Search Results for megantereon

Fangs for show?: making sense of Megantereon’s toothy grimace

For more than a century paleontologists have been puzzling about the function of the sabertooths’ namesake upper canines, with new studies and analyses being published every year, and a sort of consensus is emerging about the way these extinct predators would use their impressive weaponry, as discussed elsewhere in this blog. But a few decades ago it was fashionable to say that such enormous canines could not be used for killing prey and were there only for display. According to that view, sabertooths would intimidate other predators with their showy fangs at kill sites and gain access to carrion, (their primary food source, according to such theories) as well as using them to resolve disputes with rival males for the access to females.
There are many reasons why this theory is wrong, as I discuss at lenght in my book, “Sabertooth”. But the important thing I would like to stress now is that, although the primary function of the “sabers” was related to hunting, it doesn´t mean they weren´t used in display. For instance, let us look at Megantereon. When the animal was relaxed, the tips of the upper canines would protrude beyond the upper lips, an impressive but not neccesarily scary sight.

Here is a protrait of a relaxed Megantereon with the tips of the sabers protruding beyond the lips
megantereon relaxed

But when several muscles of the face contracted, pulling back the lips and moving up the nerve pads of the whiskers, the resulting display of teeth would give pause to any rival. However, the facial language of the sabertooths, like that of any cat, would be more complex and subtle than just an option between baring the teeth or not. Felids share with us primates the privilege of having more facial muscles than any other mammal, and they can convey a wide range of emotions to a conspecific with their facial expressions. Baring the teeth more or less fully, opening or closing the eyes (and pupiles), turning the ears one way or the other, all these elements combine to send totally different messages. For instance, in the illustration shown here Megantereon is not really trying to intimidate anyone; instead, it has just caught the smell of another cat in the plants around it, and is performing the “Flehmen”, a non-aggressive gesture that is produced as the animal tries to smell particles in the air with the Jacobson’s organ, located in its palate.

This illustration shows Megantereon as it bares its teeth during the Flehmen gesture
megantereon snarling

Facial expression is completed with motion. It can be sometimes difficult to read the difference between a Flehmen gesture and an aggresive snarl in a still photograph, but in the live animal there would be no possible confussion. When doing “flehmen”, the cat moves its head slowly to the sides as it tries to capture the scents in the air, but I don’t need to remind any cat owner of what kind of motions and gestures accompany the baring of the teeth in an irritated feline!
But let us not deceive ourselves, even when the teeth are used “for show” in aggresive displays, the animal must be ready to turn threats into real aggression. And that was a definitive weakness of the “display only” theory of sabertooth canine function. If paleontologists rule out the use of the sabers in hunting because of their supposed fragility, then it makes little sense to hypothesize that they were used to scare rivals away, because no display of strength can be used indefinitely without an occasional demonstration. And biting your rivals during a fight is no less dangerous for fragile teeth than it is to bite your prey. In fact, during a dirty fight with a rival cat there is even less control and more chance for breakage.
Sabers were there for show, most certainly. But not merely!

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Pleistocene Snapshots -Megantereon

In our imagination, we often picture the sabertooths engaged in savage predatory battles, but when we encounter the big cats in the wild most of the times they are just resting, or walking casually from point A to point B. I like to imagine what it would be to travel back in time to the Pleistocene and, after a long search, to come across a sabertooth cat, like this Megantereon, walking relaxedly and disappearing in the bushes after a short while… we wouldn’t need to get scared by its ferocity, just to absorb its beauty. This is one of the reasons why I think we can never pay too much attention to anatomy and proportions when reconstructing these creatures. Only in that way can we reach a bit closer to that impossible goal -to see the real animals face to face.
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Predators, Dreams, and Extinctions

I remember well the first excavations at the fossil site of Batallones-1, over a quarter of a century ago. After some teeth of the saber-tooth cat Promegantereon appeared at the site it seemed likely that, for the first time ever, a complete skull of the mysterious animal could be found. Back then, that possibility excited me so much that one night I even dreamed that we had discovered such a fossil and that I held the skull admiringly in my hands, as we drove to take it to the Madrid museum…

Reality fulfilled my dream and surpassed it by far. Today, we have so many complete skulls of Promegantereon from Batallones that they won’t fit on a large laboratory table. The sample of fossils from this site complex not only has provided complete skeletons of species that previously were known only from a few scraps, but it has yielded the remains of several species completely new to science. Dozens of academic papers and several Phd dissertations derived from the study of the sites have largely rewritten the history of the evolution of carnivorans and saber-tooth cats in particular.

These days there is a large exhibition celebrating the scientific success of the Batallones excavations. Some of the most amazing fossils that you can imagine are shown together in the hall of the Museo Arqueológico Regional de Madrid, in Alcalá de Henares. I have had the privilege to create a large series of illustrations updating the way we see the animals and environments of Madrid province some 9 million years ago.

This illustration I did for the Batallones exhibition depicts the best known carnivore species from the sites, shown at the same scale. They are shown life-size in the exhibition

In a way, this exhibition is another dream come true. My favorite beasts, the saber-tooth cats, have achieved center stage together with an amazing array of fossil carnivorans and other vertebrates. I am thrilled to think that many children will be exposed to the wonders of paleontology, as I was almost half a century ago when my imagination was set aflame by seeing a reconstruction of saber-tooth cats in their world painted by master paleo-artist Rudolph Zallinger.

But fossils are only half the story here. The fascination of saber-tooths derives from their combination of the strange and the familiar. Monstrous as they may appear to some, they were, after all, big cats. So, it is the modern big cats that have, so to speak, provided the flesh with which I have dressed the dry skeletons of their extinct relatives. As a child I dreamed of studying the fossils of saber-tooths but also of watching the big cats in the wild and learning about them first-hand. Those are dreams that I have fulfilled, and I am happy that I had the choice to pursue them. The fossils were there in the museums and in the sediments, and the living predators were there out in the wild. I could have settled to just learning about them through the pages of books (and later, computer screens), but I had the chance to go out and experience the real thing. But now I am worried about the options available to the children that will visit this exhibition.

I don’t know what are the chances that children now in school will be eventually able to pursue a career in paleontology (in Spain the prospect is already grim today), but their chances of seeing large carnivores in the wild a few years from now are deteriorating fast. To think that the last generation to enjoy the privileges we had regarding the observation of wildlife may be already alive is a sad but possible perspective. To experience an environment where big predators still rule is to renew our connection with the kind of world we are evolutionarily “designed” to live in, and to lose that connection is to enter an era of madness, a time of vicarious fulfillment of frustrated needs. And yet now we face a new variety of cynicism among some scientists, who claim that there is no point in lamenting extinctions because they have been happening for many millions of years and people eventually get used to everything, so after a while lost species will no longer be missed. A philosophy that contemplates without a tremor to deprive the next generations of the experiences that have enriched our own lives is the most depressing consequence of a world view that sees human life as little more than a balance between ingestion and excretion. Such views reveal the impoverished sensibilities of those scientists -or their will to downplay losses as a way to skip the fight to prevent them.

Now as you look to the assembly of magnificent carnivorans from the Miocene of Batallones, just imagine your grandchildren facing a similar illustration, but showing the lion, leopard, wolf, lynx, polar bear… by then completely extinct in the wild. Imagine the desolation of knowing that there is nowhere in the world where lions or tigers reign as sabertooths reigned in the distant past. Today those places still exist but if one day they disappear it will be, at least in part, because of our own idleness. Just by having a clear opinion and making it heard, or through our vote, we can make a difference. But trying to convince ourselves that extinction doesn´t matter is perhaps the ultimate sign of cowardice, and thinking that future generations will not be aware enough of their loss to reproach us is the farthest thing from a consolation. We need the fossils in the museums and the living predators out in the wild. Each thing in its place!

Swift pussy cat: Batallones’ little feline wonder revealed

The fossil sites of Batallones provide amazing insights into the predator guild of the Vallesian epoch (Late Miocene, 9,5 Ma) of Spain, and are best known for the incredible collection of fossils of sabre-toothed felids, including the leopard-sized Promegantereon and the lion-sized Machairodus. A less known fact is that the other “half” of the felid family, the felines (or “conical-toothed cats”) were already present and represented by a respectable sample of fossils at the site.
Those early relatives of our modern lions and tigers posed no threat for the sabertooths, because they were all much smaller animals. Two species are known from the site, the lynx-sized Pristifelis attica and the wildcat-sized Leptofelis vallesiensis. Years ago, in our initial description of the animal we called it Styriofelis vallesiensis because its dentition was very similar to that of earlier, Middle Miocene felines classified in the genus Styriofelis. However, our recent analysis of the postcranial bones of the small feline from Batallones has revealed unexpected differences with those earlier animals.
The middle Miocene Styriofelis turnauensis combined peculiar dental traits (in particular the retention of milk premolars in adult life) with a skeleton adapted for climbing, with short, robust limb bones. Such a skeleton can be considered “primitive” for felids, because the ancestral members of the family were mostly arboreal creatures. The small cat from Batallones shared with Styriofelis the retained milk teeth, but its limb bones now reveal a surprisingly early adaptation for fast, efficient locomotion on land. This condition almost mirrored the one seen in modern animals like the wildcat, but it most likely evolved independently, because the particular dental features preclude Leptofelis from being an ancestor of the modern species. In fact, the skeleton of the Batallones small cat is in itself a mosaic of features, including the presence of a well-developed quadratus plantae muscle inserting on the ankle bone. This muscle has an important function in climbing and it shows that in spite of being a proficient runner, Leptofelis vallesiensis could climb better than most modern cats, both to escape bigger predators and to catch small prey in the high branches. Also the hind limb was especially long and the knee articulation resembled that of modern small carnivores that are excellent jumpers and climbers, such as the genet. It is possible that Leptofelis used its leaping ability to capture small prey such as rodents and birds while foraging on the ground, like modern servals or caracals do. This unique combination of features convinced us of the need to create a new genus for this cat, and we coined the word “Leptofelis”, meaning “swift cat”.

In this illustration we show some of the preserved bones of Leptofelis superimposed to a hypothetical reconstruction of its complete skeleton.

There are many things we have learned from this study. On one hand, the early diversity of felines is greater than was thought some years ago, when virtually all fossil felines from the late Miocene were classified in the extant genus Felis. On the other hand, we see that the adaptations of feline cats for running not only appeared more precociously than thought, but in fact evolved several times independently. Also important is the fact that the postcranial skeleton, often overlooked in systematic studies, can provide decisive evidence for the proper classification of an extinct animal. And, finally, if we look at the larger picture, it seems that the combination of the small size of the early felines, the need to escape from larger predators, and the presence of vegetational cover in their environments probably provided the right adaptive pressures which led (more than once) to the evolution of the versatile body plan that we see in modern cats.

Here is a reconstruction of Leptofelis in the flesh. The coat colour patter is unknown in this animal and here it is reconstructed on the basis of species such as the marbled cat, whose coat markings appear to represent the ancestral patter for all living felines.

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You can check our original research paper in this link:

http://rdcu.be/x6Sl

A Glimpse into an Eocene Lost world

Isn´t it amazing to think that 35 million years ago, long before the sabertooth cats (or any true cat for that matter) ever evolved, there was such a specialized sabertoothed carnivore as Hoplophoneus mentalis?. This creature was about the size of a large lynx but it specialized in taking prey larger than itself, which it killed by a devastatingly efficient bite to the throat. But rhino-sized animals like Megacerops, shown in the background, had nothing to fear from this cat-like predator.

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At a first glance (and even at a second and third) the skull of Hoplophoneus mentalis is so similar to that of true sabertooth cats like Megantereon, which lived more than 30 million years later, that you would be excused for taking it for a real cat, even for a direct ancestor of such younger species. In fact, the best heads of paleontology were such deceived for many decades until the detailed study of obscure anatomical features, such as the structure of the ear region of the skull, revealed the true affinities of those cat-like carnivores, which are now classified in an independent family: the Nimravidae.
hoplophoneus-mentalis-head

The late Eocene of North America, nimravids included, is one of the most amazing “Lost Worlds” discovered by paleontology. Read more about it in my book “Sabertooth”!

ANCIENT AFRICA: SABERTOOTH KILL AFTERMATH

In a few days we will be heading for Botswana for one more edition of our “Drawing the Big Cats Safari”. Our main goal is to observe the felines, but that is only part of what we get: we actually get a ticket to ancient Africa. The big predators are one key piece in the giant puzzle of living ecosystems, and a place that still can sustain its large carnivores is a place where the laws of nature remain healthily at work, and for that reason I experience every safari as a kind of time-travel.
On each trip, as I board the safari vehicle for the first game-drive, I feel as if I enter a different dimension. Pristine landscapes, untouched vegetation and the diversity of wildlife bring my senses to a state of natural alertness -not in vain human kind evolved in the African savannah! One unforgettable first-day game drive took place in Samburu, Kenya, in 1999. We had spent most of the day driving from Nairobi, and we had only time for a short evening drive through the reserve on our way to the camp for the night. As we drove through the scenic riverine woods along the margins of the Ewaso Nyro river, we found another vehicle that had stopped on the margin of the dirt track: they were watching a female leopard that had just killed an impala ram. Althought the high branches of the palm trees were still golden with the last rays of the sun, the deep bush where the big cat stood with its prey was already in deep shadow. Everyone on board of the vehicles was silent, aware of facing a high natural drama. It was almost as if we didn´t have the right to spy on this decisive moment of life and death, as if both predator and prey were emitting a sort of primeval energy that gave us the goosebumps. We just couldn´t take our eyes from the golden spotted cat and the athletic shape and powerful horns of the fallen antelope. We remained there in awe for many minutes as the cat started to feed and its tiny cubs emerged from the bush, as did an adult-sized young from the previous year, but then we had to hurry for camp.

Here is a picture of the leopard with her impala kill from our 1999 trip to Samburu
leopard and impala samburu 1999

In later trips I have seen leopards with their prey on several occasions, and each time the prey was an impala. It seems the fates of these two species are tightly linked, but the fact is the impala is an older inhabitant of the African woods and savannahs than the leopard. In Kenya there were impalas (genus Aepyceros) rather similar to the modern ones as early as the late Miocene, some 6 million years ago, while the earliest fossils of leopard ancestors (genus Panthera) are known from fossil deposits of Pliocene age, some 5 million years ago, from the Himalayas. Some time afternwards, leopard-like cats entered Africa, where they are first recorded some 3.8 million years ago, but the ancestral impalas had not been free of predation in the meantime, because several species of sabertooth cats shared their African habitats since the Miocene.

The impala is so well adapted to the ecotone between grassland and woodland, that it has barely changed in 6 million years. Its acrobatic leaps are among the most sublime, if sometimes underrated, spectacles of Africa. We saw this impressive ram in Chobe, Botswana, in 2014
impala d

Becuase of my professional bias, during my African trips I can’t help imagining how would those same places look in the distant past, and what animals would occupy the ecological niches of the modern species. If we could travel back to the early Pleistocene, about 1,6 million years ago, we might come across a predation scene where an impala nearly identical to the modern ones would fall prey to a sabertooth cat of the genus Megantereon. About the same size as a leopard, Megantereon would behave similarly to the spotted pantherine in many ways: it would hunt its prey through careful stalking, approaching to within just a few meters before launching an explosive attack. If the kill took place in the grass, the cat would drag its prey as soon as possible to an area with good cover to hide it among the bushes. After all, big hyenas and lion-sized sabertooths (Homotherium) roamed the plains and were always eager to steal the smaller cat’s kills.
But the shape of Megantereon‘s teeth implied some behavioral differences: on one hand, the sabertooth cat would be less able to haul complete large carcasses up the trees, because of its relatively fragile canines. Lighter, partly consumed kills would be easier to handle, although the cat itself was a good climber and at least could always save its own life by climbing to the higher branches.
But the most important difference concerned the killing bite, which implied a lot more bloodshed than in the case of modern cats, and thus if we could see Megantereon within the few minutes after a kill, it would be a rather gory sight.

Here is a reconstruction of Megantereon whitei from the early Pleistocene of Africa, taking a breath as it drags its impala prey to the bush for quiet consumption. Part of my book project “Big Cats of Africa, Past and Present”
megantereon arrastrando impala

Today we still have impalas in Africa, but Megantereon is long gone, a proof of the vulnerability of extreme specialists like the sabertooths and the resilience of adaptable species like the impala -and the leopard. But that doesn’t mean I don´t miss the possibility of seeing such a magnificent beast as Megantereon was!

GIRAFFES AND SABERTOOTHS: AN UNEASY COEXISTENCE

For a medium sized sabertooth like Megantereon, remaining unseen was of the essence. With its strong, muscular physique it was not nearly a long-distance runner, so it needed to stalk within a few meters of its prey in order to catch it after a couple of spectacular leaps. But its prey, including medium sized antelopes, pigs or even horses, had a lofty ally: the giraffe.
Since the late Miocene, members of the modern giraffe lineage evolved their long necks and tall forequarters, and inevitably became the sentinels of the savannah. From the privileged viewpoint of their 5 meter height, they miss little of what goes on around them

Some 7 million years ago during the Turolian (late Miocene) the giraffid Bohlinia attica had already developed the large size and unique body porportions of modern giraffes. It inhabited the open woodlands of Europe, giving them a striking “African” touch.
bohlinia attica low res

Any good wildlife tracker in Africa takes advantage of the presence of giraffes in order to locate the big cats. As soon as a predator moves in the vicinity, giraffes stop their browsing or casual walking, and stand motionless pointing with their stare in the direction of the carnivore. That habit is as convenient for the less tall herbivores as it must be annoying for the predators.

Back in the Pliocene and Pleistocene of Africa, the giraffid Giraffa jumae was a contemporary of the sabertooth Megantereon whitei. While the cat tried to take advantage of every element of the landscape to conceal itself, it could hardly escape the stare of the giraffe, which surely ruined many an attempted stalk!
megantereon stalking

SABERTOOTH HYGIENE: SURVIVAL OF THE CLEANEST

All cats devote a large chunk of their time to fastidiously taking care of their hygiene and if anything, sabertooths had even more reason to do so.
Right after killing a large prey, as it should do at least once a week, a sabertooth of the genus Megantereon would look like a bloody mess -literally. Unlike modern cats, its killing bite relied mostly on causing massive blood loss to ensure a rapid death of its victim, and that implied a somewhat untidy spectacle. But if we could find the sabertooth at any random moment, chances are we would find a pretty clean animal, which is only what you would expect from a cat.

If we could time-travel and come across the sabertooth Megantereon, it is very likely that it would be grooming itself
!
megantereon self cleaning

A sabertooth smeared in old blood would be pestered by flies, it would be prone to skin conditions, and would be more easily smelled by potential prey, so it needed to be quite conscientious about its grooming.
But of course there are other benefits to grooming, incluiding the fact, obvious to any cat owner, that it feels good and decreases stress! Social species would benefit from mutual grooming but a solitary animal, as Megantereon most likely was, would need to twist a bit around. But then its long and flexible neck would be a welcome aid in reaching those difficult spots!

VOLCANOES, SABERTOOTHS AND EXCEPTIONAL FOSSIL PRESERVATION

More than 3 million years ago, somewhere in Northeastern Spain, a volcano exploded and created a large crater. With time the crater was occupied by a lake, known by geologists as a maar. Today, the lake is dry and the crater walls have been largely levelled by erosion, but an exceptional fossil site bears witness to the events that took place there. The site, called Camp dels Ninots, is close to the village of Caldas de Malavella in Girona.
Maar lakes often create remarkably beautiful scenery, with their encircling walls covered by forests, but some of them hide something more sinister. One example is lake Nyos in Cameroon, which in 1986 emitted a large cloud of carbon dioxide that killed thousands of people and animals. Such toxic gas emmissions occur because the pocket of magma beneath the lake leaks the carbon dioxide into the water, and perturbations such as those caused by a landslide can cause the gas to emerge with disastrous effects.
Three million years ago around the Camp dels Ninots maar lake, a rich fauna thrived in a subtropical environment, much warmer than today. Rhinos, tapirs and heavy antelopes crossed the forests and came down the crater walls for a drink at the lake margin. But several of those animals died mysteriously at the shore, and their bodies floated into the lake where they ultimately sank. Their skeletons, completely articulated, got exquisitely preserved as fossils, without any trace of having been disturbed by scavengers. A likely explanation is that the lake belched a cloud of toxic gas, instantly killing all the animals along the shore.
As often happens, the animals’ doom was the paleontologists’ blessing, since the death of so many creatures led to their pristine preservation. As the local authorities planned for an on-site exhibit, I was asked to create several reconstructions of the Pliocene environments and fauna of Camp dels Ninots, including a scene showing the hypothetical origin of the fossil accumulation.

This is a preliminary pencil sketch of the Cap dels Ninots death scene
camp dels ninots tafonomia boceto

And here is the finished painting that can be seen today at the fossil site

Can Argilera

Can Argilera

The scene shows antelopes of the genus Alephis and a rhinoceros of the genus Stephanorhinus on the shore, already showing signs of intoxication, while a tapir (Tapirus) lies on the ground, already dead. The bodies of other antelopes float in the lake while a pair of cormorants that happened to fly too low are already falling.
This and other reconstructions of the fauna and environments of Camp dels Ninots can be seen as part of the outdoors exhibit. Thanks to the findings made at the site, we now know a lot more about the anatomy of several species of Pliocene mammals, but, alas, no sabertooth skeleton has been found there as yet. But I don´t give up hope.

In central France, another Pliocene maar lake fossil site, a little younger in age than Camp dels Ninots, has been known for about a century, and it has yielded the most complete skeletons known to science of the typical sabertooth cats of the Pliocene: Homotherium and Megantereon. For several decades the French site, known as Senéze, was exploited without any serious excavation methodology, so we don´t know much about the taphonomy of those early finds. But since the 1990s, new field campaigns have yielded more accurate data, suggesting that the fossil mammal skeletons from Senéze accumulated as landslides coming down the crater walls trapped the animals and dragged their bodies to the lake.

I took this photo of the mounted skeleton of Megantereon from Senéze, exhibited at the Natural History Museum of Basel (Switzerland), back in 1990. All other findings of Megantereon fossils are more fragmentary, so it is only thanks to the exceptional conditions of the volcanic maar lake that we know nearly every bone of this sabertooth cat.
megantereon skeleton seneze basel

Only a portion of the ancient lake shore at Camp dels Ninots has been excavated this far. Sabertooths like Dinofelis diastemata, whose skeleton is mostly unknown, probably inhabited the area at the time, so it is not impossible that the next excavation will yield an amazingly preserved specimen…who knows?

Volcanic eruptions have provided the right conditions for the preservation of some of the best sabertooth fossils known to science, and not only beacuse of maar lake sites. If you want to know more about volcanoes and sabertooths, read my book “Sabertooth”! http://www.amazon.com/Sabertooth-Life-Past-Mauricio-Ant%C3%B3n/dp/025301042X/ref=sr_1_1?s=books&ie=UTF8&qid=1459250381&sr=1-1&keywords=sabertooth

Visit the on-site exhibition at Camp dels Ninots:
http://www.caldesdemalavella.cat/component/k2/272?Itemid=779

AMPHIMACHAIRODUS COMING YOUR WAY –DON’T PANIC!

When you spend time in the African savannah you are surprised to see how relaxed the herbivores can be in the proximity of the big cats. Zebras and antelopes don’t stampede at the mere presence of a lion, but they rather observe it. Information flows in both directions, and the ungulates know how to read the body language of predators. A feline walking casually is no cause of panic, and the potential prey just look at it cautiously until it disappears. And it makes sense, because if zebras had to be in a constant state of panic at the possible presence of a predator, stress would kill them even before the predators would. For us humans as well, irrational fear of predators is probably something that developed once we abandoned our life as hunters-gatherers to become Neolithic farmers. Before that, cautious respect and a keen interest in the predators would be a far more useful attitude than panic.
As a sabertooth freak, I often imagine what it would be like to travel back in time and meet my favourite predators from the past. If I were in a B movie, a succesion of screams and chases would follow, and my survival would depend on being the star of the film or a mere sideshow, so I would really stand little chance of survival! But in the natural world, I should rather try to follow the example of the zebra and read the cat’s body language before running.
To see an adult Amphimachairodus walking your way would be in impressive sight by any standards. Tall as a lion, it would walk with a cat-like supination of the forepaws, although less exaggerated than in a lion: the structure of its elbow and wrist joints tell us that much. Free-swinging shoulder blades would move up and down as the cat stepped towards us. But the animal’s head would be subtly different from any modern cat’s. The face was narrower, with slightly smaller eyes looking less frontally, somehow intermediate between a lion’s and a wolf’s in terms of relative size and position. The muzzle was long and high but also narrow, with blade-like upper canines showing discretely beyond the upper lips.

Reconstruction of Amphimachairodus in frontal view. The animal was as tall as a lion with a distinctly cat-like walk, but it had a peculiar narrow head with a high and narrow muzzle and blade-like upper canines showing beyond the upper lips
amphimachairodus-front low res

Concerning body language, if the cat is walking upright and with a casual cadence, you have reason to think it is just minding its business rather than stalking you. A relaxed mouth further indicates lack of aggression, but ears pointing slightly backwards are less simple to read. In the general context of a relaxed animal they don’t mean much, but it could be the sabertooth is not very happy about something. Better observe it for a few seconds and see what those ears do, but remember that any part of the animal’s body generally works as a part of the whole, and if the cat is in an aggressive mood there will be other signs apart from those ears to show it…
Amphimachairodus lived a little too long ago for our bipedal hominin ancestors to have come across it, but other sabertooths, such as Homotherium and Megantereon, were familiar elements of their world. I am sure that body-language reading was more important than panic as a reaction to their presence. But timing is everything, and surely there was a right time to panic as well!