Nimravus
Name: Nimravus
(Ancestral hunter).
Phonetic: Nim-ray-vus.
Named By: Edward Drinker Cope - 1879.
Synonyms: Archaelurus debilis, Dinictis
major, Nimravus altidens, N. bumpensis, N. confertus, N.
debilis, N. gomphodus, N. major, N. meridianus.
Classification: Chordata, Mammalia, Carnivora,
Nimravidae.
Species: N. brachyops (type),
N.
sectator.
Diet: Carnivore.
Size: 1.2 meters long.
Known locations: USA - California, Nebraska,
Oregon, South Dakota.
Time period: Oligocene through to Aquitanian of the
Miocene.
Fossil representation: Multiple specimens.
Nimravus - Not a Cat!
Nimravus
is the type genus of
the Nimravidae group of mammals that are better known as the ‘false
sabre-toothed cats’. This is because while Nimravus
and others like
it looked like the big cats, they actually evolved from a different
line of mammals than the true cats that are members of the Felidae
branch. Despite the differing lineage Nimravus
developed a body form
that was so much like a predatory big cat it leads to the conclusion
that the big cat body form is so well suited to the predatory lifestyle
that it has independently repeated itself in nature.
Although
palaeontologists who
specialise in mammals and particularly big cats can identify a number
of features different between Nimravus and true
cats, there are two
main areas that anyone can spot if given the opportunity to study the
paws and the inner cranium. First are the paws which are shaped more
like those of dogs rather than a cats, something which may imply that
Nimravus spent most of its time on the ground rather
than climbing into
trees like some cats do.
The
second identifying
feature that tells us Nimravus is not a cat is the
construction of the
auditory bullae. This is where the inner and middle ear is located,
and each one is a hollow chamber on either side of the skull for each
ear. In true cats this chamber is divided up into two by a bony
septum, but in Nimravus and other nimravids the
chamber is preserved
as a single hollow. Of course in life this chamber may have still
been divided by a cartilaginous septum that did not preserve but it is
still enough of a difference to be a defining characteristic between
nimravids and true cats.
Nimravus the
predator
Nimravus
was active from the
Oligocene to early Miocene and throughout this time it would have been
one of the key predators of North America. Perhaps the most
interesting predatory adaptations are the enlarged forward canines that
look so similar to the sabre teeth of the much more well-known
Smilodon
(better known as the sabre-toothed cat). Considering
that these two genera are not just separated by several million years
but also a differing family line, this is a remarkable case of
convergent evolution. The fact that Nimravus had
these teeth, as
well as other unrelated mammals such as the marsupial Thylacosmilus,
suggests that hunting with these teeth was a particularly effective
way for predators to hunt medium to large mammalian prey, and briefly
bringing Smilodon back into the frame, was a
technique that
disappeared comparatively recently only ten thousand years ago.
Because
the feet of Nimravus
were more similar to canids and with partially retractable claws,
Nimravus is not envisioned as a hunter that crossed
vast expanses of
ground while chasing after prey. Instead it probably exhibited more
cat-like behaviour by lurking amongst long grasses and approaching prey
animals from downwind. Once the target animal was close enough
Nimravus could launch its attack and cover most of
the ground between
them before the prey realised what was going on. How the prey was
killed remains a controversial subject as there are a number of
theories about how a predator with oversized canines like Nimravus
could have killed prey without damaging the teeth.
It
is possible that in the
surprise of the initial attack Nimravus could have
delivered a quick
but deep bite to an area like the neck that opened up an artery which
caused the prey to bleed to death in a few minutes. Alternatively
Nimravus may have tried to grapple or cling onto
prey and deliver a
penetrating bite to an area like the throat that closed the windpipe.
However Nimravus was of a more lightweight gracile
build than some
other nimravids and true cats, so it may have been restricted to
tackling smaller and more lightweight prey animals, or the juveniles
of larger animals.
Possible
prey animals for
Nimravus could have included camels like Poebrotherium,
or possibly
even smaller individuals of oreodonts like Eporeodon.
Nimravus was
not the only carnivore of these times however as it also would have had
to share its habitat with enteledonts, powerfully built carnivores
with massive mouths thought to be related to modern pigs, as well as
other nimravids. In fact one Nimravus skull has
been found with
puncture marks from another carnivores teeth in it, and one analysis
suggests that they were caused by another nimravid called Eusmilus.
The
disappearance of Nimravus
at the start of the Miocene period coincides with a shift in the
climate of North America towards a dryer and more open expanse of
plains. This caused prey animals to change to better cope with this
new environment which typically suited longer legged, faster animals
that were likely beyond the ability of Nimravus to
catch.
Additionally, new predators such as ‘bear dogs’ like Amphicyon
had made their way across the Bering land bridge into North America,
and were starting to displace the older predatory forms from their
positions at the tops of the food chains. In this new ecosystem
Nimravus found itself obsolete and unable to adapt,
its last remains
entering the fossil record at the start of the Miocene.
Further reading
- A Lower Miocene fauna from South Dakota. - Bulletin of the American
Museum of Natural History 23(9):169-219. - W. D. Matthew - 1907.
- The species of Nimravus (Carnivora, Felidae). -
Bulletin of the
American Museum of Natural History 118(2):71-112. - L. Toohey - 1959.
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