Caithness Field Club

An Agnathan, Cephalaspis magnifica, from Spittal Quarry
by Jack Saxon

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).

The Cephalaspidomorphs
The Cephalaspidomorphs range from the Upper Silurian to the Upper Devonian and are the best-known of all fossil Agnathans. They are usually rather small in size. Their apparent absence from the Orcadian Old Red Sandstone was a matter of some speculation until Cephalaspis magnifica (Traquair, 1893) was found at Sittal Quarry in Caithness. The only text figure known is a lithograph of the actual specimen in Traquair’s paper. The specimen itself is very fragile and is housed in the National Museum of Scotland, Chambers Street, Edinburgh.

Cephalaspis magnifica (Traquair,1893)
By comparison with all the other Cephalaspids the Spittal specimen is enormous; it must have been about 600 mm in length. Being crushed flat, the contour of the head shield must appear broader than it would have been in life. The left cornua at the rear of the shield is complete but the right one is broken. If it had been complete the head shield would have measured 300 mm in breadth. The edges of the shield are not very well defined, and, behind it, there are some obscure remains of scales. The snout is blunt, the left cornua is short and broad and there are no denticles along the inner margin. The orbits are oval and spaced well apart. Neither the pineal nor the nasohypophysial opening have been preserved.

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
It is probable that it foraged for food on the bottom of the lake. Whether it was parasitic like its extant cousins is impossible to know, but its large size would suggest that it had a healthy appetite. It was probably not fast enough to be a normal predator, but would have to rely on other means of feeding, such as filter feeding. Perhaps these fields served a rather more active purpose such as stunning its prey, like the electric eel.

Moy-Thomas, J A and Miles, R.S. Palaeozoic Fishes, 2nd Ed. Chapman and Hall, 1971.
Traquair, R. H., On Cephalaspis magnifica from the Caithness Flagstones

Published in 2000 Bulletin