Hyaenodon
Name: Hyaenodon
(Hyena tooth).
Phonetic: High-ee-no-don.
Named By: Often credited as Joseph Leidy, Laizer
and Parieu named the type species in 1838.
Synonyms: Alloeodectes, Neohyaenodon,
Pseudopterodon.
Classification: Chordata, Mammalia, Creodonta,
Hyaenodontidae.
Species: H. leptorhynchus
(type),
H. bavaricus, H. brevirostris, H. chunkhtensis, H.
cayluxi, H. crucians, H. eminus, H. exicuus, H.
gigas, H. horridus, H. incertus, H. megaloides, H.
microdon, H. milloquensis, H. mongoliensis, H. montanus,
H. mustelinus, H. pervagus, H. pumilus, H. raineyi, H.
venturae,
H. vetus, H. weilini, H. yuanchensis.
Diet: Carnivore.
Size: Largest species (H. gigas)
just over
3 meters long and about 110 centimetres high at the shoulder.
Smallest species (H. microdon)
much smaller
at around forty to fifty centimetres long.
Known locations: Species known from North America,
Asia, Africa and Europe.
Time period: Late Eocene through to Early Miocene.
Fossil representation: Multiple remains, although
usually of only partially preserved individuals.
Although
Hyaenodon translates as ‘hyena tooth’ the only
similarities between
Hyaenodon and hyenas are the facts that they are
both mammals and they
both eat meat. Beyond this they are completely different animals with
hyenas being more closely related to cats and Hyaenodon
actually being
a creodont, a long extinct group of mammals that did not survive the
Miocene.
Hyaenodon
was the top predator of its day with the larger forms
dominating the landscapes throughout the Oligocene periods. Key to
this success was the large head that compared to today’s animals
looks too large to fit upon the body. Indeed the neural spines of the
forward dorsal vertebrae are enlarged to allow for an increased surface
are for the attachment of neck muscles powerful enough to support the
enlarged skull.
The
skull itself has attachments that allow for the fixing of immensely
powerful biting muscles would enable the jaws to easily crush prey
animals in its mouth. Usually this would involve Hyaenodon
biting the
head or neck area of the animal. Evidence for this comes from a
comparison of predator skulls with the skull of a nimravid called
Dinictis which has puncture holes in its cranium
that closely match the
tooth pattern of Hyaenodon. Additionally
coprolites which have been
interpreted to belonging to Hyaenodon also have
fragmentary parts of
other animal skulls in them.
Hyaenodon
did not rely upon just crushing animals in its jaws however.
Once the prey was down it had to be eaten and for this Hyaenodon
had
specially adapted slicing teeth at the back of these jaws. The really
interesting thing about these teeth is that as the animal grew older
these slicing teeth would rotate against each other. This constant
grinding against each other meant that Hyaenodon
maintained a cutting
edge across these teeth for much longer than other carnivores and
possibly had a longer life expectancy because of it. This also
allowed Hyaenodon to slice meat into smaller pieces
rather than gulping
down large chunks, something that would allow for faster and more
efficient digestion. This is very simply explained by the fact that
smaller pieces would result in a larger surface area exposed to the
digestive acids in the stomach than would happen if it were a single
large piece. A modern analogy would be chemists placing a solid into
a solution will almost always use a powdered form of that solid for
this very reason.
The
slicing teeth that did most of the work in eating where placed at
the back of the mouth where they were closer to the point of jaw
articulation. This is actually a very common area for the placement
of the actual eating teeth because it places them nearer the fulcrum of
the jaw mechanism that allows for far more force to be focused through
these teeth. This is also why the teeth in the front of your mouth
are shaper for biting off a piece of food, while the flatter molar
teeth that you use to actually chew your food at nearer the back.
Spending
more time processing food in the mouth meant that Hyaenodon
would not have been able to breathe through its mouth while it was full
with whatever it had just killed. But Hyaenodon
had a very simple
adaptation to deal with this and this was a bony palate that extended
well beyond the back teeth of the upper jaw. This supported the nasal
passages so that they continued to carry air to and from the lungs even
when the mouth was otherwise blocked.
Out
of all its senses, smell seems to have been the most important to
Hyaenodon due to CAT scans that reveal a
well-developed olfactory
bulb. Include the larger skull that would have meant a nasal area
with a proportionally larger surface area than a smaller skulled
animal, and its reasonable that Hyaenodon tracked
prey by scent,
possibly identifying which animals were sicker and weaker than the
others.
However
there is fossil evidence that Hyaenodon did not
just randomly
roam across the countryside hoping to pick up a scent. A fossil site
of what was once a watering hole has been found that contains large
numbers of herbivore as well as Hyaenodon remains.
While this could
be interpreted as Hyaenodon just drinking,
predators in other parts
of the World such as Africa have been observed to frequent watering
holes while hunting for prey. This is simple but also very
intelligent behaviour as it is pointless to expend energy on the chance
of maybe finding prey some distance from a watering hole, when by
staying put you know that your prey will have to eventually come to you
in order for the chance to drink.
By
being in those locations Hyaenodon would have
waited for its target
to settle down and drink. With its guard down, Hyaenodon
could
ambush it from the undergrowth and be upon its prey before it had time
to react, with the crushing jaws clamping around its preys head to
bring a swift kill. Further fossil evidence for a variety of other
animals from small oreodonts, rhinos, camels to even primitive
horses like Mesohippus
show signs of being fed upon by Hyaenodon which
suggests that this predator was not especially selective about what it
hunted.
Like
with so many of the Cenozoic predators, Hyaenodon
seems to have
succumbed to the effects of climate change rather than a specific
extinction event. As the Miocene period progressed the world’s
climate became drier due to global cooling that also resulted in a
reduction of sea levels. The result on land was that forests and
scrub were being replaced by vast expanses of grassland that many of
the herbivores such as the oreodonts were not suited to eating,
meaning that the oreodonts that were one of the more common animals
disappeared from the menu. Additionally other herbivorous animals
were beginning to grow larger with longer legs so that they could
better cope with this new kind of environment. As such the types of
animals that Hyaenodon used to prey upon like the
horses and the camels
were becoming too fast for Hyaenodon to catch,
with greatly reduced
amounts of cover denying Hyaenodon the chance to
use ambush tactics to
sneak up on prey.
The
final occurrence that completed the demise of Hyaenodon
was the
emergence of new carnivores such as the bear
dogs, and while they are
more often associated with the old world (such as Asia, Europe,
Africa), they were able to spread into North America by crossing
the Bering land bridge that was created by the aforementioned falling
sea levels. The most famous of this group is Amphicyon,
but all of
these new predators were more suited to hunting across open ground as
well as having larger and more powerful bodies. Hyaenodon
could not
compete with these new hunters and was quickly relegated to a
subordinate position to them. No longer being able to kill its own
food, or even scavenge the kills of the more powerful bear dogs,
meant that by this stage Hyaenodon was living upon
borrowed time.
Further reading
- Review of the Miocene Wounded Knee faunas of southwestern South
Dakota. - Bulletin of the Los Angeles County Museum of Natural History,
Science 8:165-82. - J. R. Macdonald - 1970.
- Paleobiology of North American Hyaenodon (Mammalia, Creodonta). -
Contributions to Vertebrate Evolution 1:1-134. - J. S. Mellet - 1997.
- Hyaenodon venturae (Hyaenodontidae, Creodonta, Mammalia) from the
early Chadronian (latest Eocene) of Wyoming. - American Geological
Institute. - Alexander V. Lavrov & Robert J. Emry - 2004.
- Hyaenodonts and carnivorans from the early Oligocene to early Miocene
of the Xianshuihe Formation, Lanzhou Basin, Gansu Province, China. -
Paleontologica Electronica 8(1):1-14. - X. Wang, Z. Qiu & B.
Wang - 2005.
- Differences in the tooth eruption sequence in Hyaenodon (‘Creodonta’:
Mammalia) and implications for the systematics of the genus. - Journal
of Vertebrate Paleontology 31(1) - Katharina Bastl, Michael Morlo,
Doris Nagel & Elmer Heizmann - 2011.
- First evidence of the tooth eruption sequence of the upper jaw in
Hyaenodon (Hyaenodontidae, Mammalia) and new information on the
ontogenetic development of its dentition. - Pal�ontologische
Zeitschrift - Katharina Bastl & Doris Nagel - 2013.
- New material on small hyenodons (Hyaenodontinae, Creodonta) from the
Paleogene of Mongolia. - Paleontological Journal. 53 (4) - A. V. Lavrov
-2019.
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