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What to expect You might be lucky enough to come back with a "perfect" specimen or two, similar to those on display in museums and fossil shops, but "imperfect" fossils are far more common and are just as good for elucidating the evolutionary sequence of the Jurassic sea-creatures that we are about to consider. When you can identify these creatures from mere fragments of their fossilized remains then you have graduated from a collector of curiosities to a palaeontologist, the name given to a scientist who studies fossils.
Bear in mind that anyone exploring the beaches around Lyme Regis is walking over countless tons of Jurassic and Cretaceous debris that is turned over with every tide and that the fossil hunter is merely scratching its surface. Your chances of finding something spectacular are as good as anyone else's, assuming of course equal knowledge and powers of observation. |
Ammonites evolved into a great variety of species which came and went, some of which are used for dating the relative ages of the sediments in which they were laid down. The first in the sequence of "zone ammonites" used for Jurassic sediments (Psiloceras planorbis) need not concern us because the base of the Blue Lias lies well beneath Monmouth Beach. The earliest zone ammonite you might encounter is Schlotheimia angulata. This distinctive species is by no means common because the top of its range crops up above sea-level only along a limited stretch of the beach where the Blue Lias arches up. The shell of the ammonite is decorated with ribs that skirt the rim of the shell in the manner of a herringbone or the treads of a tyre. Loose body chambers derived from shale are unmistakable from this pattern alone. The suture lines occasionally show up on limestone. They are extremely complicated, resembling the outlines of feathers or ferns for which they are sometimes mistaken. The impressions of large nautiloid shells are common in the boulders of Monmouth Beach but they are easily mistaken for those of ammonites. In contrast to most ammonites the walls that separate the air chambers of nautiloids are beautifully curved like the blades of a turbine engine. However, the principal feature that distinguishes a nautiloid from an ammonite is the manner in which a thread of tissue resembling a bootlace ran through the air chambers. This tissue was almost certainly involved in regulating the amount of air within the shell. It was enclosed in a shelly tube known as a siphucle. In the ammonite the siphuncle was closely applied to the periphery the shell (running through the keel if it had one) whereas in the nautiloid it pierced the centre of each septum.
Another characteristic fossil of Regis is a bullet-shaped mass of calcite called a “guard”. This occupied the rear end of a belemnite, a shellfish which resembled a squid. The calcite was built up in concentric layers and this gives rise to a pattern not unlike the growth rings of a tree when a freshly broken guard is viewed end-on. This feature together with its smooth and streamlined surface makes an unworn guard easy to tell apart from any other object on the beach. Guards are tough but brittle and loose specimens derived from shale are invariably broken up into fragments as they are washed out of the cliffs. These pieces can be categorised as follows: a pointed end, a middle section (or sections), and a squashed end into which fitted a cone divided into air chambers by vertical partitions of calcite. This cone (or phragmocone, to give its proper name) is equivalent to the chambered shell of an ammonite. The guard can be viewed as an extension of the light phragmacone about which it acted as a counterweight. Ammonites, nautiloids and belemnites make up part of the molluscan kingdom called cephalapoda meaning “head-footed’. They were extremely common during the Jurassic Period as testified by their abundant fossilised remains around Lyme Regis. Ammonites and belemnites disappeared with the dinosaurs and the cephalapods of today are somewhat poorly represented, consisting of octopuses,squids,cuttlefish and the like. Nautiloids are still with us but restricted to a few species living in the Indo-Pacific ocean. The molluscs that have stood the test of time are those that were early adapted to a sedentary life on the sea-bed, These are the "ordinary shellfish" of today - bivalved molluscs - equipped with two shells which enclose and protect their fleshy parts: oysters, mussels, cockles and their kin. Common Jurassic species found on Monmouth Beach are pictured below. Bivalve molluscs are somewhat conservative in evolutionary terms. The shell of a Jurassic scollop, for example, is similar to that of a scollop dredged up today from the bottom of Lyme Bay. In contrast, the shells of ammonites evolved into a great variety of forms throughout the Jurassic period as if they were experimenting in ways to escape and hide from the reptiles that hunted them.
The best known Jurassic bivalve is an oyster popularly known as the "Devil's Toenail" or Gryph (pronounced "griff") after its Latin name Gryphaea arcuata. This species flourished over an enormous period and its remains are very common in the Blue Lias. It is a splendid example of how fossils can provide a clue to conditions on ancient sea-floors in so far as many of the oysters of today thrive in muddy seas. The lower shell of the Gryph is a tough cup upon which rested the upper flat and lid-like shell. The two shells were linked together at their narrow ends by ligaments which quickly rotted away after death - complete Gryphs are uncommon. The lower shell is twisted upwards at one end. It is decorated with concentric and overlapping ridges representing phases of growth. This ornamentation is soon lost when the shell is tossed about on the beach but the shell is still easy to recognise from its domed and slipper-like appearance. Those shells which are buried in limestone and which have been worn across show up as crescent-shaped impressions that resemble toe clippings. These impressions are exceptionally common on the surface of a layer of limestone which is nicknamed “The Devil’s Pavement”.
All the characteristic fossils of Lyme Regis and where to find them are described in “Fossil hunting around Lyme Regis”, one of a series of modestly priced books written and illustrated by Colin Dawes.
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