Scientists have long known that cetaceans (whales, dolphins, and
porpoises) descended from four-footed land mammals. Cetaceans still
have some features of land mammals; they use lungs to breathe air and give
birth to young that are nursed by milk produced by the mother.
Modern cetaceans cannot live on land, and look very different from land
mammals in most respects.
Cetaceans evolved rapidly, and the entire transition from land mammal to
obligate marine whale took less than 8 million years. These Eocene
cetaceans are often called archaeocetes, and they can be divided into six
Basilosauridae. It is only since the 1990’s that relatively
complete skeletons of the archaeocetes have been found in abundance and
that the transition from land-to-water could be studied in detail.
Until the early years of the 21st century, most paleontologists thought
that cetaceans were most closely related to mesonychians (The Mesonychian
Hypothesis). Mesonychians are an extinct (Paleocene-Oligocene) group of
hoofed mammals from the Northern Hemisphere. They varied in size from that
of a weasel to a grizzly bear, and may have eaten carrion or meat. Unlike
paleontologists, most scientists studying DNA were of a different opinion.
They considered hippopotamids as the closest relatives to cetaceans (The
Hippopotamid Hypothesis). Hippopotamids (including the recent Hippo and
the Pygmy Hippo) are included in a group of mammals called even-toed
ungulates or artiodactyls. Other artiodactyls are: pigs, peccaries,
camels, llamas, giraffes, deer, goats, sheep, cattle, and antelopes.
Skeleton of a mesonychian.
In 2001, two important skeletons of pakicetids, found by the
Thewissen-lab, were described (see Pakicetidae
for images). These skeletons helped to change our opinions on what the
closest land relatives of cetaceans were. Data gathered from the new
pakicetid skeletons show that mesonychians are not the closest relatives
of cetaceans (sister groups in scientific lingo). However, these new data
also disagree, less strongly, with the sister group relation between
hippos and cetaceans championed by the molecular scientists. Instead, the
new data are most consistent with close relationships between whales and
all of artiodactyls, the Artiodactyl Hypothesis. To understand the
differences between these hypotheses, let's compare the relations of these
groups of animals to those of a number of female members of a human
family. Let's say that we are trying to describe the relation of Celia to
her relatives Heidi, Arlene, and Megan, and we know that all are from the
It could be that Celia is a first cousin to Megan, and that Heidi and
Arlene are sisters of each other, but only distantly related to Celia and
Megan. If we indicate maternal relations only, the following branching
diagram would apply:
It could also be that Celia is a sister to Heidi, and that Arlene is their
first cousin. Megan is more distantly related.
Finally, it could be that Heidi and Arlene are sisters, Celia is their
first cousin, and Megan more distantly related.
These diagrams explain the essence of the differences between the three
competing hypotheses for cetacean relations. Substitute Cetacea for Celia,
Hippos for Heidi, other artiodactyls for Arlene, and mesonychians for
Megan, and a simplified version of the scientific argument surrounding
cetacean origins emerges.
The First Costal Whales
The skeleton of Ambulocetus natans
(approximately 12 feet long)
To see a flesh-and-blood reconstruction go to Carl
Ambulocetids are large, powerful animals, with short limbs, but big feet,
and a strong tail. They are only found in northern Pakistan and western
India in rocks that indicate that the environment was nearshore marine and
swampy. These rocks cannot be dated with great accuracy, but they are
clearly younger than the sediments in which pakicetids
are found. Although ambulocetids could walk on land as well as swim, it is
clear that they were not fast on either terrain. The post-cranial skeleton
of ambulocetids is well known thanks to the discovery of a very complete
skeleton of the species Ambulocetus natans. The name of this
whale means “the walking and swimming whale,” and indicates that it was
amphibious. Ambulocetus was first described by Thewissen et al.
(1994), and later, in more detail, by Thewissen et al. (1996)
The bones of Ambulocetus natans arranged in its approximate body
shape. These are the original fossils, all pertaining to a single
individual. Note hammer for scale.
Three genera of whales make up the family Ambulocetidae: Ambulocetus
, Gandakasia , and Himalayacetus . Gandakasia
and Himalayacetus are only known from a single lower jaw
fragment each, and cannot be compared in detail with Ambulocetus .
Head of Ambulocetus as drawn by Marion Lipka
Some publications on ambulocetids:
Thewissen, J.G.M., S.T. Hussain, and M. Arif. 1994. Fossil evidence for
the origin of aquatic locomotion in archaeocete whales. Science
Thewissen, J.G.M., S. I. Madar, and S. T. Hussain. 1996. Ambulocetus
natans, an Eocene cetacean (Mammalia) from Pakistan. Courier
Forschungs-Institut Senckenberg, 190:1-86.
Thewissen, J.G.M., and F. E. Fish. 1997. Locomotor evolution in the
earliest cetaceans: functional model, modern analogues, and
paleontological evidence. Paleobiology 23:482-490.
Madar, S.I., J. G. M. Thewissen, and S. T. Hussain. 2002. Additional
holotype remains of Ambulocetus natans (Cetacea, Ambulocetidae), and their
implications for locomotion in early whales. Journal of Vertebrate
Reconstruction of Basilosaurus cetoides (top) and Zygorhiza
kochii (not to scale). There are several inaccuracies in this old
reconstruction (Kellogg, 1936), such as the exact number of vertebrae and
the shape of the forelimbs. Discovery of new specimens has made it
possible to correct these.
Basilosaurids and dorudontids lived in the late Eocene, approximately 35
and 41 million years ago. They are mainly known from the eastern United
States and from Egypt, but were probably worldwide in their distribution.
Basilosaurids were enormous (possibly up to 60 feet long) and had long
snake-like bodies. They had a tailfluke, but they probably swam using
sinuous movements with their bodies. it is not clear whether that was the
main propulsive organ. Fossilized stomach contents in one Basilosaurus
indicates that it ate fish, including sharks. Dorudontids are closely
related to basilosaurids but were proportionally more like dolphins. They
probably swam using their fluke. Both basilosaurids and dorudontids had
complete hindlimbs, that included a mobile knee and several toes. However
these extremities were tiny, so small that they were certainly not
important in aquatic propulsion.
Two skull views of a dorudontid in the Natural History Museum of London
Skeleton of a right whale from Weber, 1928. B indicates bones of pelvis
Modern whales, dolphins, and porpoises and all their fossil relatives are
together included in Cetacea.
Modern Cetacea fall in two categories, called suborders: the Odontoceti
(toothed whales) and Mysticeti (baleen whales).
There are about 10 modern species of baleen whales, including the right
whale, blue whale, and humpback whale. These whales lack teeth, and
instead have a curtain of baleen hanging down from their upper jaw. Baleen
is a hair or horn-like substance which forms a sieve that filters water.
Also, their skull is characteristically modified by the anterior
displacement of the bones that form the back of the skull combined with
the posterior displacement of the bones that form the front of the skull.
Baleen whales are known for their long wailing songs which carry hundreds
of miles. They include the largest animal ever living on earth, the blue
Many species of mysticetes have been driven to near extinction by whalers.
Recent international treaties have been somewhat successful in saving
these species, although some are still quite rare (balaenids). The oldest
mysticetes are about 40 million years old and are derived from Eocene
The suborder Mysticeti includes the baleen whales, such as right whales,
blue whales, and humpback whales. These whales lack teeth, and instead
have a curtain of baleen hanging down from their upper jaw. Baleen is a
hair or horn-like substance which forms a sieve which is used to filter
water. Baleen whales are known for their long wailing songs which carry
hundreds of miles. They include the largest animal ever living on earth,
the blue whale. Many species of mysticetes have been driven to near
extinction by whalers. Recent international treaties have been somewhat
successful in saving these species, although some are still quite rare
(balaenids). The oldest mysticetes are about 40 million years old and are
derived from Eocene Cetaceans.
Characters of modern mysticetes
Bones from back of skull moved forward (telescoped)
TWO BALEEN WHALES, NOT TO SCALE
Right Whale Balaena glacialis
Length up to 58 feet ( 18 meters)
Weight up to 100 tons (90,000 kg)
Blue Whale Balaenoptera musculus
Length up to 90 feet (28 meters)
Weight up to 200 tons (140,000kg)
Skeletons of two dolphins, the Indo-Pacific Hump-Backed Dolphin, Sousa
chinensis (top) and Risso's Dolphin, Grampus griseus.
Redrawn from Howell (1930)
Odontoceti (Latin for toothed whales) include dolphins, porpoises, as well
as a number of larger whales such as sperm whales, beaked whales and
belugas. Most odontocetes live in the ocean, but a few species of dolphins
live in rivers. There are coastal and deep-sea species, some are tropical
and others polar. Some odontocetes, such as the killer whale, inhabit all
climate zones of all oceans.
Odontocetes are active hunters of large prey items. Prey may include fish,
squid (sperm whale), or mammals such as sealions and cetaceans. Most
odontocetes have teeth, and these are spatulate or peg-like. The skull of
odontocetes is modified: it is asymmetrical and the bones of the forehead
are displaced posteriorly.
Unlike mysticetes, odontocetes echolocate; they emit high frequency sounds
and determine shapes in their environment by listening to the reflections
of these sounds. To accomplish this, they have a sophisticated sound
emission system located in the forehead, and consisting of a number of
air-filled cavities that are connected to the nasal cavity. In addition,
the forehead has a fatty lense-shaped organ called the melon. This organ
gives the dolphin head its characteristic shape and functions as an
acoustic lense. The ears of odontocetes are highly specialized for the
reception of reflected echolocation signals The largest odontocete is the
sperm whale. The oldest odontocete is approximately 40 million years old
and is derived from Eocene cetaceans.
Killer whale Orcinus orca
Length up to 31 feet (10 meters)
Weight up to 9 tons (8200 kg)
Sperm whale Physeter macrocephalus
Length up to 60 feet (19 meters)
Weight up to 50 tons (45,000 kg)
Ambulocetids show more aquatic adaptations than pakicetids, and probably
filled an ecological niche similar to modern crocodiles. They are found in
near shore environments and probably ambushed part of their prey in the
shallows. They could move both on land and in water, and had robust jaws
and teeth to handle large struggling prey. The post-cranial skeleton of
ambulocetids is well known thanks to a nearly complete skeleton of the
species Ambulocetus natans that was found in northern Pakistan.
Ambulocetids are only known from Eocene deposits of Pakistan, 49 million
years ago. Some current research on ambulocetids focuses on their locomotor
Pakicetids were the first cetaceans, their fossils are only found in northern
Pakistan and western
India, and the best fossils are from a locality in the Kala Chitta
Hills in Punjab, Pakistan. The Indian subcontinent is probably the region
where cetaceans originated. Pakicetids did not look like whales at all,
and resembled land mammals. However, the skulls of pakicetids have an ear
region that is highly unusual in shape, and only resembles that of modern
and fossil whales. These features are diagnostic for cetaceans, they are
found in all cetaceans, and in no other animals. These features are main
why pakicetids are considered whales. In many other features, pakicetids
are also similar to some whales, but those features are not shared by all
whales. An example of the latter is the dentition. Pakiceid teeth look a
lot like those of fossil whales, but are unlike those of modern whales.
Pakicetids did not live in the sea. The rocks in which their fossils are
preserved indicate that the bones were buried in a shallow stream, and
that the climate was hot and dry. It is likely that pakicetids waded in
these streams. Their bones are unusually thick, possibly an adaptation to
make the animal heavier counteracting the buoyancy of the water.
Photograph (above) of bones of Pakicetus (large animal) and Ichthyolestes
(small animal), and a line drawing of the same skeletons. The hammer
indicates the size of the skeletons, and shows that Pakicetus was
approximately as large as a wolf, and Ichthyolestes is as large as
The family Pakicetidae (pakicetids in English) consists of three genera, Pakicetus,
Nalacetus, and Ichthyolestes. Of these, Pakicetus
was the largest, and Ichthyolestes the smallest. In overall
shape, these three are not very different. The skeleton of pakicetids was
described by Thewissen et al. (2001). In evolution, pakicetids were
followed by ambulocetids.
The skulls of two pakicetid whales flank the skull of a modern coyote ( Ichthyolestes
on the left, Pakicetus on the right). These skulls were
described by Thewissen et al. (2001).
Pakicetid fossils were important in determining what whales were related
to, and they were used to rebuke the idea that whales were closely related
to an extinct group of hoofed mammals called mesonychians. These fossils
confirmed what was already suggested by scientists studying the DNA of
modern whales: that whales’ closest relatives are even-toed ungulates
(artiodactyls, such as pig, hippo, camel, deer, and cows) Current research
in the Thewissen lab attempts to determine how sound was transmitted
through the pakicetid ear
and how pakicetids locomoted
water and on land.
Some publications on pakicetids from our lab:
Maas, M.C. and J. G. M. Thewissen. 1995. Enamel microstructure of Pakicetus
(Mammalia: Archaeoceti). Journal of Paleontology, 69:1154-1163.
Thewissen, J. G. M., and S. T. Hussain. 1998. Systematic review of the
Pakicetidae, Early and middle Eocene Cetacea (Mammalia) from Pakistan and
India. Bulletin of the Carnegie Museum 34:220-238.
Thewissen, J. G. M., E.M. Williams, L.J. Roe, and S.T. Hussain. 2001.
Skeletons of terrestrial cetaceans and the relationship of whales to
artiodactyls. Nature, 413:277-281.
High-resolution versions of these images can be found at URL: http://www.neoucom.edu/Depts/Anat/Thewissen/publ.html
First whales to disperse widely across the world's oceans
Protocetids are a heterogeneous group of whales that lived in the Eocene,
from approximately 48 to 35 million years ago. Protocetids postdate
pakicetids and ambulocetids and predate most basilosaurids and
dorudontids. In Pakistan and India, protocetids are found in combination
with remingtonocetids. Protocetids were the first whales to leave the
Indian Subcontinent and disperse to all shallow (sub)tropical oceans of
the world. In addition to Indo-Pakistan, they are also known from Africa
and the eastcoast of North America.
Protocetids for which most of the skeleton is known had large feet which
they used in swimming, similar to Ambulocetus. However, it is possible
that other protocetids had a tailfluke. Unlike remingtonocetids and
ambulocetids, protocetids had large eyes that were laterally placed in the
head under a large bony extension of the frontal bone (the supraorbital
Whales Endemic to the Indian Subcontinent
Remingtonocetids are fossil whales that lived in South Asia, approximately
from 49-43 million years ago. Their heads were very different from other
whales. Their eyes were small, their snout long, and they had ears that
transmitted sound similarly to those of modern whales. Remingtonocetids
had long fore- and hind limbs.
The best known remingtonocetid is Remingtonocetus, a whale named after the
well-known American whale researcher Remington Kellogg.
Skulls of Remingtonocetus, still embedded in the rock in which
they were found. These skulls were found and excavated by a joint team led
by Dr. Sunil Bajpai and Dr. Hans Thewissen in Kachchh, India.
Thewissen, J. G. M., and E. M. Williams. 2002. The early evolution of
Cetacea (whales, dolphins, and porpoises). Annual Review of Ecology and
Thewissen, J. G. M., and S. Bajpai. 2001. Whale origins as posterchild for
macroevolution. BioScience 5:1037-1049.
Thewissen, J. G. M., and S. Bajpai. 2001. Dental morphology of the
Remingtonocetidae (Cetacea, Mammalia). Journal of Paleontology 75:463-465.
Bajpai, S., and J. G. M. Thewissen. 2000. A new, diminuitive whale from
Kachchh (Gujarat, India) and its implications for locomotor evolution of
cetaceans. Current Science (New Delhi) 79:1478-1482.
Thewissen, J.G.M., and S.T. Hussain. 2000. Attockicetus praecursor, a new
remingtonocetid cetacean from marine Eocene sediments of Pakistan. Journal
of Mammalian Evolution 7:133-146.
SOME MODERN WHALES
Adapted from figures in Peter Evans' The Natural History of Whales
& Dolphins, 1987