Geology Reference
In-Depth Information
were, however, a few exceptions. One is a Middle Jurassic-
age aquatic mammal found in China that measures about
50 cm long, which also has the distinction of being the oldest
known fossil with fur. The other is an Early Cretaceous-age
mammal called
Repenomamus giganticus
, also from China,
that was about 1 m long, weighed 12 to 14 kg, and had the
remains of a juvenile dinosaur in its stomach. Nevertheless,
most other Mesozoic mammals were about the size of mice
and rats, and they were not nearly as diverse as they were
during the Cenozoic Era. Furthermore, they retained reptile
characteristics, but had mammalian features, too. The Tri-
assic triconodonts, for instance, had the fully differentiated
teeth typical of mammals, but they also had both the reptile
and the mammal types of jaw joints. In short, some mam-
mal features appeared sooner than others (again, recall the
concept of
mosaic evolution
from Chapter 18).
The early mammals diverged into two distinct branches.
One branch includes the triconodonts and their probable de-
scendants, the
monotremes
, or egg-laying mammals such as
the spiny anteater and platypus of the Australian region. The
other branch includes the
marsupial
(pouched)
mammals
and the
placental mammals
and their ancestors, the eupan-
totheres (
landmasses. The proximity of these landmasses, however, is
not suffi cient to explain Mesozoic biogeographic distributions
because climates are also effective barriers to wide dispersal.
During much of the Mesozoic, though, climates were more
equable and lacked the strong north and south zonation char-
acteristic of the present. In short, Mesozoic plants and animals
had greater opportunities to occupy much more extensive
geographic ranges.
Pangaea persisted as a single unit throughout most of the
Triassic (Figure 22.1a), and the Triassic climate was warm-
temperate to tropical, although some areas, such as the pres-
ent southwestern United States, were arid. Mild temperatures
extended 50 degrees north and south of the equator, and even
the polar regions may have been temperate. The fauna had a
truly worldwide distribution. Some dinosaurs had continuous
ranges across Laurasia and Gondwana; the peculiar gliding liz-
ards were in New Jersey and England, and reptiles known as
phytosaurs lived in North America, Europe, and Madagascar.
By the Late Jurassic, Laurasia had become partly frag-
mented by the opening North Atlantic, but a connection still
existed (Figure 22.1b). The South Atlantic had begun to open
so that a long, narrow sea separated the southern parts of Af-
rica and South America. Otherwise, the southern continents
were still close together.
The mild Triassic climate persisted into the Jurassic.
Ferns, whose living relatives are now restricted to the tropics
of southeast Asia, are known from areas as far as 63 degrees
south latitude and 75 degrees north latitude. Dinosaurs roamed
widely across Laurasia and Gondwana. For example, the giant
sauropod
Brachiosaurus
is known from western North America
and eastern Africa. Stegosaurs and some families of carnivorous
dinosaurs lived throughout Laurasia and in Africa.
By the Late Cretaceous, the North Atlantic had opened
further, and Africa and South America were completely
separated (Figure 22.1c). South America remained an island
Figure 22.31).
Although the history of the monotremes is uncertain,
fossils of several Mesozoic animals are relevant to the evolu-
tion of marsupials and placentals. In fact, the divergence of
marsupials and placental mammals from a common ances-
tor took place during the Early Cretaceous (
◗
◗
Figure 22.32).
PALEOGEOGRAPHY
Fragmentation of the supercontinent Pangaea began by the
Late Triassic and continues to the present, but during much of
the Mesozoic, close connections existed between the various
Triassic
Jurassic
Cretaceous
Cenozoic
Docodonts
Monotremes
Triconodonts
Cynodont
ancestor
Multituberculates
Symmetrodonts
Marsupials
Eupantotheres
Placentals
◗
Figure 22.31
Mammal Relationships Relationships among the various recognized groups of early
mammals and their descendants. Mammal evolution proceeded along two branches: one leading to
today's monotremes, or egg-laying mammals, and the other to marsupial and placental mammals.
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