sive predators might have caused the extinction of most ammonite species
before the Cretaceous was over.
All shelled cephalopods were absent in the Cenozoic seas except the
nautilus. It is found exclusively in the deep waters of the Indian Ocean. The
nautilus is the ammonite’s only living relative, along with shell-less species,
including cuttlefish, octopuses, and squids (Fig. 162). Squids competed directly
with fish, which were little affected by the extinction. Other major marine
groups that disappeared at the end of the Cretaceous include the rudists,
which were huge, coral-shaped clams, and other types of clams and oysters.THE ANGIOSPERMS
The Mesozoic was a time of transition, especially for plants.They showed lit-
tle resemblance at the beginning of the era to those at the end, when they
more closely resembled present-day vegetation. The gymnosperms, including
conifers, ginkgoes, and palmlike cycads, originated in the Permian and bore
seeds without fruit coverings.The true ferns prospered in the higher latitudes,
whereas today they live only in the warm tropics.
The cycads, which resembled palm trees, were also highly successful.They
ranged across all major continents, possibly contributing to the diets of the plant-
eating dinosaurs. The ginkgo, of which the maidenhair tree in eastern China is
the only living relative, might have been the oldest genus of seed plants. Also
dominating the landscape were conifers up to 5 feet across and 100 feet tall.Their
petrified trunks are especially plentiful at Yellowstone National Park (Fig. 163).
About 110 million years ago, vegetation in the early Cretaceous under-
went a radical change with the introduction of the angiosperms, flowering
plants that evolved alongside pollinating insects.The plants offered pollinators,
such as honeybees and birds, brightly colored and scented flowers, and sweet
nectar.The unwary intruder was dusted with pollen, which it transported to
the next flower it visited for pollination. Many angiosperms also depended on
animals to spread their seeds, which were encased in tasty fruit that passed
through the digestive tract and dropped some distance away.
The sudden appearance of angiosperms and their eventual domination
over other plant life has remained a mystery. They might have originally
exploited the weedy rift valleys that formed when Pangaea broke apart. The
earliest angiosperms living in the lush rift valleys appear to have been large
plants, growing as tall as magnolia trees. However, fossils discovered in Australia
suggest that the first angiosperms there and perhaps elsewhere were small,
herblike plants. Within a fewmillion years after their introduction, the effi-
cient flowering plants crowded out the once abundant ferns and gym-
nosperms. Angiosperms possessed water-conducting cells called vesselHistorical Geology