TOEFL Reading Practice
Reading Task 2
Factual Information,
Vocabulary, Reference, Summary and Insert
Questions
THE ORIGINS OF CETACEANS
It should be obvious that cetaceans—whales,
porpoises, and dolphins—are mammals. They
breathe through lungs, not through gills, and
give birth to live young. Their streamlined
bodies, the absence of hind legs, and the
presence of a fluke1 and blowhole2 cannot
disguise their affinities with land-dwelling
mammals. However, unlike the cases of sea otters
and pinnipeds (seals, sea lions, and walruses,
whose limbs are functional both on land and at
sea), it is not easy to envision what the first
whales looked like. Extinct but already fully
marine cetaceans are known from the fossil
record. How was the gap between a walking mammal
and a swimming whale bridged? Missing until
recently were fossils clearly intermediate, or
transitional, between land mammals and
cetaceans.
Very exciting discoveries have finally allowed
scientists to reconstruct the most likely
origins of cetaceans. In 1979, a team looking
for fossils in northern Pakistan found what
proved to be the oldest fossil whale. The fossil
was officially named Pakicetus in honor of the
country where the discovery was made. Pakicetus
was found embedded in rocks formed from river
deposits that were 52 million years old. The
river that formed these deposits was actually
not far from an ancient ocean known as the
Tethys Sea.
The fossil consists of a complete skull of an
archaeocyte, an extinct group of ancestors of
modern cetaceans. Although limited to a skull,
the Pakicetus fossil provides precious details
on the origins of cetaceans. The skull is
cetacean-like but its jawbones lack the enlarged
space that is filled with fat or oil and used
for receiving underwater sound in modern whales.
Pakicetus probably detected sound through the
ear opening as in land mammals. The skull also
lacks a blowhole, another cetacean adaptation
for diving. Other features, however, show
experts that Pakicetus is a transitional form
between a group of extinct flesh-eating mammals,
the mesonychids, and cetaceans. It has been
suggested that Pakicetus ted on fish in shallow
water and was not yet adapted for life in the
open ocean. It probably bred and gave birth on
land.
Another major discovery was made in Egypt in
1989. Several skeletons of another early whale,
Basi
losaurus, were found in sediments left by
the Tethys Sea and now exposed in the Sahara
desert. This whale lived around 40 million years
ago, 12 million years after Pakicetus. Many
incomplete skeletons were found but they
included, for the first time in an archaeocyte,
a complete hind leg that features a foot with
three tiny toes. Such legs would have been far
too small to have supported the 50-foot-long
Basi
losaurus on land. Basi
losaurus was
undoubtedly a fully marine whale with possibly
nonfunctional, or vestigial, hind legs.
An even more exciting find was reported in 1994,
also from Pakistan. The now extinct whale
Ambulocetus natans ("the walking whale that
swam") lived in the Tethys Sea 49 million years
ago. It lived around 3 million years after
Pakicetus but 9 million years before
Basilosaurus. The fossil luckily includes a good
portion of the hind legs. The legs were strong
and ended in long feet very much like those of a
modern pinniped. The legs were certainly
functional both on land and at sea. The whale
retained a tail and lacked a fluke, the major
means of locomotion in modern cetaceans. The
structure of the backbone shows, however, that
Ambulocetus swam like modern whales by moving
the rear portion of its body up and down, even
though a fluke was missing. The large hind legs
were used for propulsion in water. On land,
where it probably bred and gave birth,
Ambulocetus may have moved around very much like
a modern sea lion. It was undoubtedly a whale
that linked life on land with life at sea.
1. Fluke: The two parts that constitute the
large triangular tail of a whale
2. Blowhole: A hole in the top of the head used
for breathing