The archaeocetes are a paraphyletic group of primitive cetaceans that include the earliest, terrestrial 'whales'. The group consists of six families:
Pakicetidae, Ambulocetidae, Remingtonocetidae, Protocetidae, Basilosauridae and Dorudontidae, although some scientists include the latter two in one single family Basilosauridae.
The graph below shows, how these families are related to each other.
In order to understand the biology of the archaeocetes, it is important to first consider
what sort of terrestrial animals cetaceans originally evolved from. For a long time,
palaeontologists speculated that whales were in fact close relatives of mesonychians, an
extinct group of hoofed carnivores that indeed included the largest terrestrial carnivore
(Andrewsarchus) ever. They based this assumption on the rather limited fossil
material they had of both the mesonychians and early whales.
The relations of early whales (archaeocetes) to artiodactyls and the two extant groups, odontoceti and mysticeti. Tree by Felix G. Marx, University of Bristol. Images of cetacenas adapted from National Geographic's The evolution of whales by Douglas H. Chadwick, Shawn Gould and Robert Clark
Re-illustrated for public access distribution by Sharon Mooney ©2006. Open source licence CC ASA 2.5
But in 1994, Dan Graur and Desmond Higgins, two scientists working on cetacean gene sequences dropped a bombshell.
Reviving a claim that had been made as early as the 1950s, they suggested that whales were in fact most closely related to
modern artiodctyls. Although further molecular studies supported their results, many scientist were rather reluctant to accept this novel claim and a lively dispute
as to whether or not mesonychians should continued to be viewed as the ancestors of whales kept many cetologists busy. But in 2001, a crucial bit of evidence came to light that ended the discussion.
One of the most diagnostic features found in the skeleton of artiodactyles is the shape of
the astragalus, which somewhat resembles that of a double-pulley. Exactely this type of bone,
which has never been found associated with mesonychian remains, was found to be part of two archaeocete skeletons.With both molecular and morphological evidence now supporting a cetacean-artiodactyl relationship,
the traditional order Cetacea was merged with Artiodactyla, thus forming the now widely accepted order Cetartiodactyla.
Astragali of the Eocene protocetids Rodhocetus balochistanensis (left) and Artiocetus clavis (right), as compared to that of the pronghorn Antilocapra americana (centre). Note the distinct double-pulley shape. © Philip D. Gingerich 2001, available online on his web site
Ironically, the long disputed fact that cetaceans are indeed derived from early artiodactyls can
now even help us to enhance our understanding of the latter. For example, fossils of the protocetid Rhodhocetus
have been found that include nicely preserved hand and feet. Whereas the
feet are paraxonic,
as one would expect in an artiodactyl, the hands tuned out tob be mesaxonic, allowing us to see how and when the transition from
the primitive five-fingered (pentadactyl) limb to the more derived four- and two-fingered limb typical of artiodactyls occured.
Discoveries of ever more archaeocete fossils now demonstrate the transition from these early, terrestrial, hoofed and artiodactyl-like whales
to the fully aquatic animals we know today and the six families constituting the group will now be described in turn:
Pakicetidae (Gingerich and Russell 1990)
The pakicetids are the most primitive cetaceans we know to date. Their overall
appearance was that of a carnivorous, hoofed and, most importantly, terrestrial animal (although some charaters, such as osteosclerotic longbonesm have been proposed as aquaric adaptations), living
in what is now Pakistan about 48-50 million years ago. The group comprises three genera,
Ichthyolestes, Pakicetus and Nalacetus, all of which were found in fluvial
deposits. Although their skeleton implies a cursorial lifestyle, stable isotope analysis
has reveled that pakicetids foraged in freshwater. Being the most basal of all cetaceans, pakicetids also the oldest, with the sole expection of Himalayacetus, which was found in 53 million year old sediments. However, the age and nature of this specimen are still being disputed.
Pakicetus (a) and Ichthyolestes (b). Taken from: Dr. Thewissen's website, The Thewissen Lab
Beautiful reconstructions of Pakicetus and other prehistoric mammals can be found on Carl Buell's web site.
Ambulocetidae (Thewissen, Madar and Hussain 1996)
Having been about 47-48 Ma ago, ambulocetids are the oldest archaeocetes for which the whole skeleton is known. Apart from Ambulocetus, the group includes the genera
Gandakasia and the arguably oldest archaeocete specimen known so far, Himalayacetus. It is likely that amulocetids led a truly amphibiuos lifestyle, as they are normally found
in marginal marine deposits, as are formed by ancient estuaries or bays. Since stable isotop analysis indicates a ambulocetids were also at least partially dependent on freshwater,
a substantial freshwater influence, such as a river, can be inferred for these freshwater settings.
Skeleton of Ambulocetus natans. Adapted from National Geographic's The evolution of whales by Douglas H. Chadwick, Shawn Gould and Robert Clark
Re-illustrated for public access distribution by Sharon Mooney ©2006 . Open source licence CC ASA 2.5
Unlike pakicetids, ambulocetids show obvious bodily adaptations to an aquatic lifestyle, such as reduced limbs and very large feet suitable for paddling. Their hindlimbs, also strong enough to support their weight on land,
and possibly acting together with a dorsoventral undulation of the spine, probably were their main organ of propulsion in the water.
A tail fluke had not yet evolved. Because hind-foot
paddling is not a particularly effective way of swimming, it is unlikely that the carnivorous ambulocetids actually chased their prey. Rather, they probably ambushed it from the water, in a way strangely similar
Remingtonocetidae (Kumar and Sahni 1986)
The amphibious remingtonocetids lived about 42-47 million years ago. Like pakicetids ans ambulocetids, they, too, were still spatially confined
to what is now Pakistan and India. The group, originally thought to be closely related to the extant odontocetes, but now considered and specialised offshoot of cetacea and evolutionary dead end,
comprises the genera Remingtonocetus, Attockicetus, Dalanistes, Andrewsiphius and Kutchicetus, with Attockicetus being he oldest genus, equal in age to Ambulocetus.
Remingtonocetids are characterised by a long and narrow snout, a long body and small, widely spaced eyes and are typically found in nearshore marine deposits. Stable isotope analysis suggests that, although they foraged on marine food
resources, some of them they still at least occasionally required access to low-salinity waters.
Protocetidae (Stromer 1908)
Protocetids, the oldest group to include the first fully marine cetaceans, lived about 39-47 million years ago. They were also the first cetaceans that finally left
India and, indeed, the ancient Tethys Sea, and specimens of various protocetid genera have been found in
both Africa and North America. As may be expected, protocetids from the Indian subcontinent include the oldest
members of the family and comprise the genera Indocetus, Rodhocetus, Babiacetus, Takracetus, Makaracetus and Artiocetus. Protocetus, Eocetus
and Pappocetus occur in Africa, whereas Georgiacetus, Eocetus and Nachitochia
are found in North America.
Skeleton of Rodhocetus balochistanensis
© Philip D. Gingerich 2001, available online on his web site
The limbs of Protocetus were short, but their hands and feet were long and probably webbed. A tail fluke had not yet evolved in this genus, and the main mechanism of propulsion
of Rodhocetus probably still resembled that of more primitive archaeocetes. Nonetheless, the protocetids are most
likely the group from which the two most derived archaeocete families, basilosaurids and dorudontids, the ancestors of modern cetaceans,
evolved. Like the archaeocetes as a whole, the protocetids as a group are thus paraphyletic.
Basilosauridae (Cope 1868)
Tibia, fibula, ankle, foot and toes of Basilosaurus isis, the foot being approximately 12 cm in length. Note the reduction to three digits.
© Philip D. Gingerich 1991, available online on his web site
The basilosaurids are a family of large, piscivore Late Eocene Cetaceans (about 35-37 million years ago). Due to the elongation of their lumbar vertebra their body form is long and serpentine (snake-like), with recorded length of up to 16 metres. Basilosaurids are likely to have possessed a tail fluke, though it was not their main means of propulasion, as in extant cetaceans. Rather,
they relied on undulations of the back in order to move through water. Their hindlimbs were still present, but tiny, with a foot reduced to three toes, and the pelvis had become detatched from the vertebral column. Thus, they could no longer suppurt their weight on land and the function of the hind limbs probably lay in interlocking their long bodies during copulation. Basilosaurids have been found in Pakistan, the Middle East, especially in Egypt, and North America, in marine sediments between 35 and 38 million years old.
The family is closely related to the dorudontids described below and comprised only two genera, Basilosaurus and Basiloterus, though the validity of the latter has been disputed. Basilosaurus, meaning "king lizard", was originally misidentified as a giant reptile and although the mistake was rectified later, the name first give to the genus remained.
Skeleton of Basilosaurus; the tiny hind limb is shown enlarged. Adapted from National Geographic's The evolution of whales by Douglas H. Chadwick, Shawn Gould and Robert Clark
Re-illustrated for public access distribution by Sharon Mooney ©2006. Open source licence CC ASA 2.5
Dorudontidae (Miller 1923)
Dorudontids are closely related to both basilosaurids and the two families of extant cetaceans, mysticeti and odontoceti, and indeed probably gave rise to both. They were considerably smaller than basilosaurids and, rather than having a greatly elongated body shape, had proportions resembling that of modern cetaceans.
Similar to basilosaurids, their pelvis no longer attached to the vertebral column and their hind limbs tiny, and could thus no longer support their body weight on land. A distinct ball-shaped vertebra in their tail indicates that they possessed a tail fluke and their mode of locomotion is likely to have been similar to that of modern whales. They are found in 35-38 million year old fully marine deposits all over the world, indicating both a wide distribution and complete
independece from freshwater. The family includes the genera Dorudon, Saghacetus, Ancale-cetus, Chrysocetus, Pontogeneus, Zygorhiza and possibly Gaviacetus.
Skeleton of Dorudon atrox; © Philip D. Gingerich 1996, available online on his web site