THE RADIOACTIVE FOSSIL FISHES OF THE NORTH OF SCOTLAND

J. Saxon

ABSTRACT

The bones of Homostius milleri from the Middle Old Red Sandstone of the Orcadian cuvette are highly radioactive. The radioactivity is due to the decay chains of U238 and Th232. The evidence suggests that the uranium and thorium was ingested during life yet no somatic or genetic effects are observed.

INTRODUCTION

In the Devoniain period a large tract of the north of Scotland lying north and east of the crystalline Caledonian highlands was a basin of deposition and thick deposits of sedimentary rock, the Old Red Sandstone, were laid unconformably on the extremely rugged terrain. The combination of a climatic cycle and the last movements of the Caledonian orogenic epoch produced a cyclic sedimentary pattern repeating itself typically every 15 metres. This pattern was established probably in the late Emsian and continued until the end of the Givetian, quite a long time in geological terms. Each cycle has its own fish bed which represents the deep water phase of a lacustrine environment (Saxon, 1978).

The greatest extent of this lake basin probably occurred roughly at the start of the Givetian. This is seen in the Achanarras horizon which is recognisable at many localities stretching over hundreds of kilometres in the north of Scotland, the Orkney and Shetland Islands. During this period one of the most bizarre fishes first appeared in the fossil record of the Orcadian Middle Old Red Sandstone. This was Homostius milleri, first discovered by Robert Dick, a baker of Thurso and a competent natural scientist, and first described by Hugh Miller, mistakenly, as Asterolepis.

THE RADIOACTIVITY OF HOMOSTIUS BONES

Homostius (Heintz, 1934) was a large placoderm which was flattened dorso-ventrally and covered in formidable dermal armour on the head and thorax. The armoured area often exceeded 300 mm in length and 200 mm in breadth and the bone was up to 15 mm in thickness. The jaws, which were weak, suggest that the fish probably scavenged for its food on the lake bed.

A single plate of Homostius in the Geological Survey and Museum in London (GSM89090) was reported to be unusually radioactive (Bowie and Atkin, 1956). The activity in this specimen was largely due to the decay chain of Th 232.

A further plate of Homostius from Clardon Haven, about 5 km west of Thurso, in the author's possession, was also found to be radioactive to the extent of 1.2 x 104 gamma/min per gram (Diggle and Saxon, 1965). The energies of the gamma rays emitted were measured using a gamma scintillation spectrometer in conjunction with a photomultiplier, a high-gain amplifier, and a 512-channel pulse height analyser. It was evident that the radioactivity in this specimen was largely due to the decay chain of uranium with some contribution from the thorium decay chain.

To determine the distribution of the activity a section of the bone was polished and an autoradiograph was obtained using industrial grade X-ray film. The heavily blackened film showed that the alpha radiation distribution was not uniform but was concentrated into a few small areas.

A quantitive examination was made using a Hilger spectrometer and the constituents of the fossil bone were: major, calcium; minor, strontium, phosphorus and magnesium; heavy trace, barium.

Specimens of the dipnoan Dipterus valenciennesi and the crossopterygian Thursius pholidotus were also examined but showed no unusual radioactivity.

The tentative conclusion drawn was that Homostius had probably ingested the uranium and thorium during life, and it was probably that every specimen of Homostius from this palaeo- geographical region had ingested uranium and thorium.

THE AGE OF THE URANIUM

If ingestion ceased with the death of the individual then, it was thought, the decay chains of uranium and thorium might be used to determine the age of the bone, as in the classical case of C14. Five specimens were collected and it was noted (Bray, 1974) that not only were the bones of Homostius radioactive but also the surrounding rock matrix. From this it was assumed that the soft tissues as well as the bone contained uranium and thorium, the activity having passed into the rock matrix after the death of the individual

Thorium predominated over uranium in these five specimens. Three different methods were chosen to obtain a dating for the fossil bones. An age of 700 million years was found for the fossil. The conclusion drawn was that we were seeing the age of the radioactive material and not that of the fossil. This ought to have been foreseen.

THE EFFECTS OF RADIATION ON HOMOSTIUS

We now knew that every specimen of Homostius so far examined was radioactive and that this could only be the normal state for this fish. It seemed unlikely that this radioactivity had been acquired after death since other genera of fish of the same age showed no unusual radioactivity, and there was no obvious reason why uranium and thorium should have any preference for one particular genus.

All living creatures are subjected all their lives to radiation and it is possible that evolution and even the origins of life itself might have been impossible without it. Yet, if Homostius really had incorporated the uranium and thorium in life (one specimen was estimated to contain 0.52% of thorium) then the fish could be considered to have been subjected to chronic radiation exposure due to alpha, beta and gamma emitters fixed in the body tissues.

All modern experience suggests that somatic and genetic effects ought to have been suffered by this genus. There are no observable abnormalities in individuals, such as failure of the nucleus of a bony plate to ossify, and no observable genetic effects on a breeding population throughout its fossil record, which consists of the entire Givetian period. This is even more surprising than to find that the bones were radioactive.

FURTHER WORK

It would be interesting to examine Homostius sulcatus (Heintz, 1934) from Estonia and Tityosteus rieversi (Gross 1960) from the Hunsruck slates of West Germany to see if these bones, too, were radioactive. Specimens, however are not likely to come into my hands so this work must be carried out by other workers.

CONCLUSIONS

The bones of Homostius milleri from the Middle Old Red Sandstone of the Orcadian cuvette in Scotland are highly radioactive due to the decay chains of Th232 and U238. The evidence suggests that the uranium and thorium was ingested during life in quantities sufficient to give rise to chronic radiation exposure. No somatic or genetic effects are, however, observed.

R E F E R E N C E S