codonts, which also gave rise to the crocodiles and dinosaurs.They thrived in
the late Triassic, evolving quite rapidly, but apparently did not survive beyond
the end of the period.
Near the close of the Triassic, when reptiles were the leading form of
animal life, occupying land, sea, and air, a remarkable reptilian group called the
crocodilians appeared in the fossil record. The crocodilians originated in
Gondwana. They comprised the alligators with a blunt head, the crocodiles
with an elongated head, and the gavials with an extremely narrow head.
Members of this group adapted to life on dry land, a semiaquatic life, or an
entirely aquatic life with a sharklike tail, a streamlined head, and legs remolded
into swimming paddles.
As with many land animals living in the Cretaceous around 100 million
years ago, crocodiles grew to giant sizes as indicated by a 40-foot-long fossil
found in southern Brazil. A fossil of a gaviallike monster from the lower Cre-
taceous in Niger,West Africa, measured about 35 feet long.A monstrous three-
ton, 30-foot crocodile terrorized the Cretaceous swamps and might have
preyed upon medium-sized dinosaurs.
The crocodilians diversified considerably over the past 200 million years.
They spread to all parts of the world and adapted to a wide variety of habi-
tats. Crocodile fossils found in the high latitudes of North America (Fig. 121)
indicate a warm climate during the Mesozoic. They belong to the subclass
Archosauria, which literally means “ruling reptiles,” along with dinosaurs and
pterosaurs.They are the only surviving members to escape the great dying at
the end of the Cretaceous.
Reptiles that returned to the sea to compete with the fish for a plenti-
ful food supply included the sea serpent–like plesiosaurs, the sea cow–like pla-
codonts, and the dolphinlike mixosaurs. The pachycostasaurs, meaning
“thick-ribbed lizard,” were carnivorous marine reptiles 9 feet or more long.
They r esembled plesiosaurs, with thick, heavy ribs possibly used to house very
large lungs and used for ballast in order to hunt at great depths.
The sharklike ichthyosaurs (Fig. 122), whose name is Greek for “fish
lizard,” were fast-swimming, shell-crushing marine predators that apparently
preyed on ammonites.The ichthyosaur would first puncture the shell from the
victim’s blind side, causing it to fill with water and sink to the bottom, where
the attack could be made head on. Puncture marks on fossil ammonite shells
are spaced the same distance apart as fossil ichthyosaur teeth, suggesting these
highly aggressive predators might have hastened the extinction of most
ammonite species by the end of the Mesozoic.
The placodonts were a group of short, stout marine reptiles with large,
flattened teeth.They probably fed primarily on bivalves and other mollusks.
Several other reptilian species also went to sea, including lizards and turtles
that were quite primitive. Many modern giant turtles are descendants of those
PERMIAN REPTILES