Paleontology (continued)
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Second, remains of the first birds, including imprints of feath-
ers, are found in fine-grained Solnhofen Limestone of Jurassic
age from Bavaria, Germany. These animals apparently evolved
from small, feathered theropod dinosaurs, which were relatives
of the large predatory tyrannosaurids.
In terms of plant life, the Triassic was an age of ferns and
early conifers, but the succeeding Jurassic Period is sometimes
called the "Age of Cycads," a group of related gymnosperms
with short, stocky trunks and palmate leaves. Later, midway
through the Cretaceous Period, a very important new group of
seed-bearing plants—the angiosperms or flowering plants—
appeared (Fig. 8); their key breakthrough, double fertilization,
led to accelerated reproduction. These plants coevolved with
new groups of pollinating insects, including butterflies and
moths. Dinosaurs continued to flourish during the Cretaceous
and included the largest terrestrial carnivores of all time, such
as Tyrannosaurus, which may have weighed 6 tons (5500 kg).
In response to these predators, herbivorous dinosaurs evolved
varied defenses, including armor in ankylosaurs and horns
in the ceratopsians, such as Triceratops, the youngest known
dinosaur. Dinosaurs, pterosaurs, and many marine animals (for
example, swimming reptiles and ammonoids) became extinct
at the end of the Cretaceous Period. A great debacle in the form
of an asteroid impact occurred at this time, as evidenced by en-
richment of iridium and shocked quartz in Cretaceous-Tertiary
boundary clays about 65 million years ago, and this contributed
to the demise of these organisms.
The demise of dinosaurs in terrestrial ecosystems may have
opened ecological space and cleared the way for an extremely
rapid adaptive radiation of mammals during the Cenozoic Era,
often called the "Age of Mammals." However, this era could
equally be called the "Age of Angiosperms," the "Age of Frogs,"
the "Age of Snakes," or the "Age of Songbirds," as all of these
groups also evolved and radiated during the Cenozoic. The
evolution of C-4 plants, with their more efficient photosyn-
thetic pathways, including the constantly growing grasses, was
closely tied to the rise of ungulates, that is, hoofed, grazing
mammals, during the mid-Cenozoic. These included cloven-
hoofed, even-toed ungulates (pigs, camels, and cattle), as well
as rhinos and horses, a group of odd-toed ungulates, whose
evolution of long legs, reduced side toes, and abrasion-resis-
tant high-crowned teeth is well documented.
Finally, about 6 million years ago, the hominid family evolved
in Africa. Fossils document the early members of the genus
Australopithecus, which gave rise to the larger brained genus
Homo about 2 million years ago. Subsequently, Homo sapiens
evolved about 200,000–100,000 years ago also in Africa, and
then spread across the Old World and eventually across the Ber-
ing land bridge into North America. This immigration may have
ultimately led to a wave of extinction, especially of large mam-
mals such as mammoths and mastodons, perhaps as a result
of overhunting by the one species (humans) whose activities
continue to alter the terrestrial ecosystem in many ways.
