For a little over twenty million years between the mid Eocene to early Miocene periods the entelodonts roamed across North America and Eurasia. Often considered to be ancient pigs, the exact relationship between enteledonts and modern pigs is an uncertain one that varies upon differing opinions.
What are entelodonts?
Although
entelodonts are today usually interpreted as being pig-like
(particularly warthogs), most palaeontologists are of the opinion
that they are more closely related to mammals like hippos. This
instead explains the similarity of entelodonts to pigs as a case of
convergent evolution, a phenomena where two groups of animals evolve
similar characteristics to cope with similar survival requirements.
This might sound odd, but it is an idea not without precedent, in
fact another group of mammals active at the same time as entelodonts
were the nimravids which were very much like cats even though they
evolved long before the first known members of the Felidae (true
cats).
Other
mammals of the time have also been considered to be related to the
enteledonts, with the best known and perhaps most controversial case
being Andrewsarchus.
Often dubbed the biggest mammalian predator on
land of all time, this mammal is so far only known from the skull,
and most popular reconstructions are based upon another mammal called
Mesonyx
being used as a stand in for the missing body. Today however
the overwhelming majority of palaeontologists agree that Mesonyx
is not
a reliable stand in, and so the body of Andrewsarchus
remains unknown
in form. However the skull of Andrewsarchus does
bear a striking
similarity to the skulls of entelodonts with a long snout, wide
jugals and similar tooth arrangement and fossil remains are known from
the same time period and approximate location of other entelodont
remains. This had led to some cautious speculation that Andrewsarchus
may be a closely related to entelodonts if not a different form of
one, but without more complete confirmed Andrewsarchus
remains being
recovered it is impossible to prove this with certainty.
Physical features
Entelodonts
look weird. When you see a skeleton of an entelodont you can
immediately pick up on the solid robust skull and the box like body,
but the legs are so thin they seem unable to support a heavily built
creature. The skull is carried forward from the body to the point
where if you measure from the tip of the snout to the pelvis, the
forelegs would actually be close to the midpoint of the measurement.
The neck vertebrae do not look strong enough to carry such a large and
heavy skull so far from the body, but it should be remembered that
this was just part of the hard frame of the creature. In life these
vertebrae would have been supported both above and below by incredibly
powerful muscles, evidence for which comes from the enlarged neural
spines of the forward dorsal vertebrae above the shoulders, and a
robust sternum below reinforced by tightly packed ribs. These neck
muscles would have been so well developed that they would have easily
allowed entelodonts to move their heads around.
The
skull is noteworthy for being long with most of the width coming from
two greatly enlarged jugals (cheek bones). Popular thinking is
that these jugals would have allowed for the placement and attachment
of incredibly powerful jaw closing muscles. An additional theory
however is that the jugals would have made it difficult for a rival
entelodont to close its jaws around the head of another entelodont.
Both ideas are plausible, and it’s conceivable that both may apply
to entelodonts in general.
Entelodonts
are also noted for having all four kinds of mammalian teeth, these
being incisors at the front, followed by canines, then pre-molars
until finally a rear battery of molars. The canines are the most
striking teeth due to their proportionately large size to the other
teeth to the point that they are virtually tusks. The incisors while
small are actually a little more interesting since they point forwards
instead of touching one another to form a shearing edge, but more on
this in the next segment. As with all mammals the molar teeth are at
the back of the mouth to take full advantage of the jaw closing
muscles. By being towards the rear of the mouth the molars would be
nearer the point of jaw articulation which meant that forces from the
jaw closing muscles would be more focused upon whatever the entelodont
was trying to grind or crush in its mouth.
Although
an actual entelodont brain has never been preserved it is still
possible to reconstruct the external physical dimensions by studying
the cavity in the skull where the brain would have been. Studies have
revealed two important things, one being that the brain was quite
small when held in proportion to the rest of the body, two, the
brain itself was quite primitive in its layout. Entelodont brains
also seem to have been wired for making sense of external stimulae
since for example the olfactory region (smell) seems to have been
quite well developed in comparison to other areas. Altogether this
has led to the idea that entelodonts were not problem solvers and
instead lived by instinct rather than reason.
Entelodonts
are classed within the Artiodactyla, a group better known either as
‘even-toed ungulates’ or more simply as the ‘hoofed mammals’.
The actual hoofed feet of entelodonts were just two toes touching the
ground while another two were raised off the ground. These toes not
touching the ground are what are termed vestigial which means still
present but no longer serving a practical purpose. The legs
themselves are quite short when compared to the size of the body and is
also the most gracile part. It’s realistic that over short distances
entelodonts would have been competent runners capable of high speed;
however they were simply not built for long distance running.
What did entelodonts eat?
This
is probably the one question that has sparked the most interest about
entelodonts. Due to the pig-like description so commonly applied to
entelodonts, they have often been depicted as herbivores, probably
using their large canines to root up buried plant parts such as
tubers. More modern study of entelodonts however has yielded
startling revelations about the true diets of entelodonts, namely
that they absolutely were not strict herbivores.
Looking
at the teeth again and the first clue that entelodonts were not
dedicated herbivores comes from the types of teeth, in that they were
all there, incisors, canines, premolars and molars. Strictly
herbivorous mammals usually have a predominance of just incisors at the
front and molars at the back, usually with a gap in-between where the
canines and premolars would normally be called the diastema.
Additionally the incisors that entelodonts have do not form a single
shearing edge for cropping off plants and instead angle slightly
forwards rather than just down. This would actually allow the
incisors to easily project past the canines and physically grip a chunk
of meat or a bone on a carcass so that it could be pulled free.
Another factor to consider is the sheer bite force offered by the wide
jugals and their connecting muscles, which would simply be beyond
that necessary for a herbivorous animal.
The
‘smoking gun’ evidence that suggests entelodonts were meat eaters
comes from numerous remains of Eocene to early Miocene era mammals from
primitive rhinos and camels to even a chalicothere femur that still
bear tooth marks in their preserved bones. When these tooth marks are
matched up to the dental arrangements of other known mammals of the
time, entelodonts often end up being the closest match and hence most
likely animal to have caused them. There is even a collection of
primitive camel remains that suggests that some entelodonts (in this
case Archaeotherium)
displayed caching behaviour which basically
means they would horde animal parts for later consumption.
The
one area that still cannot be ascertained with certainty regarding the
diet of entelodonts is the proportions. The dentition of entelodonts
was still capable of dealing with either a diet of meat or plants,
and it’s not impossible that both types of food were eaten making
entelodonts omnivores. One scenario could be that of entelodonts
rooting through the ground for suitable plants, while also
occasionally scavenging carcasses and bones to supplement their diet,
perhaps to make up for a nutritional deficiency from the lack of a
certain nutrients presence in the plants they were eating. It could
also be however that entelodonts never ate plants at all but were hyper
carnivores (eating absolutely nothing but meat). Alternatively
the exact ratio between plants and meat may have depended upon the
entelodont genus in question with some leaning more towards meat. All
that can be said for certain is that at least some entelodonts did eat
meat, but that now leads to ideas of how they acquired it.
Are entelodonts predators or
scavengers?
Although
many people have described entelodonts as being ugly looking animals,
one thing that cannot be denied is that they were built for power.
Their box-like bodies and robust heavy skulls would have been
surrounded by solid muscle, and for their size they were probably one
of the heaviest types of mammal on the landscape. Most importantly
this made the largest entelodonts a match for nearly every other type
of herbivore and predator they were likely to come into contact with.
One
idea is that entelodonts were actually dedicated predators of other
animals. Here they could have lurked around denser growths of
vegetation and/or watering holes where they could ambush unwary
herbivores. They could then either use their bulk to knock their
target off their feet or close their massive jaws around a critical
area like the neck. Additionally the eye sockets in entelodont skulls
are angled to face slightly forwards which means that entelodonts
probably possessed a strong degree of depth perception that allowed
them to gauge distances of objects, or perhaps in this context prey,
that were in front of them. Although not entirely unknown in
herbivores, such vision is more typical to predatory animals.
However
not all palaeontologists are convinced about this kind of predator
scenario and instead suggest that entelodonts were more likely
scavengers. One particular point is that as hoofed animals,
entelodonts were not capable of grabbing hold of prey. This would be
a solid argument if not for the fact that not all predators try to hold
their prey. Additionally once a prey animal was dead say for a bite
from the jaws there would be no need to hold onto it and assuming that
an animal was capable of delivering such a devastating bite, say a
crocodile or one of the South American terror
birds or even an
entelodont, it is plausible.
Many
fossils that bear entelodont-like tooth marks on them are skulls but
these could be interpreted as supporting either hunting or scavenging.
Most predators will try to hit a vulnerable area such as the neck and
occasionally the head to try and kill their prey quickly. The head
however does not have as much soft tissue on it as the rest of the
body, and since a predator probably cannot eat a similarly sized
animal in one go it will eat the softer areas such as the flanks and
thighs first. The areas with less meat on such as the head and
extremities like the feet do not normally get eaten until the more
meaty areas have been finished, since the closer proximity of hard
bone in these body parts increases the chance of tooth damage. It’s
the less meaty areas that scavengers usually evolve to deal with since
they are the areas most likely to be left. This is where the extreme
bite force of entelodonts would be a significant advantage since they
could crack the carcass bones and get at the bone marrow within,
abilities that may have been beyond the original predator that brought
it down.
More
insight into how entelodonts acquired their meat comes from trackways
that have been located in Toadstool Geologic Park, Nebraska, USA.
Here there are many trackways of different mammals including
herbivores like primitive rhinoceroses and carnivores such as
Hyaenodon.
What is even more interesting however are the trackways of
entelodonts (probably of the genus Archaeotherium)
that not only
seem to follow some of these other tracks but do so in a zigzag pattern
rather than a straight line. A zigzag pattern is no choice for
chasing down an animal but it does allow an animals to track and zero
in on another by scent.
What
we do know about entelodonts is that they had reasonably good senses of
smell, and by turning towards one direction and then another it could
then sample the air for the strength of the scent. When the scent was
stronger in one direction over the other, the entelodont would know
to turn in that direction, steadily narrowing down the options until
it was able to get a visual fix upon the source. Once found the
entelodont could begin feeding upon the carcass, and if another
carnivore was already there, possibly drive it away so that it could
have the carcass to itself. Although not as exciting as the depiction
of entelodonts as predators, if an entelodont could drive off other
predators by intimidation alone it could live and eat with a reduced
amount of energy expenditure as well as experience a significantly
reduced risk of injury.
Temporal and
geographical distribution of entelodonts
Currently
known entelodonts first appear around the Lutetian period of the Eocene
with genera such as Brachyhyops.
This genus is known from North
America, though another genus from China called Eoentelodon
has been
speculated to be a synonym to Brachyhyops. This
blurs the point of
origin theory for entelodonts to possibly be Asia, though it’s
perfectly possible that entelodonts could cross between Asia and North
America over a land bridge that existed between these two continents at
this time called Beringia. As such the precise point of origin for
entelodonts remains uncertain, though future discoveries may help to
clarify the issue.
During
the late Eocene and Oligocene eras entelodonts seem to have been at
their most numerous in terms of different genera. In addition they
are known to have ranged across Eurasia and North America. However by
the early Miocene entelodonts were on the decline and currently the
latest known remains are dated to around the Burdigalian stage of the
Miocene. This disappearance seems to have been global all at once
with Burdigalian era remains being attributed to Asian and North
American genera.
The
popular conception of entelodonts regarding their size is that the
earlier genera were small while the later genera grew larger, a
pattern that is repeated in most animal groups. To an extent this is
true for entelodonts since two of their largest genera (Paraentelodon
and Daeodon)
are known from Late Oligocene to early Miocene
deposits. However to contradict this claim the type genus of the
Entelodotnidae Entelodon
is of a large size almost as big as these
two later genera, yet Entelodon is dated farther
back to the late
Eocene to early Oligocene. This clearly indicates that even at this
earlier stage in the entelodont timeline, there was still a niche for
an exceptionally large genus.
This
in turn could indicate that entelodonts evolved from more moderately to
larger sized ancestors, they could quickly alter in terms of size of
successive generations to even the possibility that we have yet to find
the earliest entelodont ancestors. If the latter is true then future
discoveries of entelodonts might be yielded from the earliest Eocene to
possibly even the late Paleocene. Shifting to the other end of the
scale and it’s not completely out of the question that entelodont
fossils may one day be discovered later into the Miocene.
Extinction - Demise
of the Entelodonts
When
looking for the exact cause as to why an animal that lived millions of
years ago disappeared you have to consider all possibilities rather
than get distracted by just looking for one single reason. In the
case of entelodonts their disappearance seems to be tied in with
on-going climate change of the Oligocene and Miocene epochs, but here
it is other changes that occurred as a result of this climate change
that more directly affected the entelodonts.
The
main sequence of the climate change of this time is that of global
cooling with the ecosystems on land changing from more tropical forests
to open grasslands. If the entelodonts did eat plants or select parts
of them such as tubers, then these changes in ecosystems could have
triggered a decline and possibly a large scale disappearance in
suitable plant types. Other groups of herbivores during the Oligocene
to Miocene such as primitive horses were able to make the shift from
browsing vegetation (that was slowly decreasing in density) to
grazing grass (that was becoming more common) as can be seen from
their changes in such features as teeth. Entelodonts as a group
however display no such change which indicates that they either could
not shift to eating new plant types, or alternatively didn’t eat
plants.
This
leads back to entelodonts being meat hunters, but let’s look at the
idea of predation first. In the Eocene and Early Oligocene the vast
majority of herbivores were browsers, and particularly in the Eocene
there would still have been a lot of growth. This afforded both cover
as well as making it unnecessary for herbivores to travel vast
distance. This is why most herbivores had shorter legs that were less
well adapted to running than their future descendants; the need for
speed just wasn’t there. Entelodonts probably could run at high
speeds over short distances, and could also use the cover of the
extensive vegetation to approach herbivores. Even if a herbivore
managed to spot an entelodont, the resulting chase would have
probably been quite even.
The
on-going shift towards more grassy areas however meant that the more
successful herbivores were developing longer legs that were better able
to carry them over distances, something that also allowed them to run
faster. Additionally the increase in open grassy areas meant that
there would have been less cover for entelodonts to approach meaning
that they if they did hunt then they would have to run for longer after
animals that were much better adapted to running than they were.
Eventually this would mean expending more energy than they were
getting from food resulting in starvation.
The
third scenario which many consider to be the most likely is that
entelodonts were scavengers that searched for carcasses while relying
upon their bulk to drive off other predators that they might come
across. Additionally entelodonts might have followed other predators
and waited for them to make a kill just to drive them off afterwards.
Scavenging like this might have always been a factor in entelodont
behaviour, however the entelodonts, particularly later ones, may
have been driven towards it due to their inability to hunt new types of
faster herbivores or forage certain plants. During the Eocene and
Oligocene entelodonts may have occasionally been going up against
creodonts, nimravids,
mesonychids, and early amphicyonids
amongst
others, all powerful predators, but most of these genera would
have come up second when against the larger entelodont genera such as
type genus Entelodon.
Other
big genera such as Daeodon thrived into the early
Miocene but now they
were living in times that saw newer predators that were larger,
meaner and perhaps most important of all, more intelligent than
their predecessors that they had already largely replaced. Many of
these predators such as the large bear dogs and another group of false
sabre-toothed cats called the barbourofelids
were more capable hunters
that would have been able to offer much more of a fight even to the
larger entelodont genera.
By
the mid Miocene the remaining entelodonts were relics of an older and
very different time. In this new era that they found themselves in
they lacked a complete set of predator skills that would allow them to
be deal with a variety of new herbivore types, and even with their
immense physical power they could no longer bully the other meat eaters
into giving up their food. The change in plant types may have also
made it difficult for them to switch to a more herbivorous diet that
would take them out of direct competition with the newer and more
competent predators, many of which now likely had the jaw power to
process all of a carcass leaving little to nothing for the entelodonts
to clear up. Outclassed and not adapting to a new lifestyle meant
that it would only be a matter of time before the entelodonts vanished.
Archaeotherium Brachyhyops Cypretherium Daeodon (Now also includes Dinohyus) Entelodon (Now also includes Elotherium) Eoentelodon Paraentelodon |