but it is assumed that they act like the lateral line on the fishes, i.e. they are sensory organs. The ventral mozaic may also have a similar function.
In reconstructing the dorsal view I have assumed that the lateral fields resemble those of other Cephalaspids and the openings between the orbits are also similar. In the ventral view the size and position of the mouth and gill openings are assumed to be like those of other Cephalaspids. I have also indicated the pectoral fins and part of the body shape.
Since the head shield of the Cephalapids has no sutures it is a matter of speculation that the shield only ossified from a probably leathery condition when it ceased to grow. No other speculation, such as moulting, seems reasonable.
Mode of Life