Contents Updated: Monday, September 13, 1999
Dinosaurs so dominated the landscape in ancient times that they have dominated the minds of Palaeontologists too. Now people are. taking a closer look at the tiny lizard-like reptiles that shared the world of the dinosaurs. Dr Nicholas Fraser wrote this when a research fellow at Girton college and working in the Zoology Department of the University of Cambndge.
To many people the dinosaurs are the most fascinating group of animals ever to have inhabited the Earth. The cause of their sudden death about 65 million years ago is the subject of a vigorous debate, but little attention is paid to their origins towards the end of the Triassic period, about 185 million years ago, and the environment of that time.
Palaeontologists found the earliest bones attributed to a dinosaur in Brazil, in deposits that are approximately 230 million years old. From the late Triassic onwards, the dinosaurs spread rapidly to become the dominant terrestrial animals with four legs until their extinction at the end of the Cretaceous. A wealth of smaller reptiles existed, but palaeontologists have tended to overlook these forms. In the literature at least, dinosaurs dominate the creatures that lived in their shadows.
These "microvertebrates" have small delicate bones which the processes of fossilisation are likely to damage or destroy. But finds from deposits in the Bristol Channel area in Glamorgan and Avon allow us a glimpse of the interesting world that existed at the feet of the early dinosaurs. A fascinating subject in their own right, micro-vertebrates also help us to understand the environment in which some of the first dinosaurs lived. More importantly, some of them can shed considerable light on the evolutionary relationships between dinosaurs and pterosaurs.
During the late Triassic, the sea partially inundated southwest Britain. By the early Jurassic, 200 million years ago, only the highest outcrops of rocks remained above the water as islands. At times the sea receded, exposing more land before it flooded back. It was under these conditions that a diverse array of small vertebrates lived.
Caverns and underground fissures opened in the exposed Carboniferous limestone. As the water level rose towards the end of the Triassic, rainstorms washed sediments down into these caves, choking them. It is in the sediments filling these crevices that we find the remains of microvertebrates. Some of the animals may have fallen to their deaths down the cracks, where the sediment quickly covered them. Others may have drowned in flash floods, which then swept their bodies into caverns.
Some small vertebrates may have been cave dwellers; others might have been dragged into caves by predators. Many more died on the surface, where scavengers scattered their bones. The effects of wind, floods and rain completed the disintegration of the bodies, sweeping the remains into crevices in the limestone. Rainstorms washing through the middens of predators would also enrich the sediment with bones.
It is extremely difficult to date the various deposits beyond placing them in the late Triassic and early Jurassic. However, the data we have indicate that abrupt changes in the micro-vertebrate terrestrial faunas did not coincide with the great sea floods. As the amount of land above water level shrank, there was a loss of diversity. But even though the numbers of mammals and mammal-like reptiles grew, non-mammal-like reptiles remained the most abundant small vertebrates well into the Jurassic in southwest Britain.
Paiaeontologists have recovered some well-preserved articulated skeletons over a number of years. We have also found hundreds of thousands of bones jumbled together in the limestone. After freeing these from the stone by dissolving it with acetic acid, we have found a spectacular range of bones, including tiny jaw bones less than 2 millimetres long, complete with needle-sharp teeth.
When you analyse fossils, articulated specimens are relatively easy to restore and describe. Disassociated remains require a completely different and more exacting approach. After we had released the bones from their limestone tombs, we were left with a huge pile to identify and sort. It was like being asked to piece together jigsaws from several sets, some of which lack pieces, and with no pictures to help you. Many of the bones are broken; many closely resemble each other. We have managed to reconstruct, more or less completely, only the most common animals from this collection. However, the jaw bones of many others testify to their dietary habits so we can now start to formulate some ideas about their way of life.
The commonest group was the sphenodontids—or chisel-toothed reptiles. Only one member of the family, the tuatara (Sphenodon punctatus), survives today on a few remote islands off New Zealand. Research on sphenodontids found in fissure deposits has shown that the tuatara is a specialised form, so the frequent references to it as a living fossil are incorrect. In appearance, it is much like a lizard but for a long time zoologists thought it a relative of the extinct rhynchosaurs as it had a beaklike snout, common to both groups. Now they recognise the sphenodontids as a sister group of lizards and snakes.
Sphenodontids appeared at the end of the Triassic and span a large part of the fossil record, stretching into the Jurassic and Cretaceous periods. One of the ways you can identify sphenodontids is by their teeth. Typically, these are fused to the summit of the jaws. The teeth were not replaced after hatching and continuous use may have worn them down to the bone. By contrast, lizards' teeth are not fused to the jawbone, and they stay sharp throughout their lives because they are continually replaced as they wear out or break, growing from the inner wall of the jaw. (Living crocodiles as well as many extinct groups of reptiles, such as dinosaurs and marine plesiosaurs have a third kind of teeth. Set in deep sockets, new teeth take the place of old, ensuring a sharp set at all times.)
Sphenodontids have another toothy characteristic. An enlarged row of teeth on the palate runs parallel to the teeth in the jaw, effectively forming a double row of teeth in the upper jaw. The teeth in the lower jaw fit between the upper rows of teeth to give a powerful shearing bite. Although many adults might have lost some of their teeth, the arrangement of teeth provided a self-sharpening mechanism which acts on the bony margins of the jaws. Backwards and forwards movements of the jaw, combined with the shearing up and down action, increased the efficiency of the bite in some sphenodontids, including the tuatara. This might also have helped to get rid of food stuck in the groove between the two upper rows of teeth. However, the shearing bite does not seem well developed in the smallest animals we found in the caves.
A commonly found sphenodontid, Planocephalosaurus, has a head little more than 2.5 centimetres long. Its front teeth are wedge-shaped cones, while an array of small teeth cover its palate. This—and its small size—shows that it fed rather like many modern lizards. The sharp front teeth impaled small insects. Then, by moving its head up and back, it would swallow its prey with little attempt at chewing. In the Triassic, large sphenodontids, such as Clevosaurus had skulls about 4 centimetres long. Their teeth were also cone-shaped but broader and their jaw bones were deeper. They could tackle larger and tougher prey, perhaps feeding on young Planocephalosaurus, perhaps even on their own young. Tuatara make crunching sounds when they eat, so Clevosaurus may have chewed its prey into pieces that were easy to swallow. But this particular species was probably not wholly carnivorous. The cheek—or side—teeth have keels at the front and back, reminiscent of the teeth of modern herbivorous lizards, such as Uromastix and some iguanids. Clevosaurus may have also fed on plants.
Sigmala, another small Triassic sphenodontid, probably fed entirely on plants. It had short deep jaws with closely packed teeth. This combination would make an efficient plant-shredding machine. In mature animals, the small front teeth are worn down to the bone, leaving a cropping "beak", a bit liked that of modern turtles and tortoises.
Other reptiles must have been insectivores. They were generally small with light skulls and sharp marginal teeth. Kuehneosaurus was one of the most unusual of this group and may have occasionally reached nearly 1 metre in length. Palaeontologists find it fascinating because it had greatly elongated thoracic ribs that could function as a gliding membrane, like that of the modern flying dragon, Draco. Kuehneosaurus could not have been "the swallow of the Jurassic skies", catching insects on the wing, as it would have needed powered flight and a sophisticated nervous system. But it could have evaded predators by jumping off rocks and gliding out of reach. On the other hand, new finds uncovered this year point to the presence of a small pterosaur which was presumably much better suited to the aerial realm.
Insectivores also include the small thecondontians, the group from which dinosaurs descended, and the early shrew-like mammals, Morganucodon and Kuehneotherium. Remains of these mammals are abundant in deposits from the early Jurassic found in South Wales. Even earlier are the two teeth from Kuehneotherium found in Somerset. Discovered in Triassic sediments, these may be the oldest record of a true mammal. Palaeontologists believe that both these early mammals foraged for grubs and insects among fallen plant debris. The shape of their skeletons suggests that they may have been able to clamber up plants to look for insects in the bark or leaves. Some of the thecodontians may prove to be invaluable in unravelling the relationship between pterosaurs and dinosaurs. Kevin Padian of the University of California at Berkeley believes that a small thecodontian called Scleromochlus can provide us with information about the link between pterosaurs and the ancestors of the dinosaurs. Unfortunately, palaeontologists know Selerom ochius from only a few poorly preserved natural casts from sediments near Elgin, Scotland. Abundant new remains like those of the seleromochlids from the fissure deposits have given us the chance to find out more about the ancestry of pterosaurs and dinosaurs.
Early mammals and insectivorous reptiles had similar eating habits and their sizes were comparable. The mammals may have avoided direct competition with the reptiles by adopting a nocturnal—or partially nocturnal—way of life. Among the small insectivorous reptiles, it is likely that each kind had slightly different feeding strategies, much as modern lizards do. The thecodontians, for instance, were probably active hunters with a catholic taste, scuttling round the rocks for any tasty morsel. On the other hand, some of the sphenodontids may have adopted a sit-and-wait approach, possibly even perching at different heights to avoid competitors in the same way as the present day anoline lizards. They may also have been more selective in their choice of prey.
To support such a wealth of insectivores there must have been a diverse and rich invertebrate fauna. We know that many invertebrates had established themselves by this time. They include annelid worms, spiders, centipedes and millipedes, grasshoppers, beetles, flies and cockcroaches. Until recently, we had found only a few remains of beetles and some branchiopod crustacea in the fissure deposits. The sediments are generally too coarse to preserve the delicate remains of insects. The discovery last year of some beautifully preserved millipedes in Gloucestershire is important, therefore—particularly since Mesozoic millipedes are rarely found as fossils anywhere.
Herbivores among these small vertebrates, apart from the sphenodontids, include procolophonids, tritylodontids and an early prosauropod dinosaur. Procolophonids, whose closest relatives today are generally believed to be the turtles and tortoises, were squat animals, little more than 50 centimetres long. They possessed a battery of cheek teeth, transversely broadened, which made an efficient grinding apparatus. At least one member of the family had bony spines projecting from its cheeks. These would have given predators a nasty surprise, as the moloch with its similar body spines does today.
The tritylodontids, mammal-like reptiles, also had a battery of molar shaped teeth suitable for grinding plants. Hans-Dieter Sues of the Smithsonian Institution, Washington, suggests that the forelimb of tritylodontids was ideally suited for digging. The animals could have grubbed up roots and bulbs for food. But Beverly Halstead at the University of Reading, however, thought that this shape in a foreleg also suited an aquatic habit. The tritylodontids could have led a life similar to that of modern water rats and voles.
An early dinosaur, Thecodontosaurus, has left its remains in fissure deposits in England and Wales. It is the largest reptile found in these Mesozoic sediments. Many of the remains may be those of young animals but even the adults were little more than 3 metres long—dwarves compared with their Jurassic and Cretaceous relations. Their teeth are leaf shaped, leading Peter Galton of Bridgeport, Connecticut, to argue forcefully that all the members of this group are herbivores. They had long necks and probably supported themselves on their powerfull hindlegs and tail to reach taller plants for food. Their small skulls would have reduced the leverage on their long necks.
Just as the remains of insects are rare because of their fragility, so the evidence for pollen and other plant remains is scarce. Some deposits in South Wales have yielded spores that show shrubs and trees like modern conifers. We have also found traces of ferns and cycads.
With so many small insectivores around, predators thrived. Gracile terrestrial crocodiles and early carnivorous dinosaurs fed on the insectivores. These crocodiles bore little resemblance to their living descendants. Triassic crocodiles did not lie sluggishly submerged in water waiting for prey. They were active quadrupedal hunters, thin and slender-about the size of a large cat. An impressive array of serrated, knife like teeth lined their jaws—ideal for cutting and slicing the flesh of the small sphenodontids. In the deposits from the Mesozoic, we have found the remains of small theropod dinosaurs with similar teeth.
The erect gait of the terrestrial crocodilians, with their legs drawn underneath the body in a manner similar to that of mammals, contrasts sharply with the sprawling posture of modern reptiles. Is it possible that such active forms were warm-blooded? The theory that the dinosaurs were warm-blooded has been championed by Robert Bakker of the University of Colorado at Boulder. He bases his claims partly on their erect gait, presumed high levels of activity and the possession of a completely divided four chambered heart. Apart from the crocodiles, all living reptiles have a heart with three chambers. Palaeontologists assume dinosaurs had four chambers because a large animal would require high pressure to pump blood to the head and body. In a dinosaur such high pressures would cause massive haemorrhaging in the thin vessels of the lung, so the lungs must have had a completely separate low pressure blood supply. Although modern crocodiles are cold-blooded, could their four chambered heart be a relic of a warm-blooded ancestry?
These rich deposits continue to yield new finds, each adding to the vivid picture of life 200 million years ago in southwest Britain. Palaeontologists are also beginning to piece together the evolution of these lizard like reptiles by comparing them with remains found at similar sites else where in the world. For example, we discovered that several small reptilian jaw bones from Wales resemble those found in middle Triassic deposits in fissures in Poland. Recently a team of palaeontologists from Harvard University excavated large quantities of early Jurassic sediments in Nova Scotia, uncovering the remains of mammal like reptiles, sphenodontids, crocodiles and dinosaurs. As work progresses on all these deposits we should find ourselves in a better position to comment on the early evolution of mammal and dinosaur.
The Mesozoic deposits in caves and gulleys in Britain are providing us with an excellent opportunity to observe the microvertebrates that inhabited the land at the end of the Triassic. Their descendants lived on into the late Jurassic and Cretaceous—when dinosaurs reached their acme but did not, as might be supposed, reign alone.