lock together into a zigzag pattern that cuts into the flesh of the unlucky
victim (figure 47). Pakicetids have the same number of teeth as other
basal placental mammals: 3.1.4.3 in both upper and lower jaw. The
lower molars have a low and a high part (talonid and trigonid), and the
upper molars have three large cusps (figure 34). Crushing basins and
crests on the molars are reduced, and they lack the cutting edges that are
found in carnivores; instead, their wear pattern is similar to that in
other Eocene whales: steep wear facets that indicate that pakicetids
chewed their food in very unusual ways.13 That wear pattern does not
occur in any modern mammal, and it is difficult to make sense of it.
In general, the amount of tooth wear in an animal depends on the
kinds of food it eats, its age, and the way it uses its teeth.14 From the
position of wear facets on the teeth, one can determine how teeth rubbed
along each other and how they interacted with food. Some wear, near
the tips of the cusps, is caused by tooth–food–tooth contact before the
upper and lower teeth contact each other as the jaw closes. This type of
wear is called abrasion (figure 48). During chewing, abrasion is the first
wear to occur. After that, cusps from opposing upper and lower molars
slide past each other and cause a type of wear called attrition, resulting
from tooth–tooth contact. There are two phases to this attritional move-
ment. During phase I, teeth are coming into full contact as lower teeth
shear along the upper teeth, moving up and somewhat toward the side
of the tongue (lingually). Phase I ends when the upper and lower teeth
come into full interlocking contact. In phase II, lingual movement con-
The River Whales | 147
figure 47. The dentition of the left upper and lower jaw of Pakicetus. Tooth
crowns are known for all teeth shown. After L. N Cooper, J. G. M. Thewissen, and
S. T. Hussain, “New Middle Eocene Archaeocetes (Cetacea:Mammalia) from the
Kuldana Formation of Northern Pakistan,” Journal of Vertebrate Paleontology 29
(2009): 1289–98.
tinues as the lower teeth slide further toward the tongue, but now the
jaw opens slightly. At the end of phase II, upper and lower teeth lose
contact, the jaw opens further and the cycle is repeated.
This precise interlocking of the teeth occurs only in mammals, and
attritional wear facets are a characteristic of them. However, modern
cetaceans are an exception to that mammal rule. They do not chew and
do not occlude their teeth very precisely. There are hardly ever attri-
tional facets in a modern odontocete. Most tooth wear in odontocetes
is caused by contact with food: abrasion. This kind of tooth wear can be
spectacular. Some killer whales wear their teeth down to flat stubs. It
has been shown that those individuals suck in water with their prey
and that they eat mostly small fish and occasional seals and large fish.
148 | Chapter 11
Right lower molars
The artiodactyl Indohyus
The cetacean Pakicetus
Abr.
Abr. Abr. Ph. I Ph. I
Abr.
Ph. I
Ph. I
Ph. I
figure 48. Three-dimensional reconstructions based on laser scans of lower
molars of an archaic artiodactyl ( Indohyus, discussed in chapter 14) and the
ancient whale Pakicetus, showing the tooth crown morphology. Artiodactyl lower
molars are characterized by three types of wear: abrasion (Abr.), phase I attrition
(Ph. I), and phase II attrition (Ph. II). Pakicetus and other whales have teeth with
simpler crowns and show nearly exclusively phase I wear. Compare with figure 34
to see how whale molars changed in evolution.
Counterintuitively, killer whales that specialize on feeding on large
whales have barely any abrasional tooth wear.15 Wear is equally impres-
sive in beluga whales, another suction feeder mostly feeding on squid
and bottom fish. When beluga teeth erupt, they are sharp prongs as in
other odontocetes, but then the teeth quickly wear down to be nearly
flat stubs (figure 49). This kind of sloppy abrasional wear is very differ-
ent from the earliest whales.
Basal members of the artiodactyls are the closest land relatives of
cetaceans, and they have unspecialized teeth that display all three types
of tooth wear—abrasion, phase I attrition, and phase II attrition—in
comparable amounts (figure 50). Tooth wear in early whales is extremely
specialized. There is no phase II attrition and barely any abrasion. Phase
I attritional wear dominates these teeth. It is not clear what this means.
Was their prey special and in need of unusual ways of processing, or
was this just the way that pakicetid ancestors chewed, without there
being anything particularly good about chewing in this way? The early
families of whales lived in a variety of environments, from freshwater to
oceanic, but their wear patterns are similar, and the particular diet or
food-processing mode was ubiquitous, regardless of environment. To
understand what went on, we need to know exactly what live prey the
early whales ate, and what whales’ ancestors ate. We might be able to
The River Whales | 149
coin is 18 mm
Young individual in which teeth had
in diameter
not broken through the gums
Old individual in which front teeth
are lost and back teeth are strongly worn
due to use related to suction feeding
figure 49. Lower jaws of a young and old beluga whale, with a penny for scale.
In life, the teeth in the young individual had not erupted from the gums.
track diet by doing more in-depth isotope work. As for the ancestors,
we need to sort through artiodactyls, in particular those from the time
and place where the early whales lived: Asia in the Eocene. Artiodactyls
are clearly critical to solving this puzzle.
Sense Organs. Clues regarding prey also come from the position of the
eyes. In Pakicetus, they are close together and raised above the rest of
the skull near the midline, and they face up, dorsal (figure 51). This dif-
fers from Ambulocetus, remingtonoc
etids, and basilosaurids (figure 52).
The pakicetid position occurs in animals that live underwater but that
watch what goes on above the water-line. Crocodiles, for instance, may
sneak up on their prey with eyes and nose emerged but body and head
hidden underwater. In hippos, the eyes are also elevated above the skull,
enabling them to stay submerged while looking out above the water. It
is likely that pakicetids lay in wait, hunting animals that came close to
the water’s edge. As discussed, the bone-conducted sound of the foot-
steps of prey may have been an important sensory cue.
The unusual position of the eyes affects the other sense organs. The
nose and the nerves going from it to the brain are located between the
150 | Chapter 11
Exclusively
phase II wear
Eocene artiodactyls with
primitive teeth
The Eocene artiodactyl Indohyus
25%
Most Eocene whales
Increase
The Eocene whale Babiacetus
in phase I
wear
50%
75%
Exclusively
phase I
Exclusively
wear (100%)
apical wear
75%
50%
25%
Increase in phase I wear
figure 50. Diagram summarizing tooth wear on lower molars in
ancient artiodactyls and whales. Surface areas of apical abrasion,
phase I wear, and phase II wear are measured, and then recalculated
as a percentage of total surface-area wear. These three kinds of wear
are then plotted on axes that make up the three sides of this triangle,
with the corners representing teeth with exclusively one kind of wear.
In most Eocene whales (red oval), phase I wear dominates on the
teeth, but most basal artiodactyls are closer to the center of the
triangle (yellow triangle), indicating that they had all three kinds of
tooth wear. Redrawn from Thewissen et al. (2011).
eyes and their nerves. In animals with large eyes that are close together,
the structures related to vision seem to encroach on the space for olfac-
tion. This is the case in humans: the nerves to the nose are moved to the
area above the eyes, and they are small. This may be part of the reason
why humans have excellent vision but a poor sense of smell. The same
is true in pakicetids: the closely set eyes make the interorbital region
(the area between the eyes) very narrow. For the fossil collector, this has
the unfortunate consequence of creating a zone of weakness where
most pakicetid skulls break during fossilization; and for the animal it
had the consequence that the nerves coming from the nose that carry
information about smells must be small as they pass through this nar-
row passage. The sense of smell of these first whales was limited. For
reasons that are not clear, the interorbital region is not just narrow but
also long, and as a result, the olfactory nerves and the bony tracks that
they reside in are long. That feature is present in all early whales, and
The River Whales | 151
figure 51. Skull of Pakicetus attocki, the most archaic whale,
known from Pakistan. The circle is the size of a penny, 19 mm in
diameter. Reconstruction based on H-GSP 18467 (braincase and
orbit), 18470 (maxilla), 96231 (premaxilla), 30306 (maxilla), 1694
(mandible), and 92106 (tip of mandible).
can be easily seen in Remingtonocetus (“tract for olfactory nerve” in
figure 35).
Pakicetids have a long snout,16 but not nearly as long as in ambu-
locetids or remingtonocetids. The nose opening was near the tip of the
snout, and bone in this area is perforated by many small holes through
which, probably, nerves traveled. Nerves in this area usually relay infor-
mation from the snout and whiskers back to the brain, and it is likely
that pakicetids had a sensitive snout with many whiskers. Modern seals
use their whiskers to detect vibrations in the water,17 and it is possible
that pakicetids did the same.
Walking and Swimming. The position of the orbits is not the only feature
that suggests an amphibious lifestyle for pakicetids. The bones of the skel-
eton also indicate it. Limb bones of mammals usually have a large marrow
cavity, surrounded by bone. The bone here has the shape of a cylinder; its
outside is massive and is called the cortical layer. It is thinner in animals
that need to be light, like bats, and thicker in those that need strong bones,
like buffalo. In aquatic animals, the bones can be a source of ballast, allow-
ing the animal to stay down and counteract buoyancy, so their cortical
layers are often extra-thick. This is true for hippos and sirenians, for
instance, and is called osteosclerosis (discussed before in chapters 2 and 3).
Osteosclerosis does not occur in aquatic mammals, such as dolphins, for
whom speed is important, because the weight would slow them down.
Unlike most modern whales, pakicetids are osteosclerotic—their cortical
s
the
hales
dontoceti
elphinapteru
O toothed w modern beluga D
Basilosauridae
Protocetidae
ysticetiM baleen whales fetal bowhead whale Balaena mysticetus
Drawings are not to scale and thus not a good
ingtonocetidae
Remingtonocetidae Remingtonocetus harudiensis
e
White oval indicates position of the eye.
Ambulocetida Ambulocetus natans
icetidae icetus attocki
Pak Pak
Hippopotamus
tyla
Cladogram showing the evolution of the position and orientation of the eyes (as indicated by its bony socket, Indohyus
e 52.
n
rtiodac
r
other even-toed ungulates A
u
40 million years ago
50 million years ago
moder
fig
orbit) in some ancient and modern whales.
indicator of the size of the eye.
The River Whales | 153
layer is extremely thick. The osteosclerosis of the limb bones suggests that
pakicetids spent time in the water, but were not fast swimmers.18
So, how much did they move? On land, they could certainly walk.
Their body proportions were similar to those of a wolf, but, given that
the bones were so heavy, their locomotion was probably lumbering and
slow. Just like land artiodactyls, the back was relatively immobile—the
vertebrae of the lower back had interlocking joints that limited move-
ment—whereas the joints of the legs allowed a lot of mobility in
the
front-to-back direction, and less in the side-to-side direction.19 They had
five toes on the hand, and four on the foot, with no indication that there
was webbing. All fingers ended in a small hoof, betraying their ancestry
as ungulates, but when they walked they were not up on their hooves,
but instead had the entire finger touch the surface, like a dog, a pattern
called digitigrady. The osteosclerosis of the limbs would have prevented
fast swimming. Two features reveal a bit more about aquatic locomo-
tion in pakicetids: the pelvis and tail. The pelvis of most four-footed
mammals has a long part in the front, the ilium, and a shorter part in
the back, the ischium. Those length relations are reversed in pakicetids:
the ischium is proportionally long and has a large expanse for the
attachment of the hamstring muscles. Hamstring muscles are large in
animals that kick back their legs, such as seals. That may indicate that
pakicetids did some swimming.
In addition, pakicetids have relatively large tail vertebrae. They are not
as large as in Kutchicetus, and this is a tricky subject to study. Since there
are no associated skeletons of pakicetids, it is not known how many tail
vertebrae they had, and it is thus impossible to know exactly how long
the tail was. Many tail vertebrae were found at locality 62, so it is likely
that the tail was long. Furthermore, the number of tail vertebrae in the
artiodactyls that were relatives of whales ( Messelobunodon, twenty-four)
and in cetaceans slightly younger than pakicetids ( Maiacetus, twenty-
one) are similar, so it is reasonable to assume that pakicetids had slightly
more than twenty vertebrae too. From the shape of the fossil vertebrae
that we found, we know that the tail was muscular. Hind limbs and tail
may have been used to give the animal a burst of speed at the moment of
attacking its prey, but it is unlikely that they were sustained swimmers.
Habitat and Ecology. When we first figured out which bones from
locality 62 belonged to pakicetids, the most impressive thing about
them was how gracile the limb bones were. They do not look like the
stocky limbs of their closest relatives, other Eocene whales, but instead
154 | Chapter 11
resemble those of more distant relatives, the running artiodactyls. As we