2. The evolution of dinosaurs
By Hema Sura and Jennifer Hale

3. DINOSAUR RADIATION
The dinosaur radiation began in the middle Triassic 230 million years ago.
By the end of the period, dinosaurs were globally dominant.
The first dinosaur is believed to have been the archosaur.
“ the ruling lizard”
They were carnivores that were initially quadrupeds.
Eventually they evolved into bipeds.
It is hypothesized that they adapted this trait in order to reach higher for feeding.
They were low slung with long jaws and tails.
These first dinosaurs were bipedal predators that were one to two meters long.
All other dinosaurs evolved from the bipedal archosaur.

4. Three Main Clades of Dinosaurs
From the archosaur, evolved three main clades of dinosaurs:
Ornithischians
Sauropodomorphs
Theropods
Each clade evolved different characteristics that set them apart from the rest.

5. The Rise of the Dinosaurs
There are 2 possible explanations for the rise of the dinosaurs:
Dinosaurs out-competed their rivals.
Had superior design compared with the animals of the time period.
However, the dinosaurs did not become dominant until the late Triassic.
This means they slowly evolved their superior designs and better adaptations.
Opportunistic replacement
The dinosaur radiation began after the great Permian extinction.
The dinosaurs took advantage of a land left vacant after so many animals became extinct.
This is the more plausible explanation.

6. The Ornithischians By the end of the Triassic, some acquired armor.
One reason for the horns, plates, and spikes was for defense. These characteristics might have made the dinosaur appear bigger than it really was in order to scare off predators.
These dinosaurs were believed to have been the prey of the Theropods
The second possible reason was to attract mates.
During the Jurassic period some evolved into quadrupeds.
The adaptation of quadrupedalism is due to an increase in body size.
Four legs were needed in order to hold the large body up.

7. The Ornithischians
Received the name because the ischium in the pubis points down and towards the tail.
This adaptation evolved in order to support the dinosaur while moving.
These dinosaurs evolved into herbivores acquiring tooth-to-tooth occlusion and a holding space that functioned as a cheek.
There was a wide variety of plants like ferns, cycads, ginkgophytes, and gymnosperms during the Mesozoic era.
Because of this wide variety of plants, it is believed that these dinosaurs evolved into herbivores.
                                                               

10. The Sauropodomorphs
The first Sauropodomorphs evolved into herbivores with the increase in the variety of plants.
These dinosaurs also increased their body size.
In order to escape predation from their predators, the theropods, the Sauropodomorphs adapted a larger body size.
They also evolved into quadrupeds due to the increase in body size.
Most Sauropodomorphs acquired long necks and forelimbs.
This adaptation was either for high browsing among the trees or having a large range for feeding horizontally. They also aided in feeding without having to move their massive bodies.
The long tails are believed to have helped the dinosaurs to not fall on their faces. They helped them to balance.

11. The Theropods
The Theropods evolved bones with large medullary cavities.
This adaptation made these dinosaurs have light body masses so they could run quicker.
This allowed for the evolution of birds.
They evolved a three-fingered hand.
This adaptation allowed the Theropods to grasp with a partially opposable thumb.
Theropods were carnivorous.
They did not evolve into herbivores possibly because they could get all their resources from their prey.
They were bipeds.
Two legs enabled these dinosaurs to run quicker and out run their prey

14. Other Reasons for Diversification Among Dinosaurs
During the middle Jurassic, the supercontinent Pangea was beginning to break apart.
At this time, dinosaur faunas were relatively similar.
There was still land links to allow dinosaurs to go back and forth.
By the Cretaceous period, Pangea was increasingly breaking up.
As a result, dinosaur became highly differentiated.
The separation of northern Laurasia and southern Gondwana, resulted in ten major land areas.
The differentiated fauna of dinosaurs can be explained by three processes: vicariance, regional extinction, and dispersal.

15. Other Reasons for Diversification Among Dinosaurs
Vicariance is the splitting of lineages in response to the emplacement of a geographic barrier.
The breakup of Pangea could have caused differentiated fauna.
However, the fauna must have lived during the time of the breakup.
Regional extinction is the disappearance of a taxon from one or more geographic regions whose former presence was clearly seen by fossils.
Smaller bodied dinosaurs were quickly replaced in certain areas by larger bodied dinosaurs.
Dispersal is the crossing by a taxon of geographic barriers.
Dispersal between Northern Laurasia and southern Gondwana occurred during the Cretaceous time period.
There was also the Bering passage that allowed occasional passage of a few individuals of a species.

16. Reasons for Extinction
Most of the dinosaurs became extinct during the Cretaceous-Tertiary Extinction 65 million years ago.
Only five families of crocodiles and most non-avian dinosaurs (birds) survived the extinction.
Hypothesis for reasons for the extinction:
Extensive volcanoes
Huge asteroid impacts
Decrease in sunlight which reduced photosynthesis
An infectious disease killed the dinosaurs.
Parasitic protozoans have been found in some dinosaur fossils.
It eats away at the jaw and can cause the host to starve to death.

17. Works Cited
"Evidence indicates that a single impact killed the dinosaurs." World Disease Weekly 19 Dec. 2006: 38. Health Reference Center Academic. Web. 22 Feb
Farlow, James O. "Dinosaur Biology." Annual Review of Ecology and Systematics 26 (1995):
Padian, K., and J. R. Horner. "The evolution of 'bizarre structures' in dinosaurs: biomechanics, sexual selection, social selection or species recognition?." Journal of Zoology (2011): Academic Search Premier. EBSCO. Web. 22 Feb
Sereno, Paul C. "The Origin and Evolution of Dinosaurs." ANNUAL REVIEW OF EARTH AND PLANETARY SCIENCES 25 (1997):

18. Questions?