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Dinosaurs - Jurassic Gallery

208 million - 144 million years ago
The Jurassic Period was the height of the Age of the Dinosaur and it lasted about 64 million years. By this time, the super continent Pangaea was breaking up and North America shifted to become a continent now called Laurasia, a combination of North America, Europe and Asia. These massive geologic events saw the western region drop in elevation. The western states of North America were at first cover by ocean water, with the Pacific shore lapping up against what is now the Sierra Nevada, making Arizona and Nevada nearly oceanfront land. The Southwest grew drier, deserts formed on the near sea-level land. Mountain ranges began to rise up as a result of the pressure of shifting landmasses, creating dry ridges surrounded by large river basins.

Millions of years passed. In the Late Jurassic the Southwest's elevation rose and tilted, running the broad rivers from mountains in Wyoming in the north, down across the Southwest, to reach the sea at the Gulf of California. This became the Colorado River and its many tributaries in rivers carved the remarkable canyon lands we see today. Far to the west, volcanoes were actively pushing up through the earth's crust and low seas. There were still desert areas in the Southwest, but more and more the climate once again grew wet and humid, encouraging more tropical and forest plants to grow. Sequoias, ponderosa pines and giant ferns flourished.

When is a Dinosaur not a Dinosaur?

The Pleiosaur

For many years, no one had ever found a dinosaur bone in California. It was assumed that because these westernmost of the southwestern lands had been underwater, there never would be a dinosaur find. But a geologist, Richard Hilton, searching for ammonite seashell fossils in the foothills of the Cascades, just west of Mt. Shasta, came upon some large bones that he took back to Sierra College and stored in his lab. A few years later, the college hired a paleontologist from Utah who took one look at the bones and knew the enormity of the discovery. These were bones dating from the Jurassic period. Further investigation revealed that this 50 foot long, paddle-swimming creature was a sea-going reptile as large as a dinosaur, but not actually a dinosaur. This Pleisaur was a good swimmer, enjoyed the seas, and probably kept company with ichthyosaur that were the dolphin shaped marine mammals of the time.

Jurassic Dinosaurs
On other continents many dinosaurs from the early Jurassic have been found, but only three dinosaurs from the early Jurassic have ever been found in North America. All three were discovered in Arizona. These were two legged, meat eating Theropods. Presumably the large ocean covering and drier climate in the Jurassic made it hard for more dinosaurs to survive in the southwest. But also, there are only a few areas of exposed early Jurassic rock in the southwest. This proves that access to the right geologic strata - rock layers - is important in finding fossils from all periods of the Mesozoic. We only know what we can see .. and think how much still lies buried waiting to be discovered.

Scientists have access to more Late Jurassic rock in North America, and they have been able to discover over 17 dinosaurs from this later period. In these finds, they have discovered the giant sauropods with four huge legs that were long necked plant eaters. These creatures became the dominant animals of the later Jurassic in the Ancient Southwest.

In addition to the lizard-hipped dinosaurs found in the Jurassic, there have also been a few of the bird-hipped dinosaurs found - called ornithischians - in the later Jurassic.

The Allosaurus was a huge theropod that terrorized the Ancient southwest in the late Jurassic. He was a towering giant at 15 feet tall. He weighed 4 tons and was 35 feet long nose to tail. Standing on massive hind legs, his hips were eight feet above the ground and he could outrun a man today. He had fierce claws on his hind feet and hands and sharp 2-4' serrated teeth for ripping meat that filled his three-foot long head. He had very large eyes for a dinosaur and a convenient protruding brow that protected the eyes from the sun, increasing his long distance vision. The Allosaurus lived in the mountain and jungle regions that became common in the Late Jurassic.

In spite of its fearsome presence, the Allosaurus may have provided very thoughtful care to its young in nurseries. Scientists think a newly hatched Allosaur would have been less than 20 inches long, completely unable to fend for itself, and was most likely raised in protective nurseries.

The smaller Prosauropods of the Triassic were replaced by the huge Sauropods by the mid Jurassic in the Southwest. Sauropods hade four massive tree trunk-like legs much like an elephant. They had very long necks and long strong whip like tails. Their heads were small and their blunt teeth made them well suited for eating plants, not flesh. They were suited to live on land and swim in swamps and rivers, allowing them to reach a variety of land and water plants for food. In fact, paleontologists have found twigs and needles from sequoias, pine, and fir trees in the fossilized Sauropods stomach.

In addition to studying dinosaur bones, scientists called Ichnologists also study their tracks. An example is a group of fossilized Sauropod tracks found in Texas. These showed that they moved in herds, probably keeping their small offspring in the center (as elephants do) surrounded by the larger adults. Ichnologists have also found fossilized impressions of Sauropod skin - it had coarse granular skin like a lizards.

One of the largest dinosaurs of all time was the Apatosaurus, often called the Brontosaurus (meaning thunder lizard). The correct scientific name first given this find was Apatosaurus. The Apatosaurus could measure 75 feet in length and 15 feet at the hip! She could weigh 30-40 tons and have a tail 30 feet long. The Apatosaurus neck was 20 feet long and it enabled the dinosaur to reach high into the treetops for food. How high? Add the shoulder height of 10 feet to the neck length of 20 feet, and then add the two or more foot might of the head to the front teeth, and you have a feeling for her height.

The Apatosaurus head was about the size of a horse's head and her teeth could pull off but not rip or grind its plant food. Moving in herds across the Southwest, it lived as far North as Montana and south down into Baja California. The Apatosaurus footprint was nearly three feet long and over two feet wide - each!

Another type of Sauropod to roam the Southwest in ancient times had a feature different than this relative the Apatosaurus. The Apatosaurus had front legs and shoulders that were closer to the ground than its hind legs, causing it to lean forwards when walking. In contrast, the Brachiosaurus had front legs and shoulders that were much higher of the ground raising its shoulder up higher for a better view of the landscape. Brachiosaurus means 'arm lizard' and denotes its long front legs.

While named after its long arms, it had another remarkable characteristic. The Brachiosaurus was one of the largest land animals ever to live on the earth. Measuring 75-80 feet in length, the top of its head could tower 40 feet above he ground. Weighing up to 80 tons, it was twice as big as the Apatosaurus. Scientists think the Brachiosaurus may have lived for 200 years or more as a normal life span in order to grow to reach this huge size.

The Other Dinosaurs
The Jurassic Period is when the other type of dinosaurs - the bird-hipped - lived in the Ancient Southwest. There are four types of these Ornithischians: Ornithopods that walked on two legs most of the time; Stegosaurus, the plated dinosaur; Ankylosaurus, an armored dinosaur; and Ceratopsians, a horned dinosaur. Only the Ornithopods (two legged) and Stegosaurs (plated) lived in North America.


This was the most common dinosaur of its type in North America, and it roamed the west in large herds, browsing on low plants. They came in many sizes, from as small as a turkey to 17 feet long and 7 feet in height. Camptosaurus means 'bent lizard' and this refers to the suggestion that while walked and ran on his hind legs keeping his body vertical, he then bent over and used his five fingered hands to gather food. His hind legs were powerful and he was probably a swift runner helping it survive into the Cretaceous Period. He did not have front teeth, but had a beak instead! In fact his descendents are the duck billed dinosaurs of the Cretaceous. The Camptosaurus roamed the open Jurassic plains in herds, in company with the Stegosaurus.


The most famous of the Ornithischians is the plated Stegosaurus. Stegosaurus walked on all four legs, a peaceful plant eater roaming he highlands of the west in small herds, grazing on low ground plants such as horsetails and ferns. She could be 25 feet nose to tail, weigh 2-3 tons, and stood 11 feet at the hip. But this fair sized beast appears to have had a brain the size of a golf ball in a head only 16 inches long! Two rows of thin bony plates ran from the back of the head, down along the spine, down to the tail. The tallest plates were 2 feet in height, positioned over the hips. Scientists speculate about what these plates were for - to look larger? To attract a mate? To help her cool off? What do you think they were used for? The end of her tail had four spikes, each a foot long. She could swing this tail and maim her predators, and important asset since her legs did not allow her to run fast.

Scientists have discovered tow small Stegosaurus baby fossils at the dinosaur National Monument in Utah. The babies did not have plates - perhaps they grew them when they were older? Or maybe our she-Stegosaurus was actually a he-dinosaur, and the females did not from the plates at all? If so, maybe the babies were females? They lived for 50 million years to the end of the Jurassic in Colorado, Utah and Wyoming.


Another bird-hipped dinosaur of the Southwest is the Dryosaurus, a peaceful little plant eater. This creature was graceful, slender, and while it walked on hind legs, its body was carried horizontally. Its hind legs were slender with a three toed, forward facing foot, adding to its ability to run swiftly. The Dryosaurus, 'oak lizard', is named for the shape of its plant eating teeth shaped much like an oak leaf. It had a slim snout, large eyes, and more delicate features than many of its Jurassic neighbors. The Dryosaurus lived in open glades near woodlands in Jurassic Colorado and Wyoming, a peaceful seeming creature equipped to run, not fight.