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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.
Allosaurus
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.
Sauropods
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.
Apatosaurus
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!
Brachiosaurus
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.
Camptosaurus
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.
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.
Dryosaurus
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.
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