EVOLUTION OF THE DINOSAURS

What follows is a passage through nearly 350 million years in the history of the planet earth. The various geological periods mentioned below are not just strange names; they represent the divisions of geological time through which the evolution of the dinosaurs and their ancestors can be traced through the rock record... Click below to select the period that interests you most, or just scroll down through the geological time periods...

THE DEVONIAN PERIOD (about 410 to 360 million years ago)

THE FIRST LAND ANIMALS EVOLVE

The ancestry of all land animals, including the dinosaurs, can be traced back over 400 million years to the Devonian Period. During this period some animals moved to land from the water. The reason for this is that the land had food resources that had not been exploited. A Devonian lobe finned fish called Eusthenopteron evolved the limb bones in its fins that were later necessary for the transition to land. Early amphibians, such as Ichthyostega and Acanthostega were probably closely related to the lobe finned fish Panderichthys, (perhaps the ancestor of all land-living animals). The evolutionary tree below shows the possible relationships of these early fish.

THE CARBONIFEROUS PERIOD (about 360 to 290 million years ago)

THE TETRAPODS RADIATE

The main phase of evolution for the tetrapods occurred during the Carboniferous. A wide range of early tetrapods are known, such as the aquatic Diadectes and the bizzare Diplocaulus with its boomerang shaped head. Large terrestrial amphibians such as Eryops also evolved. Reptiles evolved from amphibians when some forms such as Seymouria 'split' from the amphibian lineage. This 'splitting' pattern explains how amphibians and reptiles were able to evolve seperately along different 'branches', and is the reason why reptiles and amphibians exist together today, even though one evolved from the other. Seymouria possessed characters that were intermediate between amphibians and reptiles. A major evolutionary step occurred with the development of the amniotes, animals able to lay shelled eggs, as this allowed animals to reproduce out of water for the first time. The evolutionary tree below shows the relationships of the early tetrapods.

THE PERMIAN PERIOD (about 290 - 250 million years ago)

REPTILES FIRST DOMINATE THE LAND

During the Permian, reptiles populated the land. At this time, most of the continents were at high, rather cold latitudes. The 'mammal-like' reptiles (pelycosaurs) included such well known forms as the 'sail backs' Dimetrodon and Edaphosaurus (up to 3m long) and were the most common tetrapod in the Early Permian fauna. Other groups living at this time include the therapsids, which were a diverse group including Moschops (up to 5m long) and Dicynodon. It is important to realise that the lineage of animals that eventually led to the dinosaurs (known as the diapsids) were rare in the Carboniferous and Permian - they showed no sign of their later rise to dominance. They included forms such as Protorosaurus, and the remarkable Coeleurosauravus that was able to glide from tree to tree using skin covered ribs in a similar way to the modern 'flying lizard' (Draco).

At the end of the Permian, the greatest mass extinction (so far) in the history of life wiped out 75% of all tetrapod families. The cause of the mass extinction is not known for certain, but the lineage of animals that led to the dinosaurs, along with some other groups, survived. This great extinction marked the end of the Palaeozoic, one of the great geological divisions of time. The Mesozoic era that followed it was to see the evolution of the dinosaurs themselves. The Mesozoic is divided into three 'periods' of geological time when the dinosaurs lived. These periods are the Triassic, Jurassic and Cretaceous.

THE TRIASSIC PERIOD (about 250 to 210 million years ago)

THE EVOLUTION OF THE ARCHOSAURS AND EARLY DINOSAURS

During the Triassic period, all the continents of the earth were joined together to form a supercontinent called Pangaea. As the continent was so large, much of the interior was a long distance from the sea, which resulted in very dry, hot and arid climates. The first dinosaurs were to appear in the late Triassic. In the Early Triassic, however, land ecosystems were dominated by the immediate ancestors of the dinosaurs, the archosaurs.

THE FIRST DINOSAURS

Those archosaurs most closely related to the dinosaurs are forms such as Marasuchus. The detailed evolutionary relationships are still debated, but by the late Triassic, several early theropods are known, as the dinosaurs rapidly diversified. These dinosaurs, such as Eoraptor, Coelophysis and Herrerasaurus were all carnivores, and, despite their diversity, were quite rare at this time.

The rise to prominence of the dinosaur lineage, it is suggested, took place as the early dinosaurs took advantage of habitats left vacant after other extinctions at the end of the Triassic period. Environmental changes at this time affected other reptiles more than dinosaurs. This dominance of the land by the dinosaurs was to last throughout the remainder of the Mesozoic. To understand the further development of the dinosaurs, it is necessary to look at the classification of the dinosaurs.

During the Triassic, the carnivorous saurischian dinosaurs such as the sleek Coelophysis (meaning 'hollow form', reflecting the bone structure) and the larger and more robust Herrerasaurus occupied the niche previously occupied by the carnivorous archosaurs. Other saurischians showed an evolutionary trend towards greater size, such as the prosauropod Plateosaurus, and became adapted to a herbivorous diet.

THE JURASSIC PERIOD (about 210 to 150 million years ago)

DINOSAURS DOMINATE THE LAND

During the Jurassic period, the supercontinent Pangaea was beginning to break up due to the mechanisms of plate tectonics (or 'continental drift'). This allowed narrow seaways to spread between the continents, but land links still existed that allowed the dinosaurs to spread throughout the continent. The extinction of the mammal-like reptiles at the Triassic-Jurassic boundary allowed the dinosaurs to diversify rapidly in the Early Jurassic.

The mild Jurassic climate (though still warm) allowed the largest dinosaurs to evolve. The prosauropods continued into the Jurassic and showed an increase in size (a common trend in the fossil record). The 10m long sauropodVulcanodon shows numerous changes to the skeletal structure (for example, in the hip) to accomodate the great increase in weight. This animal laid the foundation for the true dinosaur giants, which evolved early in the Jurassic and became distributed worldwide. From these animals, more advanced sauropods such as Brachiosaurus evolved, which had a very long neck and elongated forelimbs, an adaptation to reach high vegetation. Other sauropods were adapted to browsing on lower vegetation. The largest of all sauropods, (such as Seismosaurus) are also of Jurassic age.

Though the sauropods were saurischian, other herbivores at this time evolved from the ornithischian line, such as Stegosaurus and Hypsilophodon. Large ornithischian herbivores such as Scelidosaurus evolved a turtle-like 'beak' for the close cropping of low plants. Primitive iguanodonts such as Camptosaurus also showed features adapted to low grazing.

The largest carnivore of the early Jurassic was the double-crested Dilophosaurus, (which DID NOT have a 'frill' or spit poison as a well known film would have us believe!) up to 6m in length, which evolved from earlier ceratosaurs such as Coelophysis. The carnosaurs were much larger predators, and include animals such as Allosaurus.

THE CRETACEOUS PERIOD (about 150 to 65 million years ago)

THE GREATEST DIVERSITY OF DINOSAURS

By the Cretaceous, the distribution of the continents was becoming more similar to the present day. The climate was very warm, the Atlantic ocean opened, and the seaways were more extensive. The separation of landmasses isolated certain groups of dinosaurs and allowed them to evolve separately from each other as they were unable to breed and exchange genes. This isolation allowed the tremendous diversity of Cretaceous dinosaurs to evolve.

The large saurischian sauropods of the Jurassic were rare, however. The majority of the herbivores that replaced them were smaller ornithischian dinosaurs which showed a greater diversity of form, the ornithopods. The replacement of the sauropods by the ornithopods probably reflects the influence of environmental stresses on the large sauropods. Perhaps the large sauropods may have had some difficulty in adapting to a diet of flowering plants that evolved in the Cretaceous. Iguanodon was the largest of the ornithopods, and in the early Cretaceous evolved a wide variety of specialisations in related to its diet. Large herbivores such as Tenontosaurus were closely related. The nodosaurs were the successors to the Jurassic stegosaurs, and, like the iguanodonts developed a horny 'beak' for grazing on low plants.

Cretaceous carnivores included the ostrich-like Ornithomimus, and the 3m long theropod Deinonychus. The teeth and bones of a Deinonychus were found near the bones of a much larger Tenontosaurus, which may suggest they preyed on the ornithopod. Deinonychus evolved several specialised structures associated with its carnivorous way of life, including a large retractable 12cm claw on the second toe, used to slash at prey animals.

During the Late Cretaceous, the largest terrestrial carnivores of all evolved, such as Tyrannosaurus (up to 14m long), the tiny, but very strong forelimbs of which were specialised; their purpose remains debated. Equally large, though less well known was Carcharodontosaurus. The great diversity of form that evolved among Cretaceous theropods is underlined by Carnotaurus, which evolved a short, bulldog-like snout, and two horns over the eyes.

Also at this time, the 'duck-billed' dinosaurs (the hadrosaurs and lambeosaurs) came to prominence. These were related to the iguanodonts and filled a similar niche, as low level grazers. Lambeosaurs became very diverse and the skull developed many different 'ornamental' crests. These elaborate crests may have been used as visual signalling devices (rather similar to the way modern peacocks use their tails). One group evolved hollow crests, and this has led some authors to suggest that they were used as a kind of 'trombone' to add sound to the visual display, for example, Parasaurolophus. The well known horned dinosaurs, such as Triceratops evolved from the beaked, Early Cretaceous Psittacosaurus. The elaborate horn development may have served as a defensive weapon, a signalling device, or may have been used during rutting contests, similar to the antlers of modern deer.

DINOSAUR PHYLOGENY

This evolutionary tree shows the relationships of the main dinosaur groups.

THE EXTINCTION OF THE DINOSAURS

The dinosaur extinction at the end of the Cretaceous is perhaps the best-known event in the fossil record. A great deal of research has gone into answering the question: why did some animals become extinct and not others? To attempt to answer the question, evidence must be considered for several factors, such as the effect of meteoritic impact, climate change and the timing and rate of extinction. The dinosaurs were in decline at the end of the Cretaceous, though Late Cretaceous dinosaur sites are heavily biased towards North America, so at the present time, the global picture of dinosaur extinction remains controversial.

To find out more about the extinction of the dinosaurs, click here

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