an introduction to dinosaurs

5. The jurassic period

The Jurassic period began around 200 million years ago and spanned 50 million years to the beginning of the Cretaceous period, 145 million years ago. The climate was warm, with no significant ice caps at either pole. Much of modern day western Europe was submerged beneath shallow tropical seas which is why we find extensive marine fossil deposits from the Jurassic at famous sites such as the Jurassic Coast along the southern coast of England and the lagerstatten of Holmaden and Solnhofen in Germany. Jurassic outcrops are much less abundant in North America, with notable exceptions such as the famous Morrison Formation.

It was during the Jurassic period that dinosaurs diversified and became the dominant land vertebrates of their time. Sauropodomorphs, small dinosaurs that walked around on two legs, evolved in the Late Triassic around 208 million years ago. The extinction event at the end of the Triassic saw some of them evolve over the course of the Jurassic into the largest land animals that ever lived, the Sauropods.

The Jurassic also saw the rise of smaller Ornithischian dinosaurs such as Stegosaurus and saurischian 'lizard-hipped' dinosaurs such as Allosasaurus and Megalosaurus. Dinosaurs also took flight during the Jurassic, with the evolution of some of the first birds.


One of the most famous Jurassic age dinosaur bearing rock formations is the Morrison Formation in North America. The formation dates to around 155 - 147 million years and has outcrops in many states, but is mostly centered around Wyoming and Colorado. Its age is similar to to that of the Lourinhã Formation in Portugal and the Tendaguru beds in Tanzania. All were closely situated in the Late Jurassic and share some of the same fauna; it's believed dinosaurs may have migrated between the three areas.

The majority of the sediments found in the Morrison Formation were deposited by rivers, lakes, swamps and mudflats.These sediments contain abundant fossils, mostly notably those include many different types of dinosaur. Dinosaur National Monument in Utah is the site of a quarry where numerous dinosaurs were found in the early 1900s, including 23 mountable skeletons. The base of the quarry which contains a large jumble of dinosaur bones remains in place and forms one wall of the monument's Dinosaur Quarry building. Many of the dinosaur fossils at the monument were found as jumbled accumulations of bones, likely the result of the carcasses being transported along rivers and streams before their burial.



One of the most famous carnivores of the Morrison Formation, Allosaurus was a large bipedal (walked on two legs) predator which likely reached over 39ft in length and had a sizeable skull equipped with dozens of razor sharp teeth. With its powerful hindlimbs and a heavily muscled tail, Allosaurus likely preyed upon large herbivors that shared its environment, such as Stegosaurus, ankylosaurs and sauropods.

Allosaurus jummadsei skull (MOR 693) (Credit: Daniel J. Chure, Mark A. Loewen)
Fragmentary skeleton of a juvenile Ceratosaurus from Bone Cabin Quarry, Wyoming


Ceratosaurus is another dinosaur found in the Morrison Formation. It was a medium sized bipedal carnivore, likely reaching lengths of up to 23ft. It lived alongside larger carnivores such as a Allosaurus and Torvosaurus, and had a skull with long blade-like teeth, and horns on its nose and above its eyes (which were most likely for display rather than combat). Ceratosaurus probably avoided competition with larger predators by preferring different types of prey or deploying different feeding strategies.


Stegosaurus was a large, herbivorous quadruped (walked on four legs) with broad upright plates lining its back and a spiked tail. Large individuals could grow up to 30ft in length. Today it is mostly agreed that the plates were used for display and to regulate heat, while the spiked tail was used to defend against predators. Among its predators were Allosaurus and Ceratosaurus. The first Stegosaurus was discovered in 1877 by Othniel Marsh and to date over 80 individual animals have been found. Stegosaurus is one of many herbivorous dinosaurs found in the Morrison Formation.

A 1901 life restoration of Stegosaurus by Charles R. Knight


Diplodocus, another famous dinosaur from the Morrison Formation, was a very large, long-necked quadruped dinosaur with a small head and a whip-like tail. Diplodocus is one of the longest known dinosaurs, with some estimated to reach over 100ft. Their necks were so long that it has been suggested these animals would have either required a huge heart (around 1/10 of their overall weight) or auxiliary "hearts" in their neck to help pump the blood. Others argue the animals almost horizontal posture would have helped supply blood to the brain.

6. The Cretaceous period

The longest and last period of the mesozoic era.

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