Charmouth 180 million years BC and the Ichthysaur.

CHARMOUTH 180 MILLION YEARS BC.
and THE ICHTHYOSAUR
By Christopher M. Pamplin B.Sc. - Geologist

The history of Charmouth from when it was under the sea during the Jurassic period when ichthyosaurs and other reptiles roamed until today.

Marine life movements between the Cretaceous, Jurassic, Mesozoic, Triassic and modern periods of Britain, inc dinosaurs.
180 million years BC, UK under sea.

Marine life movements between the Cretaceous, Jurassic, Mesozoic, Triassic and modern periods of Britain, inc dinosaurs.
Today

180 million years ago Charmouth was a very different place from the one we see today, it then lay under the sea, a sea which no longer exists, so long ago was the period of time we are contemplating.
The world then was very different. In the Early Jurassic, dinosaurs were becoming the dominant land living creatures. Whilst mammals were dwindling, dinosaurs became huge creatures. In the oceans other large reptiles swam.
The sea in question no longer exists due to plate tectonic movements. Before that period, the continents had been joined together as the super continent of Pangaea. For millions of years this area of southwest England had been a desert, but early in the Jurassic, water flooded the area to become the THETHYS sea. This would last for the remainder of the Jurassic period, encroaching and receding periodically as the sea level rose and fell.
Britain was much further south than it is today, about 15 degrees above the equator. The large landmass and different marine circulation of ocean currents caused the climate to be warmer. There was no snow at the poles, winters were hot and wet and the summers were hot and dry. High latitudes were sub-tropical whilst the equatorial rain forest extended much further north and south than it does today.
It is even thought that days were slightly shorter as the earth was spinning slightly faster than it does today.
Although the Atlantic started to widen in the southern hemisphere, America and Africa were still joined, Wales and Scotland were land attached to the old continent and islands broke the surface waters where the Mendips lie today. Dartmoor and Cornwall may have been islands to the west, but to date no evidence exists to verify this theory.

THE ICHTHYOSAUR

Ichthyosaurs or 'Fish lizards', were sleek predators of the Mesozoic seas, and ideally suited to their marine environment. Not fish, but highly developed specialised reptiles which could swim with accuracy and speed, catching their prey.

Marine life predators like dolphins that lived in Lyme Bay, Dorset, UK.

Take a look at the examples above, and you could be forgiven for thinking they were dolphins. Dolphins are however mammals, but both they and the Ichthyosaurs have at different times adapted the same form to meet the demands of the same environment. This is known to the palaeontologist as homomorphy or superficial similarities in morphology of members of different plyla. In dolphins the tail is orientated in the horizontal plane, whereas in Ichthyosaurs the tail is orientated vertically as is the sharks, but is reversed and known as the reversed heterocercal tail. Though Ichthyosaurs are now extinct, their bones and even skin can be found as fossils in the marine rocks of the Mesozoic Era. In particular the rocks of the Jurassic period. Formidable predators, their prey were the abundant cephalopods which evolved alongside them. Ammonites often have teeth marks of these reptiles when found as fossils.

Marine life predators like dolphins that lived in Lyme Bay, Dorset, UK

In the Triassic seas which once lapped against the shores of the hot lands in the area now known as Central Europe, swam the most primitive of the Ichthyosaurs, Mixosaurus carnalius. This ancestor of the later dolphin-like Ichthyosaurs was already well adapted to its marine environment. However, it had vestiges of its land origin. The limbs were converted to paddles but were internally like legs. The humerus, radius and ulna in the fore limbs and the femur, tibia and fibula in the hind limbs were normally developed. The caudal fin was smaller than those of later species and was long and narrow in shape. The eyes were small and set in a short skull. In one genus, Omphalosaurus, the teeth were blunt with fluted crowns for a diet exclusively of shellfish. All had separated unfused teeth.

The Jurassic was the heyday of the Ichthyosaurs. As seas flooded over deserts to lay down the Jurassic shales of Britain the new species followed, later to die and subsequently preserved in these rocks. They can be found in Dorset in the Lyme Bay area, right up to Whitby and in those Scottish islands which have such rocks. Much more advanced and fishlike, these reptiles were once massed into the genus Ichthyosaurs. However, many distinct types have been identified just as there are many types of shark or dolphin. Two examples are the narrow finned Stenopterygius quadridiscus and the broad finned Euryptergius sp.

In the dark, marine shales of the Lower Lias are found well preserved examples. Some show the outline of the body as a dark impression in the shale. Others have small Ichthyosaurs within them. Once it was thought that this indicated that the young had been eaten, but study has shown that these are in fact unborn babies. Some are seen to be emerging from the mothers pelvic region. This remarkable evidence tells us that the Ichthyosaurs like the dolphins gave birth to live young and not eggs as most reptiles do. The mother then gently guides the youngster to the surface for air.

Their skin was smooth wothout scales and their tails were a unique development. The upper caudal fin was unsupported and the caudal vertebrae have a natural break at one point, such that the column diverts downwards to form the lower tail.

The limbs of the Jurassic Ichthyosaurs had hexagonal or circular bones, the humerus, radius and ulna now being much adapted for marine life. Extended fingers, with extra finger joints, a feature known as hyperphalangy, this formed a paddle, ideal for steering through water. The main locomotive force was the flexible tail and powerful muscles in the hind quarter.

The skull had large eyes, a short temperal region and the long beak or jaw had the nostrils set well back, features which gave excellent sight in the depths and ease of breathing as the reptiles surfaced. By the late Jurassic the head was tucked close into the body as in the last illustrated example Nannopterygius entheciodon . The cervical spinal region was much reduced. Nannopterygius entheciodon had an extended tail and hind quarter which was a very powerful locomotive source indeed.

Ichthyosaurs become rarer in the fossil record as the Cretaceous unfolds. In the Kansas chalk other large marine creatures, such as the plesiosaurs and mosasaurs are found in abundance, but the Ichthyosaur is conspicuous by its absence. Well before the end of the Cretaceous the Ichthyosaur became extinct.