a missing chapter in insect evolution in an extraordinarily rich fossil quarry in
southern Virginia, near the North Carolina border.The site yielded well-pre-
served insects dating to the end of the Triassic 210 million years ago, includ-
ing the discovery of the oldest records of some types of insects. Insects are hard
to fossilize because their delicate bodies tend to disintegrate on burial. Later
insects were preserved in amber, which is altered tree sap that entombs the
animals and enables them to withstand the rigors of time.
The oldest dinosaurs originated on the southern continent Gondwana
when the last glaciers of the great Permian ice age were departing and the
region was still recovering from the cold conditions.The dinosaurs descended
from the thecodonts (Fig. 133), the apparent common ancestors of crocodiles
and birds, their distant living relatives. The earliest thecodonts were small- to
medium-sized predators that lived during the Permian-Triassic transition.
Onegroup of thecodonts took to the water and became large fish eaters.They
included the phytosaurs, which died out in the Triassic, and the crocodilians,
which remain successful today.
Pterosaurs were also descendants of the Triassic thecodonts.The appear-
ance of featherlike scales ostensibly used for insulation suggests that the-
codonts were also the ancestors of birds. The protofeathers helped trap body
heat or served as a colorful display for attracting mates as with modern birds.
By the end of the Triassic, the dinosaurs replaced the thecodonts as the dom-
inant terrestrial vertebrates.
Every dinosaur species could trace its origin back to a single common
ancestor called eoraptor, meaning “early hunter.” It evolved from predatory
reptiles in the lower Triassic about 240 million years ago. Early in dinosaur
Figure 133The
dinosaurs descended from
the thecodonts.TRIASSIC DINOSAURS