Geology Reference
In-Depth Information
various metallic ores. The world's largest copper deposits
were formed during the Mesozoic and Cenozoic in a belt
along the western margins of North and South America (see
Figure 2.27).
Mollusks such as cephalopods, bivalves, and gastropods
were the most important members of the Mesozoic marine
invertebrate fauna. Their rapid evolution and the fact that
many cephalopods were nektonic make them excellent guide
fossils (
Figure 22.19). The Ammonoidea, cephalopods with
wrinkled sutures, constitute three groups: the goniatites, cer-
atites, and ammonites. The latter, though present during the
entire Mesozoic, were most prolifi c during the Jurassic and
Cretaceous. Most ammonites were coiled, some attaining di-
ameters of 2 m, whereas others were uncoiled and led a near
benthonic existence (Figure 22.19). Ammonites became ex-
tinct at the end of the Cretaceous, but, two related groups
of cephalopods survived into the Cenozoic: the nautiloids,
including the living pearly nautilus, and the coleoids, repre-
sented by the extinct belemnoids (
LIFE OF THE MESOZOIC ERA
Fossils and restorations of prehistoric fl oras and faunas—
especially Mesozoic dinosaurs, marine reptiles, and fl ying
reptiles—fascinate many people. Indeed, these animals have
been popularized in topics, television specials, and movies,
including Jurassic Park (1993) and its two sequels, The Lost
Wo r l d (1997) and Jurassic Park III (2001), as well as Dinosaur
(2000). Part of the popularity of these creatures is no doubt
related to the tremendous increase in knowledge that scien-
tists have gained about the Mesozoic biota during the last
few decades.
The Mesozoic Era is commonly called the “Age of Rep-
tiles”, calling attention to the fact that reptiles predomi-
nated among land-dwelling vertebrate animals. Certainly,
the Mesozoic diversifi cation of reptiles was an important
event in life history, but other equally important, though
not as well known, events also took place. Mammals made
their appearance during the Triassic, having evolved from
mammal-like reptiles, and birds evolved probably from
small carnivorous dinosaurs by the Jurassic. And, of course,
there were a variety of reptiles in addition to dinosaurs—
flying reptiles and marine reptiles being two of the most
notable.
Important changes took place among plants when the
first flowering plants (angiosperms) evolved and soon be-
came the most common plants on land. Even though the
major land plants from the Paleozoic Era persisted, and many
still exist, they now make up less than 10% of all land plants.
Marine invertebrates such as clams, snails, and cephalopods
made a remarkable resurgence following the Permian mass
extinctions.
Although Pangaea began fragmenting during the
Triassic (Figure 22.2), the proximity of continents and mild,
Mesozoic climates made it possible for plants and animals to
occupy extensive geographic ranges. But as the fragmenta-
tion continued, some continents became isolated, especially
Australia and South America, and their faunas evolving in
isolation became increasingly different from those elsewhere.
Indeed, Australia has been a separate landmass since the
Mesozoic and even now has a fauna unlike that of any other
continent.
Figure 22.20), which are
good Jurassic and Cretaceous guide fossils, as well as by the
living squid and octopus.
Mesozoic bivalves diversified to inhabit many epifau-
nal and infaunal niches. Oysters and clams (epifaunal sus-
pension feeders) became particularly diverse and abundant,
Marine Invertebrates and Phytoplankton
The Mesozoic was a time when marine invertebrates re-
populated the seas. The Early Triassic invertebrate fauna was
not very diverse, but by the Late Triassic, the seas were once
again swarming with invertebrates—from planktonic fora-
minifera to cephalopods. The brachiopods that had been so
abundant during the Paleozoic never completely recovered
from their near extinction, and although they still exist, the
bivalves have largely taken over their ecologic niche.
Figure 22.19 Cretaceous Seascape Cephalopods such as the
Late Cretaceous ammonoids Baculites (foreground) and Helioceros
(background) were present throughout the Mesozoic, but they were
most abundant during the Jurassic and Cretaceous. They were
important predators, and they are excellent guide fossils.
 
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