| TIME OF THE TITANS-DINOSAURS OF THE JURASSIC
At the beginning of the Jurassic, dinosaurs started getting bigger. As vegetarians grew in size, so did the creatures that hunted them. Dr Jo Wright describes life in the time of the titans.
Late Jurassic Earth
Diplodocus walked on four legs, but may have reared up onto two
The earliest dinosaurs were pretty small.Eoraptor was about one metre long. Its contemporary Herrerasaurus grew no more than four metres long and Coelophysis was about three metres long. And in all cases the length was mostly tail.
However the plant-eating prosauropod, Plateosaurus, that appeared at the end of the Triassic period, was a harbinger of things to come. At up to nine metres long it was the first really big dinosaur.
Prosauropods could walk either on all fours or just on their hind legs, leaving their hands free, perhaps to grasp branches and bring them within reach of their mouths. They disappeared at the end of the Early Jurassic period and their role was taken by the sauropods which thrived during from the Middle Jurassic to the Early Cretaceous period.
Sauropods had huge elephantine bodies coupled with very long necks and tails. They walked only on all fours. Some such as Diplodocus could probably have reared up on their hind legs using their tails as props.
Diplodocus skeletons show a number of features that support this: They have high vertebral spines over the hip region, showing the creatures had strong muscles and ligaments there. They also have skid-like bones underneath their tails, which would have protected delicate nerves and blood vessels when their tales were resting on the ground.
Keep on growing
In the Early Jurassic the maximum size of both herbivores and carnivores increased. This trend continued throughout the Jurassic culminating in the staggeringly large sauropods such as Apatosaurus and Brachiosaurus. Carnivores had also increased in size, although not by as much and the 12 metre long Allosaurus was dwarfed by its sauropod prey. Allosaurus would have had to pick on the young or weak or may have hunted in packs.
Evidence based on growth rings and bone texture indicates that sauropods reached their adult size in 10-20 years. If, as was once suggested, sauropods took 70 years to reach maturity, it would be unlikely that many would survive to reproduce. The data suggesting that they grew quickly also fits that from the bones of theropods and ornithopods, who were thought to reach maturity fast.
Stegosaurus had display plates its back
Stegosaurus was an early armoured dinosaur and its defences were formidable. Later armoured dinosaurs were veritable living tanks - some even had armoured eyelids!
There are two main types of dinosaurs, named from the configuration of their pelvic bones: bird-hipped and lizard-hipped. Sauropods and theropods are lizard-hipped dinosaurs; Stegosaurus is a bird-hipped dinosaur, an early member of a group which became much more common and diverse in the Cretaceous period.
| GIANTS OF THE SKY
The Early Cretaceous Earth was home to the largest animals ever to fly. But on the ground, life was changing too - and a species of dinosaur was about to undergo a very important change. Dr Jo Wright explains why.
The largest animals ever to fly were pterosaurs. With wingspans of up to 12m, they could be as large as a small modern glider. Pterosaurs were a peculiar mixture of bird and bat. Their long beaks remind us of birds, but they had membranous wings like bats.
These wings were attached to their legs and they walked on all fours, with an erect, rather than sprawling posture. Although not necessarily very speedy on land, they walked competently. Their tracks have been found in rocks around the world.
There were two kinds of pterosaurs, ones with and others without tails. The tailed pterosaurs lived from the Late Triassic period to the Late Jurassic period, when they were replaced by the tailless ones. It is this second group, including Ornithocheirus, which grew to great sizes.
Iguanodon - Chewing gives it an evolutionary advantage.
Early Cretaceous Earth
The early Cretaceous period was also a time of great change for animal life on the land. New types of dinosaurs - the ornithopods - were appearing, and they would later be among the most numerous on land.
Small versions of these dinosaurs had actually been around since the Early Jurassic period, but only now did they come into their own. Iguanodon was one of the earliest of the ornithopods. There were several species in Europe, Asia and North America.
Although smaller than the sauropods, the ornithopods were more numerous. They were able to chew their food and hold it in their cheeks while doing so. Chewing broke their food into smaller pieces, greatly reducing digestion time. Sauropods didn't chew their food at all. They needed huge bodies so their guts could break down food over several days. As they didn't have cheeks, they had the big reptilian smile we see in modern lizards and crocodiles.
Utahraptor - Its sickle claw makes it a deadly fighter.
New kinds of theropods also appeared in the Early Cretaceous period. These were the kinds popularly known as 'raptors', such as Utahraptor and Velociraptor. These theropods were some of the most bird-like dinosaurs that ever existed. They had fairly long arms and a more bird-like pelvis. Birds are descended from theropod dinosaurs like these - over 100 characters in the skeleton link these groups. Fossil evidence from the USA indicates that these types of dinosaurs may have hunted in packs.
The power of flight
The earliest known bird - Archaeopteryx - comes from the Late Jurassic period in Germany. By the Early Cretaceous period, birds had radiated and diversified. Fossils have been found in Spain and China. Most of these early birds look very primitive and some still have teeth. But by the Late Cretaceous period they looked look far more modern and had diversified into different niches. Some wading or diving birds had even lost their power of flight again.
By the late Cretaceous, birds looked very like they do today.
A world of colour
In the plant kingdom there were great changes too. Flowering plants appeared in the Late Jurassic period, and in the Early Cretaceous period the first flowers appeared. Soon flowers and flowering plants were to become the most common plants on Earth.
The earliest known fossil flower is very small. It was found in the Wealden Formation sediments of southern England. During the Early Cretaceous period, southern England was a low-lying, richly vegetated river floodplain. The climate was humid and sub-tropical, although there may have been wet and dry seasons, as there is evidence of forest fires. By the Late Cretaceous period the world would have looked rather more familiar to us, although there were still no grasses.
| THE GIANT CLAW
Seventy-five million years ago, the Mongolian desert was home to dinosaurs of all shapes and sizes. But none were more astonishingly bizarre than Therizinosaurus. This creature was the proud owner of the largest claws of all time, which measured around 70 centimetres in length.
This animal was part of a group of dinosaurs called theropods, making it a distant relative of ferocious meat-eaters like Tyrannosaurus and Velociraptor. But unlike other theropods, Therizinosaurus was a vegetarian, a quality which also accounts for its strange appearance.
Therizinosaurus roamed the conifer forests on the edges of the desert, using its long neck to stretch for leaves growing on tall trees. It had a large stomach in order to digest all this tough vegetable matter, giving its belly a characteristic swollen appearance.
Intriguingly, scientists think its huge claws were an adaptation to a vegetarian way of life, rather than deadly weapons. Therizinosaurus may have used its huge claws to pull tree branches towards its mouth in order to feed on the leaves.
But its claws might also have been used for mating displays, and some scientists think they could have been used for defence against attack by predators.
The Mongolian desert in the Late Cretaceous was home to fearsome predators. Perhaps the most terrifying of these was Tarbosaurus, an Asian relative of Tyrannosaurus that stood a lofty five metres tall and weighed a whopping five tonnes.
Fast and deadly
Tarbosaurus prowled the broken woodland and desert fringes, pursuing lone or vulnerable dinosaurs, or lying in wait to pounce on them. Though it was heavy, Tarbosaurus' powerful leg muscles made it a good sprinter.
However, experts believe that it probably didn't try to attack Therizinosaurus, as the giant-clawed herbivore was probably too big, even for an awesome carnivore like Tarbosaurus.
A common sight around this desert habitat was Protoceratops, a stout, lumpy dinosaur about the size of a pig. Herds of these creatures were to be seen everywhere. In fact, they were so abundant that experts have dubbed them 'the sheep of the Cretaceous.'
Protoceratops was almost certainly at the bottom of the food chain and would undoubtedly have been preyed upon by almost every carnivore that stalked the Mongolian desert, including Tarbosaurus and a vicious little creature called Velociraptor.
We know this dinosaur hunted Protoceratops because an extraordinary fossil found in Mongolia in 1971 shows a Velociraptor locked in combat with a Protoceratops. It is thought that the dinosaurs were smothered in a sand storm while in mid-combat.
At two metres in length and weighing in at just 20 kilogrammes, Velociraptor was small in comparison with Tarbosaurus, but it had a deadly array of weaponry that it used to inflict lethal damage on its prey.
The most lethal of all these weapons was a set of razor-sharp claws on Velociraptor's hands and feet, including an enlarged second toe claw that was used to rip at the flesh of unsuspecting animals.
Small animals like Protoceratops had more to fear from packs of Velociraptor than from big predators like Tarbosaurus.
| New Blood - Life in the late triassic
The late triassic - image from Walking with Dinosaurs
At the dawn of the age of the dinosaurs, the world was very different. Dr Jo Wright, scientific advisor to the BBC One series Walking with Dinosaurs describes life in the late triassic, in the first of eight articles about the age of the dinosaurs.
When dinosaurs first appeared about 230 million years ago the world was very different. There were very few of the animal groups we recognise today - no mammals, no birds and no lizards. But there were some lizard-like reptiles.
What? No grass?
The difference was also apparent in the plant kingdom. Plant life would have seemed very drab, just green and brown in colour. There were no flowering plants, so nothing like most of the common trees and shrubs today. What trees there were would have looked different, though some were relatives of modern day ferns and podocarps. There was no grass. Instead, low ground cover would have been ferns and mosses.
The Triassic world was unusual for another reason. About 20 million years before the appearance of the first dinosaurs, the biggest extinction the world had ever known had occurred. Over 90% of all plant and animal species then alive on land and in the sea had died out at this time. Even in the Late Triassic the world was still recovering, and there was not the usual variety of life normally found on earth.
It took more than 10 million years before ecosystems recovered and complex systems and larger animals took even longer. Most of the dominant land animals that were around when dinosaurs evolved were products of long and established lines of descent.
A giant desert
The continents of the triassic Earth were configured differently to today. All the land masses on the planet were joined together into one huge continent called Pangaea. This stretched from pole to pole and its central region was a vast inhospitable desert. We know this because the type of rocks that were deposited at this time have sedimentary features characteristic of a dry harsh climate.
As all the continents were connected, the animals and plants found in the fossil record from that time are very similar all over the world.
Peteinosaurus caught insects in its pin-like teeth.
The Late Triassic was an innovative time in the animal kingdom. By the end of the period not only the dinosaurs had appeared but also pterosaurs (flying reptiles), various kinds of marine reptiles, the first crocodiles and turtles, and the earliest true mammals.
Towards the end of the Triassic, 220 million years ago, there was another extinction, which wiped out many of the non-dinosaurs including the dicynodonts such as Placerias and primitive archosaurs such as Postosuchus. It was after this that dinosaurs really started to radiate and diversify.
Dinosaurs gain the edge
It was often assumed that the dinosaurs survived due to their superior speed and agility. We now think they were simply fortunate because they were not hit as hard by extinction. After the extinction at the very end of the Triassic, the dinosaurs were the only large land animals left.
| SPIRITS OF THE ICE FOREST
Modern reptiles are cold blooded. However, a series of startling finds suggests there may have been dinosaurs living at the poles. What does this tell us about the possibility of warm-blooded dinosaurs. Dr Jo Wright investigates.
The idea that dinosaurs lived at the poles is based on remarkable finds made in Australia. There are two clues that Australia was once within the Antarctic Circle. Firstly, we can can determine at what latitude rocks formed from the orientation of magnetic particles within them. Secondly, evidence that the climate was seasonally cold comes from both plant fossils and sedimentary structures which form when the ground freezes.
A better picture
The fossil sites in Australia are remarkable because several different animals have been found, giving us a fuller picture of the palaeoenvironment (life in the area at the time).
At least some of the animals must have been year-round residents - Leaellynasaura were too small to have migrated hundreds of miles in and out every year. Leaellynasaura may have had a special adaptation for life in the polar regions.
Their skulls seem to have had especially large eye sockets. Their large eyes may have allowed them to see better in the continuous low light levels of the polar winter.
Hanging in there
Some of the fossil vertebrates at these sites are very important because some very primitive animals that had become extinct elsewhere seem to have survived here. Labyrinthodont amphibians, like Koolasuchus, were previously thought to have died out over 100 million years earlier. In this environment it occupied a crocodilian niche, as the climate was too cold for crocodiles.
The earliest known dinosaur found in polar palaeolaltitudes is called Cryolophosaurus (which means 'frozen crested reptile'), and was found in Antarctica. It is a meat-eating dinosaur but we do not know whether it migrated in during the summer months, or whether it lived there year round.
Dinosaurs from polar latitudes have also been found in Alaska, but they are very similar to those from further south and are probably just a migrant population.
Dwarf Allosaur - Terror of the Antarctic - the largest killer there
The existence of dinosaurs at polar latitudes is significant. It means one of two things. Either dinosaurs had to hibernate or go into an inactive state in the polar winter. Or they had some way of maintaining a high body temperature - ie they were warm blooded.
Many people think that some, if not all, dinosaurs were warm blooded. These polar discoveries are strong evidence for warm-bloodedness in at least some dinosaurs.
A CRUEL SEA
Ophthalmosaurus from the series Walking with Dinosaurs
The Mesozoic era truly was the age of giants. While huge dinosaurs dominated the land, large marine reptiles ruled the seas. Dr Jo Wright describes life in Jurassic seas.
Late Jurassic Earth
The Ichthyosaurs (fish reptiles) appeared much earlier than the dinosaurs. They are first found in the Early Triassic, and they are already very specialised, with limbs modified into flippers.
Ophthalmosaurus - Large eyes help it hunt in deep, dark waters
By the Jurassic, ichthyosaurs looked very like dolphins. They even had a dorsal fin and a big vertical tail fluke - we know this from some fossils in Germany which have the body outline preserved as a carbonised film.
Ichthyosaurs propelled themselves through the water with strong side to side movements of their tails, steering with their flippers.
The rise of plesiosaurs
Plesiosaurs arose in the Late Triassic but became very numerous in the Middle and Late Jurassic. They used their flippers to move through the water. There were two kinds of plesiosaurs. The large short-necked pliosaurs were the top predators of Jurassic seas. Long-necked plesiosaurs probably fed on small fish and other small prey. We known they ate squid-like animals because parts of belemnites have been found in their fossilised stomachs.
As well as large ichthyosaurs and giant pliosaurs, other reptilian denizens of the Middle-Late Jurassic seas were the marine crocodiles. There were a couple of different types, one of which was so adapted to life in the seas that their limbs had turned into flippers and they even had a fluke on their tails to help them push themselves more efficiently through the water.
Ammonites - Large specimens weighed up to 100 kg
Ammonites are very common fossils in Jurassic rocks. When they were alive, these molluscs looked as if a small squid had been stuffed into a spiral shell. They were very successful in the Jurassic although they were probably rather slow moving; we know from fossil stomach contents that they ate crinoids (sea lilies) - animals that are attached to the seabed.
Still a mystery
Mesozoic marine reptiles are already so specialised when they are first found in the fossil record that it is difficult to trace their ancestry. We have very little idea which groups of land reptiles ichthyosaurs and plesiosaurs descended from, but this only makes them all the more intriguing.