Biology Reference
In-Depth Information
Fig. 4.17
out sudden catastrophic events occurring against this uniform background of geo-
logical change. There is evidence of at least two major extinction events linked to
extreme volcanic activity and asteroid strikes that caused the Earth's atmosphere to
darken for periods long enough to render extinct many organisms that rely, directly
or indirectly, on photosynthesis.
Macroevolution is also inferred from the fossil record. If some species can change
into other species over long periods of time, there might to be fossils of transitional
forms between them - and there are! Transitional species are defined as those that
show a mixture of features from both their ancestors and their descendents. We now
have transitional fossils between fish and amphibians, reptiles and birds, hippos and
whales, and apes and humans. Figure 4.18 shows some examples of fossils sharing
characteristics of both dinosaurs and birds.
On the left of Fig. 4.18 is shown one of the famous
Archaeopteryx
fossils, of
which ten specimens are now known, all found in southern Germany. The first was
found in 1861, just two years after the publication of
On the Origin of Species
, and
the last in 2005. These fossils were all preserved in limestone in Bavaria, and date
to the Jurassic period, about 150 million years ago. The word
Archaeopteryx
means
“ancient wing”.
These fossils show a mixture of avian and dinosaur traits. They share with
dinosaurs, jaws with sharp teeth, three fingers with claws, a long bony tail and an
extensible second toe. They share with birds a wishbone, flight feathers and wings.
