tion lived in a narrow margin near the equator. Corals declined in the late
Paleozoic and were replaced by sponges and algae when the seas they inhab-
ited receded. This was due to extensive glaciation, whose ice caps captured
a significant amount of Earth’s water. Corals, which require warm, shallow
water for survival, were particularly hard hit as evidenced by the lack of
coral reefs at the beginning of the Triassic. When the great glaciers melted
and the seas began to warm, reef building became intense in the Tethys Sea.
Thick deposits of limestone and dolomite were laid down by prolific lime-
secreting organisms.
The mollusks appeared to have weathered the hard times of the late Per-
mian extinction quite well. They continued to become the most important
shelled invertebrates of the Mesozoic seas, with some 70,000 distinct species
living today. The warm climate of the Mesozoic influenced the growth of
giant animals in the ocean as well as on land. Huge clams grew to 3 feet wide,
giant squids were upward of 65 feet long and weighed over a ton, and tall
crinoids reached 60 feet in length.
The cephalopods were extremely spectacular and diversified. They
became the most successful marine invertebrates of the Mesozoic seas, evolv-
ing into some 10,000 species.The ammonoids evolved in the early Devonian
395 million years ago.The two forms best preserved in the fossil record were
those with coiled shells up to 7 feet wide and more awkward types with
straight shells growing to 12 feet in length. They traveled by jet propulsion,
using neutral buoyancy to maintain depth, which contributed to their great
success. However, of the 25 families of widely ranging ammonoids in the late
Triassic, all but one or two became extinct at the end of the period. Those
species that escaped extinction eventually evolved into the scores of ammonite
families in the Jurassic and Cretaceous.
Among the marine vertebrates, fish progressed into more modern
forms.The sharks regained ground lost from the great Permian extinction.
They continued to become the successful predators in the oceans of today.
Nevertheless, because of intense overfishing, some shark species appear to be
headed for extinction.
During the final stages of the Cretaceous, when the seas departed from
the land as the level of the ocean dropped, the temperatures in the Tethys Sea
began to fall. As the continents drifted poleward during the last 100 million
years, the land accumulated snow and ice, which had an additional cooling
effect on the climate. Most warmth-loving species, especially those living in
the Tethys Sea, disappeared.The most temperature-sensitive Tethyan fauna suf-
fered the heaviest extinction rates. Species that were so successful in the warm
waters of the Tethys dramatically declined when ocean temperatures dropped.
Afterward, marine species acquired a more modern appearance as ocean bot-
tom temperatures continued to plummet.
TRIASSIC DINOSAURS