Geology Reference
In-Depth Information
◗
Figure 23.27
Evolution of Whales
Pakicetus
(Early Eocene)
Protocetus
(Middle Eocene)
Basilosaurus
(Middle-Late Eocene)
Toothed Whales
(Present Day)
Baleen Whales
Protocetus
length:
up to about 6.5 m
Basilosaurus
length:
up to about 20 m
Land-dwelling ancestor
(Early Eocene)
a
Cladogram showing the relationships among some fossil and
living whales and their land-dwelling ancestors. Note that
Pakicetus
had well-developed hind limbs, but only vestiges remain in
Protocetus
and
Basilosaurus
. (Source: Reprinted with permission from Mammalian
Evolution in Major Features of Vertebrate Evolution, Short Course on
Paleontology, no. 7, Dr. R. Prothero and R.M. Schachieds)
b
Restoration of
Protocetus
and
Basilosaurus
. Although
Basilosaurus
was a fully aquatic whale, it differed considerably
from today's whales.
Because of this, there is even disagreement on the total
number of hominid species. A complete discussion of all
of the proposed hominid species and the various compet-
ing schemes of hominid evolution is beyond the scope of
this chapter. However, we will briefl y discuss the generally
accepted taxa (
about 8 million years ago and humans separated from chim-
panzees about 5 million years ago.
The next oldest hominid is
Orrorin tugenensis,
whose
fossils have been dated at 6 million years old and consist of
bits of jaw, isolated teeth, fi nger, arm, and partial upper leg
bones. At this time, there is still debate as to exactly where
Orrorin tugenensis
fi ts in the hominid lineage.
Sometime between 5.8 and 5.2 million years ago, another
hominid,
Ardipithecus ramidus kadabba,
was present in eastern
Africa. Although many paleoanthropologists think that both
Orrorin tugenensis
and
Ardipithecus ramidus kadabba
were ha-
bitual bipedal walkers and thus on a direct evolutionary line
to humans, others are not as impressed with the fossil evi-
dence and are reserving judgment. Until more fossil evidence
is found and analyzed, any single evolutionary scheme of
hominid evolution presented here would be premature.
Figure 23.31) and present some of the cur-
rent theories of hominid evolution.
Remember that although the fossil record of hominid
evolution is not complete, what exists is well documented. Fur-
thermore, the interpretation of that fossil record precipitates
the often vigorous and sometimes acrimonious debates con-
cerning our evolutionary history.
Discovered in northern Chad's Djurab Desert in July
2002, the nearly 7-million-year-old skull and dental remains
of
Sahelanthropus tchadensis
(
◗
Figure 23.32) make it the old-
est known hominid yet unearthed and at or very near to the
time when humans diverged from our closest living relative,
the chimpanzee. Currently, most paleoanthropologists accept
that the human-chimpanzee stock separated from gorillas
◗
Australopithecines
Australopithecine
is a collective term
for all members of the genus
Australopithecus.
Currently, fi ve
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