Whale Evolution
Life Science: Natural World
Cetacean
Cetacean, which literally translates to large sea mammal,
includes whales, dolphins, and porpoises.
Whale Evolution
Mesonychids
Mesonychia ("Middle Claws") are an extinct order of medium
to large-sized carnivorous mammals that were closely related
to cetaceans (dolphins and whales). They first appeared in the
Early Paleocene (67-55 mya).
Mesonychid
Pakicetus
Pakicetus is a genus of extinct cetaceans found in the
early Eocene (55-34 mya) of Pakistan. Complete
skeletons were discovered in 2001, revealing that
Pakicetus was primarily a land animal, about the size of a
wolf, and very similar in form to the related mesonychids.
Pakicetus
The first fossil, a lone skull, was thought to be a
mesonychid, but Gingerich and Russell recognized it as
an early cetacean from characteristic features of the inner
ear, found only in cetaceans. This suggests that it is a
transitional species between extinct land mammals and
modern whales.
Pakicetus
Ambulocetus
Ambulocetus ("walking whale") was an early cetacean that
could walk as well as swim. It lived during early Eocene about
46 million years ago. It is a transitional fossil that shows how
whales evolved from land-living mammals. It was clearly
amphibious, as its back legs are better adapted for swimming
than for walking on land. Chemical analysis of its teeth shows
that it was able to move between salt and fresh water.
Ambulocetus
Rodhocetus
Rodhocetus is one of several extinct whale genera that
possess land mammal characteristics, thus demonstrating the
transition from land to sea that whales went through. The first
fossils of this species were found in Balochistan Province,
Pakistan in 2001 by Philip Gingerich. These organisms lived
approximately 43 million years ago and contain a very
characteristic whale ear.
Rodhocetus
Basilosaurus
Basilosaurus was a cetacean that lived from 40 to 34 million
years ago. Fossil remains were first discovered in the
southern United States (Louisiana), and were initially believed
to be some sort of reptilian sea monster. These were
enormous creatures up to 60 feet long. They had a tailfluke,
complete hind legs, and mobile knees.
Albert Koch's 114-foot long “Sea Serpent” skeleton from
1845. Later was properly identified as the Eocene whale
known as Basilosaurus.
Basilosaurus
Dorudon
Dorudon was a genus of ancient cetacean that lived
alongside Basilosaurus 40 to 35 million years ago, in the
Eocene. They were about five meters (16 feet) long and were
most likely carnivorous, feeding on small fish and mollusks.
Dorudontids lived in warm seas around the world, fossils have
been found in North America as well as in Egypt where the
Tethys Sea once existed.
Dorudon
Aetiocetus
Aetiocetus is an extinct genus of baleen whale from the
Oligocene (about 24 mya). Found in Oregon, Aetiocetus
is believed to be a transitional fossil between Pakicetus
and the modern beluga whale.
Aetiocetus
Nostril Comparison
Intermediate
Whale Discovery: Discussion Questions
1. Which typical whale traits were the earliest to
appear?
Early whale teeth, then early whale-type ear bones;
skulls becoming long and narrow; shorter legs with
big feet.
Whale Discovery: Discussion Questions
2. Which whale traits evolved much later?
Nostrils shifted back toward top of skull; hind legs
reduced, then eliminated; forelimbs become paddle
like; strong swimming tail with flukes form.
Whale Discovery: Discussion Questions
3. What age sediments, and in what region of the
world, would you search now to get the fossils which
would shed more light on whale origins, and what
specific traits would you expect to find?
40-50 million year old sediments in the Pakistan
region. Traits showing increasing adaptations to
marine life over time.
Whale Discovery: Discussion Questions
4. Does Ambulocetus seem to fit the “predicted traits”
for an intermediate between mesonychids and
Rodhocetus?
Yes, Ambulocetus does fit well into the sequence,
showing transitional features.
Whale Discovery: Discussion Questions
5. Notice the reconstruction of Pakicetus, showing it as
a four-legged animal. What evidence, if any, would
suggest such a reconstruction? What additional
evidence would give you greater confidence in that
reconstruction?
Ear bones more like mammals; Chemical analysis
indicates dependence on freshwater. There were no
complete fossils discovered until 2001, and these
confirm the presence of four terrestrial legs, with
hooves.
Whale Discovery: Discussion Questions
6. As each new “intermediate fossil” was found, filling a
“gap”, how many new gaps were formed?
Two new gaps were formed, one above, and one
below the new find.
Whale Discovery: Discussion Questions
7. Can we make predictions about past events? Why?
Yes. Based on existing fossils and assumed
evolutionary trends, knowledge of anatomy,
geography, and geological history.
Whale Discovery: Discussion Questions
8. Explain why the absence of transitional
(intermediate) fossils is not a fair argument against
evolution.
They may not have been found yet; transition may
have been fairly rapid, leaving few fossils; many new
transitional fossils have been found in recent years.
Whale Discovery: Discussion Questions
9. Why is it very unlikely that these fossils of early
whale evolution are the direct ancestors of whales?
How are they probably related to those direct
ancestors? What is wrong with the popular “missing
link” concept of evolution.
As in living species today, there are many different
species in the same families (i.e. cat family). They
are much more likely to be close cousins to any direct
ancestors. “Missing link” suggests a simplistic linear
evolutionary chain of life, rather than the more
realistic multi-branched tree-of-evolution concept.
Whale Discovery: Discussion Questions
10. Several species of modern whales have welldeveloped rear limbs while embryos. As the embryo
continues to mature these limbs atrophy (shrink) and
become nonfunctional. Why do you suppose this
happens?
Genes for leg formation are still in the whale genome
(as expected from a terrestrial ancestry), but are
turned off during later development, since legs were
selected against in the marine environment.
Whale Discovery: Discussion Questions
11. Summarize what you have learned about the
process of science in this lesson
Testing hypotheses about whale origins (evolution),
both in form and in geography, includes the making
predictions of expected traits, age and locations,
looking for expected fossils, and finding them, which
confirms the hypothesis.
Whale Discovery: Discussion Questions
12. Summarize what you have learned about the
process of evolution in this lesson.
Fossils were discovered which show traits
intermediate between terrestrial mammals and
whales, with a continuum of earlier and later features,
which is consistent with the idea that whales have
evolved from earlier non-whales, gradually acquiring
features which enable them to survive better in a
deep ocean environment.