Conclusion

The rise of the first dinosaurs is a story of opportunity and evolutionary innovation. The first dinosaurs, after having lived in the shadows of larger reptilian kin, took advantage of major mass-extinction events at the end of the Triassic Period to move into niches once occupied by their predecessors. In the course of doing so, dinosaurs quickly radiated to widespread geographic ranges and began an evolutionary course that led to the development of two major clades of dinosaurs, the Saurischia and Ornithischia.

The Late Triassic and Early Jurassic Epochs were times of experimentation in dinosaur evolution, as the earliest herbivorous and predatory dinosaurs adapted and evolved increasingly specialized body forms and lifestyles. Among the saurischians, carnivorous dinosaurs—having begun as small, lightweight bipedal creatures such as Eoraptor—perfected a body plan that provided mobility and quickness coupled with a variety of weapons such as bladelike teeth and recurved claws on their hands and feet. This same body plan would be repeated for millions of years in the continued evolution of predatory dinosaurs, large and small.

The saurischian herbivore body plan— that of a long neck, a bulky body, and a mouth lined with plucking teeth—took shape in the evolution of the "prosauropods" and early sauropods. These dinosaurs rapidly became the largest, most common herbivores of the Early Jurassic and led to the continued perfection of anatomical features suited for supporting great bulk, height, and the ability to consume plant material at enormously productive rates.

The first ornithischian dinosaurs maintained a low profile during the Late Triassic and Early Jurassic, quietly adapting dentition

Tyrannosaurus rex. charging

and jaw designs that led to the highly complex, chewing batteries of their descendants. The diversity of later ornithischians is all the more surprising when one considers the simple origins of this clade. The few known basal ornithischians were small, bipedal animals and the first dinosaurs to develop cheek teeth. By the Middle

Jurassic, the ornithischians had spawned the first plated dinosaurs and were soon followed by the parallel evolution of the armored dinosaurs. By the Late Cretaceous, the diverse ornithischians also included enormous bipedal "duck-bill" dinosaurs, bone-headed dinosaurs, and a wide assortment of horned dinosaurs.

Dawn of the Dinosaur Age has dwelt on the foundations of the major clades of dinosaurs, their traits, their adaptations, and their lifestyles. Such were the roots of the dinosaur family tree and the natural events that led to the dinosaurs' rapid dominance of terrestrial life during the rest of the Mesozoic Era.

Dinosaurs Human

Invertebrates Fishes Land Plants Amphibians Reptiles Mammals & Birds Ancestors

65.5 100 150 200 250 300 350 400 450 500 542

NEOCENE

CENOZOIC

PALEOGENE

p

M E

CRETACEOUS

H

S

A N

z o

JURASSIC

I

E

c

TRIASSIC

R

p

PERMIAN

O

A

CARBON

Z

L

IFEROUS

E O

O

DEVONIAN

I

Z

SILURIAN

C

O

ORDOVICIAN

I

c

CAMBRIAN

Proterozoic

Precambrian

Appendix Two: Anatomical Directions

Superior (Cranial)

Anterior (Cranial)

Posterior (Dorsal)

G Infobase Publishing

Positional terms used to describe vertebrate anatomy

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