Octopi to oysters - can we make fossils live a little?

OCTOPI TO OYSTERS - CAN W E M A K E FOSSILS LIVE A LITTLE? G . D . HEATHCOTE

Fossils can be of great value to the geologist, enabling him to identify and date rock formations and even to recognise potentially oil-bearing strata.

Although Dinosaurs have long captured the public imagination, few other fossil animals have had public recognition - if only because of their intimidating scientific names. However, it is a stränge and moving experience to hold in one's hand the remains of an animal which died millions of years ago and which has never been seen before. The study of fossils is rewarding.

Unless one has had the benefit of special training it may be impossible even to guess what a long-extinct animal looked like in life from its fossil remains.

Let us consider the possible appearance of a few Suffolk animals that only survive as fossils, choosing specifically those mentioned in Mottram's article on Blood Hill which was published in these Transactions (Mottram, 1991).

Two groups of Molluscs which are now extinct appeared in Mottram's article. We can never be sure what they looked like, or how they lived, but by comparison with their living relatives we can have a good idea. Ammonites and Belemnites are numerous as fossils. They were Cephalopods, unusually active Molluscs which may have a shell (as with the Nautilus), have an internal shell (like the Cuttlefish), or be without a shell (like the Octopus).

Only under exceptional circumstances are soft-bodied creatures preserved as fossils but chalky shells commonly are. All the living Cephalopods mentioned except for the tropical Nautilus have representatives off the Suffolk coast.

Nautilus The Nautilus species are the nearest living relatives of the extinct Ammonites. The Pearly Nautilus, Nautilus pompilius L., from the Indo-Pacific region is one of the best known, but there are others. N. macrompholus Sow., with its attractive mottled shell is well known to collectors - I have a specimen from Papua New Guinea. Nautiloids are the sole living representatives of a large group of Cephalopods which were once the dominant marine invertebrates. They flourished from the early Cambrian to the late Cretaceous periods.

Nautilus has a smooth external shell divided into many Chambers by concave septa. It is held vertically over the 'head', and a thickened area of tissue where it joins the shell form a hood, acting as an operculum (trapdoor) when the head-foot is retracted. The largest Chamber is occupied by the animal and the other Chambers are gas-filled to give buoyancy. The gas content can be changed, enabling the creature to rise or sink in the water like a submarine. Nautilus has a double ring of tentacles which are adhesive but without suckers. It has no chromatophores and cannot change colour. It can crawl over rocks like an Octopus but generally inhabits moderately deep

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OCTOPI TO OYSTERS - CAN W E MAKE FOSSILS LIVE A LITTLE?

Fig. 1

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Structure of Nautilus - based on Borradaile et al., (1963).

water, feeding on the surface at night and sinking to the bottom during the day.

Ammonites Like the Nautilus, Ammonites had a coiled shell divided into Chambers.

They varied greatly but, typically, an Ammonite shell was tightly coiled with prominent ridges. The dividing septa joined the shell internally at a scar or suture, sometimes making a complex pattern. Although the suture pattern varied with the age of the animal it is very useful in helping to identify the species. Ammonites first appeared in the middle of the Palaeozoic and reached their peak in the Mezozoic period. Some uncoiled forms evolved in the Cretaceous period when they were approaching extinction.

There are many things we will never know about Ammonites. What colour were they, for example? There are, of course, more important questions than this. Many of the soft parts of Molluscs are important when it comes to their Classification, such as the number of ctenidia (leaf-like branches involved in their respiration). Nautilus has four pairs of ctenidia (it is placed in the Tetrabranchiata group), whereas the Octopus, Squids and Cuttlefish have only two pairs of ctenidia (and belong to the Dibranchiata group). Without information such as this we cannot be sure how closely the Ammonites and the Nautilus are related, but it is likely that they resembled one another in structure and external form.

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Fig. 2

(Left) Kosmoceras compressum, an Ammonite.

(Max. 41 mm across x 14 mm wide) (Centre) Gonioteuthis quadrata, a Belemnite.

(Max. 64 mm long x 13 mm wide) (Right) Gryphaea arcuta, an Oyster.

(Max. 61 mm x 47 mm) Fossils from the Ipswich Museum Collection.

Some fossil Ammonites were very large. For example, on the beach at Lyme Regis, Dorset, some form boulders large enough to sit on. They are usually much worn and not easily recognised for what they are. I was once taken by a countryman to see a fossil lizard on the South Downs many years ago. This proved to be an Ammonite from the chalk about 2!/2ft. across. He had mistaken the shell ridges for the legs of the 'lizard'.

Although the pit at Blood Hill was worked for chalk, other material had been deposited there by melting ice or carried there by rivers. Mottram mentions the Ammonites Pavlovia pallasioides and Rasenia involuta from the Kimmeridge Clay, and another derived fossil Ammonite, Kosmoceras compressum Quenstadt, which is illustrated here.

Cuttlefish The internal shell of the Common Cuttlefish, Sepia officinalis (L.), the 'cuttlefish bone', is a well-known object on our beaches. It is often put in the canary's cage. The entire animal can sometimes be found on the fishmonger's slab. It has a flattened oval body with a striped pattern, but it can change colour very rapidly. Waves of brown can flow along its body, changing its

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D e a d specimen, Brighton, 1950.

appearance in a most extraordinary way. Lateral folds undulate when it is swimming. The distribution of internal organs is similar in the Cuttlefish and the Nautilus. Both have very large eyes, but the Cuttlefish has four pairs of short tentacles covered on their inner surface by suckers, plus two long tentacles with suckers only at the free ends. Cuttlefish are common in the shallow water off the Suffolk coast.

Belemnites Belemnites disappeared during the Cretaceous period (about 63 miltion years ago) but were extremely abundant earlier during the Mesozoic. They were probably like primitive Cuttlefish, and are thought to be more closely related to them than to Squids, which also have an internal skeleton. All that remains today are hard, shiny cylinders, pointed at one end and showing growth rings and internal septa. They closely resemble bullets. It is even said that a professor of geology pronounced a specimen as a Belemnite until it was pointed out that it had a number on it. I picked up several .38 bullets in a chalk quarry which had been used as a firing ränge during the War and these could have passed as Belemnites. The largest Belemnites were thought to be 'thunderbolts' by country people in my home county of Sussex. Was this true in Suffolk also?

Belemnite structure compares so well with that of the calcified cuttlefish 'bone' and the internal horny shell of the Squid that it is difficult to imagine they looked so very different.

Squids Like Cuttlefish, Squids have four pairs of short tentacles and a long pair with suckers only at the tips which can be shot out to capture their prey. The bodies of the common squids Loligo vulgaris Lam. and Allotheutis subulata (Lam.) are relatively long, with folds on either side which act as fins. Squids

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can move very fast through the water and often swim in groups. Some occur in very deep water. In life they are often red, but appear grey when seen at the fishmonger's. I have seen squids with bodies about 1 ft. long in Bury market, and often found small specimens in batches of shrimps or prawns.

Their internal shell is horny rather than calcified.

Octopi The Lesser Octopus, Eledone aldrovandii (Rafinesque), occurs off the Suffolk coast. It is distinguished from the Common Octopus, Octopus vulgaris L., which is generally found on rocky shores, by having a single row of suckers on its four pairs of tentacles and not a double row. I have only found live octopi among the rocks on the coast of Brittany, but I remember finding 14 washed up at Black Rock, Brighton, after a storm in February 1950. Octopi have very different habits from the other Cephalopods described here. They are 'shy', usually hiding in holes in rocks from which they can dart out to capture crabs or similar prey.

The only hard part of an Octopus is its 'horny', bird-like beak, situated at the centre of its ring of tentacles. Octopi do not make good fossils, but it is wise to study the structure of all Cephalopods when trying to reconstruct the bodies of extinct species such as the Ammonites and Belemnites.

Oysters Many fossils are the remains of animals which must have been almost identical with those living today. The only other fossil Mollusc mentioned by Mottram was an Oyster, Gryphaea arcuata Lam., which closely resembles oysters living today. It is another derived fossil, left by the Anglian ice sheet.

It was a bivalve with very conspicuous growth lines on the shell. One valve was loosely coiled and the other small and flat. The shell can reach 3in. in length. It has the dubious benefit of a populär n a m e - 'Devil's toenail', which is remarkably descriptive!

The sedentary Oyster could hardly be more different from the active Octopus, but both are Molluscs. Oysters are Lamellibranches, which have flattened bodies enclosed in a 'mantle' which secretes the shell, the two valves of which are joined by ligaments and a hinge. Strong aductor muscles close the shell. A detailed description of the animal is not justified here and can be found in any zoological textbook such as Borradaile, Eastham, Potts and Saunders (1963), but the head is rudimentary, without eye or tentacles.

The enlarged ctenidia filter food particles from the water which passes over them. Many people would probably prefer not to look too closely at the structure of an oyster or mussei if they are going to eat them, and I will not go into more detail.

Conclusions Most of us do not have the specialist knowledge to reconstruct the bodies of extinct creatures and I claim no originality for this article. I spent a morning

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with a U3A (University of the Third Age) 'field studies' group sorting out what we knew about Molluscs and, sad to say, it was 'very little'. However, fossil Molluscs may be easier to reconstruct than some other fossils, particularly those that were Nature's 'failures', quickly replaced by better models. In spite of these difficulties, studying fossils can help us to understand the structure of animals living today.

Acknowledgements I thank Howard Mendel for the loan of fossils from the Ipswich Museum collections.

References Anon (1964). British Mesozoic fossils. British Museum (Natural History), London.

Borradaile, L. A., Potts, F. A., Eastham, L. E. S. & Saunders, J. T. (1963), The Invertebrata. A manualfor the use ofstudents. Cambridge University Press.

Mottram, H. B. (1991). Blood Hill, Bramford. Trans. SuffolkNat. Soc.,27, 37.

Dr. G. D. Heathcote 2, St. Mary's Square, Bury St. Edmunds, Suffolk, IP33 2AJ.

Trans. Suffolk Nat. Soc. 28 (1992)