Reptiles On The Rocks by William T. Blows
Chapter 2. The Dinosaur Dynasties
In the popular imagination the dinosaurs were slow, ponderous, somewhat dim-witted, gigantic reptiles. However, modern research is throwing new light on these animals. In fact they were an immensely successful group which dominated the land faunas of the world for some 140 million years from the late Triassic to the end of the Cretaceous, (for comparison our own species, Homo sapiens, has been around for only a brief interval of geological time, less than one million years). During this long period of time several hundred different types of dinosaur evolved and became extinct, each adapted to a particular environment and mode of life. It is now clear that some of them, especially the smaller kinds, could move rapidly and had some degree of intelligence. Some scientists now believe that the dinosaurs were warm-blooded, that is, they could control their internal temperatures in much the same way as mammals and birds can. Unfortunately, in spite of a considerable amount of work, we still do not know why they died out. It is known that towards the end of the Cretaceous period, about 70 million years ago, there was a distinct change in the world's climatic patterns and it seems likely that the extinction of the dinosaurs is in some way linked with this event.
Although dinosaurs have been found on every continent except Antarctica, there is nowhere in the world where the complete dinosaur story can be demonstrated in a single area. Nonetheless geologists have been able to piece together the evidence from various places so that the general evolutionary pattern of the dinosaurs is now reasonably well established. The Wealden Marls of the Isle of Wight have not provided such spectacular remains as the Upper Jurassic of North America and East Africa or the Upper Cretaceous of North America and Mongolia. Nevertheless the Island has produced specimens which have been vital in understanding the Lower Cretaceous part of the dinosaur story.
Although we habitually speak of "dinosaurs", in fact this term includes two completely separate types of reptiles, the Saurischia (lizard-hipped dinosaurs) and the Ornithischia (bird-hipped dinosaurs), the distinction between them being based on the different configurations of the three bones which form the pelvis. Both types are further subdivided, the Saurischians into theropods and sauropods, the Ornithischians into ornithopods, ceratopsians, stegosaurs and ankylosaurs. In spite of the fragmentary and often incomplete nature of the remains which makes identification difficult, four of the six dinosaur groups are represented in the Wealden of the Isle of Wight. We can now see where the Island types fit into this scheme.
The theropods were the only carnivorous dinosaur group. All of them were bipedal, that is they walked on their hind limbs. The theropods include two subgroups: the massively built carnosaurs with shortened forelimbs, of which the most famous example is Tyrannosaurus, and the smaller, more lightly built coelurosaurs in which the forelimbs remained long and bore long-fingered grasping hands. Both subgroups are represented on the Island, the carnosaurs by Megalosaurus and the coelurosaurs by three very poorly known forms Aristosuchus, Calamospondylus and Thecocoelurus.
In common with other dinosaur localities, the Wealden Marls of the Isle of Wight contain very few theropods and there is a good reason for this. Among all modern animal populations there are always many more plant-eaters than meat-eaters. It is in this way that nature balances the need to provide the carnivores with an adequate food supply with the need to continue the herbivore stock. Analyses of fossil animal communities show exactly the same pattern and the dinosaurs were no exception to the rule. Hence the rarity of the carnivorous theropods in the Wealden Marls and elsewhere.
The study of the relative proportions of predators to prey in fossil reptile and mammal communities has been the subject of much research in recent years. It has been shown in studies of modern populations that the predator to prey ratio is always much higher amongst cold-blooded animals than it is amongst warm-blooded animals. In fossil dinosaur communities there is always a very low predator/prey ratio, similar in fact to that shown by modern mammals. This is one piece of evidence which has been put forward to support that contention that these reptiles were warm-blooded.
The large flesh eater from the Wealden is Megalosaurus. Unfortunately the material is very fragmentary but we can get some idea of what this animal was like from better material occurring in the Upper Jurassic of England. It was about 6 metres (20 feet) long and stalked its prey on powerful hind legs. When walking the upper part of the body was tilted forward, so that the backbone would have been almost horizontal, and the tail was carried clear of the ground. The hind foot bore three clawed toes pointed forward and a fourth vestigial toe which pointed to the rear. That the latter came into contact with the ground occasionally is shown by the series of footprints which I recently discovered on the Island. The teeth of this animal were 5 cm. (2 inch) long blades with saw-like edges front and back. This type of tooth was clearly designed for cutting and tearing rather than chewing.
Aristosuchus, Calamospondylus and Thecocoelurus
All three of these relatively small carnivorous coelurosaurs are known from very fragmentary material and it is still not certain if in fact three distinct types are truly represented. Some new specimens recently discovered on the Island may help to solve this puzzle. From remains of other types of coelurosaur we know that they were agile, lightly-built animals which hunted their prey by swift running. It seems likely that the Island forms would have been about 2m (6 feet) in length and probably caught lizards and perhaps even mammals.
The sauropods were plant-eating quadrupedal forms. Most of them were very large animals with massive pillar-like legs, long neck and long tail, such as the well known North American forms Apatosaurus (the correct name for the dinosaur commonly known as Brontosaurus) and Diplodocus. It used to be thought that these huge animals could not have moved around on dry land and for this reason reconstructions usually show them at least partly submerged in lakes, the water providing some buoyancy to overcome the weight problem. Recent, although still controversial, evidence suggests that the skeleton was so constructed as to support the bulk of the body and that they were capable of moving around on dry land.
Sauropod remains have been recovered from the Island from time to time but they are always uncommon and incomplete. This situation has led to considerable confusion over the naming. Altogether ten different names have been applied to these specimens but the material has never really been studied as a whole. It seems likely that in many cases different names have been given to different parts of the same animal. Until there is a complete re-assessment of the material, all that can be said at the moment is that on size alone there appear to be two basic types. The large types, which would include material called Camarasaurus, Cetiosaurus, Hoplosaurus, Chondrosteosaurus, Eucamerotus, Pelorosaurus and Ornithopsis, were very large animals probably at least 15 metres (50 feet) in length. Although we cannot be certain at the moment, two or perhaps three separate species are probably included within this group. Very recently a single bone was discovered at Brighstone and received national publicity. The excitement was because the bone was a haemal arch, a boat shaped structure suspended beneath the tail vertebrae. This type of structure occurs only in Diplodocus, the North American sauropod which reached a length of nearly 30 metres (100 feet). The new specimen may be the only evidence to date of this animal in Europe. As well as these giant sauropods, there also appears to have been at least one smaller form. This would include specimens which have been called Pleurocoelus or Titanosaurus. The original animals would appear to have been about 4 metres (14 feet) long.
The ornithopods were plant-eating bipedal forms. They were a very successful group of dinosaurs with a great variety of forms including several species which inhabited the Island.
This is probably the best known dinosaur from the Wealden since several complete skeletons have been found, notably from Belgium. The almost complete skeleton from the Isle of Wight in the British Museum (Natural History) in London has already been mentioned, and I have collected sections of the skeleton on two occasions as described later in the book.Unfortunately because Iguanodon is such a common dinosaur on the Island, here is a tendency to label all the single bones that are found as Iguanodon. Possibly the bones of other animals have been allocated to this "Iguanodon Dustbin" if only because no other positive identification can be made.
The anatomy of this beast is quite well known. It was a large bipedal reptile reaching a maximum length of 9 metres (nearly 30 feet) and a height of about 5.5 metres (18 feet). It had three toes on the strong hind feet which supported a weight of about 7 tonnes when fully grown. The forelimbs, carried well clear of the ground, had a remarkable human-like hand bearing four fingers and a spur-like thumb, the latter probably having a defensive function. The tail was quite long and helped to balance the upper part of the body, the whole body being pivoted around the pelvis. When in motion, the tail must have been carried clear of the ground, since few tail impressions exist at the sites of fossil Iguanodon footprints.
The skull was furnished with leaf-shaped teeth and it is from these that the animal derives its name, which means "Iguana tooth". When first found the teeth were shown to resemble those of the South American lizard Iguana. In Iguanodon the teeth line the sides of the jaws, the front being toothless and rather beak-like. It seems that the animal cropped plants with the beak and then sliced up the food with the teeth before swallowing it.
Iguanodon must have lived in considerable numbers during the early Cretaceous judging by the number of bones that are found. There is some convincing evidence that they were herd animals. For example in 1878 at Bernissart, near Mons, in Belgium thirty-three complete and partial skeletons were found together. It has been suggested from the character of the deposit in which they were found that these remains represent a herd which fell into a pothole. We can therefore imagine herds of Iguanodon roaming the swamplands of Europe 120 million years ago much as modern elephants wander the plains of Africa today.
This small bipedal dinosaur is, like Iguanodon, relatively well known. The remains have been found almost entirely in the one band of rock which bears its name, the "Hypsilophodon Bed", which rises up from the beach into the cliffs west of Cowleaze Chine, near Atherfield. It was here that the Rev. William Fox first found this dinosaur which carries his name — Hypsilophodon foxii. The Hypsilophodon Bed has yielded many specimens including three nearly complete skeletons. Thanks to the recent studies by Dr. Peter Galton we now know the anatomy of Hypsilophodon in some detail.
The animal was about 1.5 metres (5 feet) long when fully grown and just over 0.6 metres (2 feet) high at the hips. Like Iguanodon, the body was tilted forward when moving with the tail held clear of the ground. The skull was about 10 cm. (4 inches) long and showed some interesting primitive features. Unlike its cousin Iguanodon, the front of the upper jaw bore teeth. The eye had a ring of bony plates around it which could be opened or closed like an iris to accommodate for variations in the light. There is evidence for a series of bony scales extending down the back, another vestige from the early days of the dinosaurs.
It was originally thought that the five-toed hind foot was adapted for tree climbing, but Dr. Galton was able to show that it was actually the foot of a fast running, ground-dweller. Hypsilophodon may have lived in small herds like Iguanodon and we can perhaps think of this animal as the dinosaur equivalent of the deer or antelope.
Vectisaurus and Valdosaurus
Amongst the ornithopod remains found on the Island are a series of bones which belong to animals intermediate in size between Iguanodon and Hypsilophodon. From this material two forms have been described but both are very poorly known. The name Vectisaurus is based on a group of eight dorsal vertebrae and a partial pelvis found near Chilton Chine. These specimens indicate an animal of modest size which closely resembled the larger Iguanodon, and in fact the two animals must have appeared very much alike in both structure and habits. The second animal, Valdosaurus, is a much smaller form approaching Hypsilophodon in size. The name is based on a partial femur from the Island, although further material has been discovered in the Sussex Weald.
Yaverlandia was described recently on the basis of a very small skull fragment found in the cliffs at Yaverland, Sandown and now on display in the Museum of I.O.W. Geology. Although such a small specimen it has been shown that it shows resemblances to a peculiar group of North American dinosaurs called pachycephalosaurs. The chief characteristic of these dinosaurs is the extremely thick skull roof and Yaverlandia shows this feature although not quite so pronounced. It is suggested that this small species, perhaps about the same size as Hypsilophodon, was ancestral to the later American animals.
Ceratopsians were rhinoceros-like quadrupedal animals characterised by a great bony frill extending over the neck from the back of the skull and often carrying one or more horns on the face. The most famous member of the group is Triceratops from the late Cretaceous of the U.S.A. Ceratopsians evolved late in the dinosaur story and none occur in the Isle of Wight.
Stegosaurs were a mainly Jurassic group which includes a variety of armoured quadrupedal types of which Stegosaurus is the best known. No stegosaurs occur in the Wealden of the island.
In the Cretaceous the stegosaurs were replaced by the ankylosaurs. In general these were squat, almost tank-like creatures with a broad armoured back and a club-like tail.
Polacanthus is perhaps the strangest of all the dinosaurs that lived on the Isle of Wight during the early Cretaceous. It was originally described from a headless skeleton discovered by the Rev. William Fox, after whom it was named Polacanthus foxi. Although the skull is missing from this skeleton, Polacanthus seems to have been about 4 metres (14 feet) long and at the most 1.2 metres (4 feet) high at the hips. The animal was therefore relatively long and low in stature. The most spectacular feature of Polacanthus was the variety of armour that it carried. Running down backwards from the neck to the pelvis was a double row of long, sharp, pointed spikes. Over the hip region lay a large shield of bone composed of a large number of irregular plates fused together. The tail was furnished with a double row of bony plates which decreased in size towards the rear. Finally there were many button-like plates or ossicles, some of which were set between the tail plates while others seem to have been scattered on the sides of the body below the spines. The ossicles seem to be the only remains of Polacanthus which have been found since the original skeleton. They can be seen in the museum at Sandown and I have myself been lucky enough to find some.
The armour of Polacanthus -shows a character intermediate between the Jurassic stegosaurs and the late Cretaceous ankylosaurs. The former had an armour of bony plates or spines running down the back, while the latter were much more heavily armoured with the whole of the upper surface of the body covered with a mass of small knobs, spikes and plates of bone. It is possible therefore to see a gradual evolutionary trend towards more and more effective protection against the attacks of the contemporary carnosaurs. For this reason some people, including myself, feel that it is unnecessary to separate the stegosaurs and the ankylosaurs. However, it must be said that there is some compelling evidence to suggest that the plates of Stegosaurus were not primarily for protection but were used in controlling the body temperature by radiating or absorbing heat according to conditions in the surrounding environment. Clearly we have a lot more to learn about the relationships between the stegosaurs and the ankylosaurs.
Polacanthoides and Hylaeosaurus
An isolated tibia, humerus and possibly a scapula from the Island have been called Polacanthoides. In the absence of more specimens we can say very little about this animal except that it appears to have been closely related to Polacanthus.
A third Wealden ankylosaur was found by Dr. Gideon Mantell in the Tilgate Forest of Sussex. These bones and others subsequently found on the Isle of Wight were described as Hylaeosaurus by Professor Richard Owen in 1832. Very little additional material of this dinosaur has been discovered since although a large spike is exhibited in the Museum of I.O.W. Geology and I have collected a single vertebrae in Sandown Bay. Although the evidence is far from complete, it is apparent that Hylaeosaurus was closer to the large late Cretaceous ankylosaurs than Polacanthus in having a more flattened body with spikes on the sides of the body rather than along the back.
This completes our brief survey of the Island's dinosaurs. A few species are now known in some detail, but the majority still require a further research. It is to be hoped that someone will take up the challenge and continue the work of painstaking description begun by Professor Owen over a century ago. One day our understanding of these creatures will improve, especially if studies on new discoveries are combined with work on all the existing fossils housed in museums and private collections around the country. It is essential, if this is to happen, that anyone who has a collection of dinosaur remains or who finds the odd bone notifies a museum, preferably either the Museum of I.O.W. Geology at Sandown or the British Museum (Natural History) in London so that a record of all the available material is kept up to date and can be used by research workers.This page was last edited on: 4th March, 2015 06:16:20