Wednesday, June 14, 2017

Smilodon: Beast of the Week

This week, thanks to multiple requests, we will be looking at a really popular prehistoric mammal.  Make way for Smilodon!

Smilodon was a prehistoric cat that lived in many parts of what is now North and South America during the Pleistocene epoch (more casually referred to as the "ice age"), from 2.5 million years ago to as recent as only ten thousand years ago.  There are currently three named species of Smilodon.  The most well-understood is Smilodon fatalis, which is known from literally hundreds of specimens found in California, which measured between five and six feet from snout to rump and a little over three feet tall at the shoulder as adults.  Smilodon populator was the largest, with canine teeth that could measure up to eleven inches long. Smilodon gracilis was from the oldest time, likely a direct ancestor of the other two, but only known from very partial remains so details of its anatomy is mostly unknown other than the fact that its teeth were more slender than those of its later relatives.  The genus name, Smilodon, translates to "Smiling Tooth" in reference to the fact that this cat had HUGE top canines that likely would have stuck out of the mouth at least partially even when the jaws were completely closed.  Assuming its diet was the same as modern cats, Smilodon would have been an obligate carnivore, eating only other animals.  (I hear you cat people out there.  "But, Chris!  My cat at home eats grass and other plants all the time!" Ah, yes.  But what does your kitty do after consuming the grass?  BARF!  In fact, because they cannot digest plant material, cats specifically eat grass to induce vomiting when they don't feel well.)

Smilodon fatalis with cub.  Reconstruction by Christopher DiPiazza.  Color scheme inspired by modern pumas.  Because of the vast span of places Smilodon remains have been found, it likely was adapted to living in a multitude of different habitats.  Pumas are known to exhibit the same success today, living in mountains, deserts, forests, and even tropical everglades.  That being said, Pumas aren't particularly closely related to Smilodon over any other modern cat.

Smilodon is a very popular and well-loved prehistoric mammal not just in the paleontology community, but also to the general public in general.  It is certainly up there with Woolly Mammoths, and even popular dinosaurs, like Tyrannosaurus and Triceratops.  However, it is only rarely referred to by its actual name in popular media, often called a "saber-toothed....tiger".   (I'm cringing just typing that.) Word to the wise; never EVER refer to Smilodon as a "saber-toothed tiger" in earshot of a paleontologist. If you do forget the proper genus name, it's always safe to refer to it as a "saber-toothed cat" instead.  This is because Smilodon was not a tiger.  In fact, Smilodon and its closest relatives appear to have diverged from modern cats, including tigers very early in cat evolutionary history, about 23 million years ago.  In other words, despite the fact that Smilodon was a cat, there is no species of living cat that we can say is any more closely related to Smilodon than any other.  The subfamily that Smilodon is part of, called machairodontinae, and the subfamily that includes everything from tigers to housecats, called felinae are as distant as they can be within the cat family.  That being said, could Smilodon have even roared?  Think about it.  Only modern cats in the specific genus, panthera, like lions, jaguars, and tigers, can roar.  Every other cat, including Cheetahs, Pumas (mountain lion), and house cats cannot.  It is very possible, since it was so distantly related, that saber cats, like Smilodon, couldn't roar either.  (Sorry if I killed your nostalgia/childhood there.  Just thinking out loud here.)

Smilodon skeleton on display at the American Museum of Natural History in New York.

Despite how popular Smilodon is, many people don't actually know that much about it, often assuming it was just another big cat that happened to have long front canines.  In reality, Smilodon had lots of really interesting features about it beyond the teeth. On the skull, aside from the front teeth, it had a particularly robust and almost blunt-looking face to accommodate the roots of those huge front canines.  So reconstructions of Smilodon that look like a just lion or tiger with long fangs are wrong.  Smilodon also had proportionally longer arms than those of other known cats, and a short, almost stubby tail.

Overall, Smilodon had a much stockier build than most cats.  This suggests that it relied on strength over speed to capture prey.  It is very possible that in life, Smilodon would have been an ambush predator, surprising animals that get too close to its hiding spot, then relying on its strong arms to overpower and contain struggling prey.  In fact, looking at the arm bones of Smilodon, it was determined that it would have had the strongest arms of any known cat!  Also remember that like all cats (except Cheetahs) Smilodon was armed with hooked, retractable claws on each limb, which were probably great at latching onto into, and immobilizing prey even further.  If you have ever played with a house cat before, you might be familiar with the behavior they do that involves holding on with their front claws, and rapidly kicking with their back feet.  As cute as this behavior is in house cats (my fiance and I endearingly refer to it as "gripp'n and kick'n"), it is actually a really effective disembowel tactic when dealing with actual prey.  Imagine how much damage a Smilodon could have done, digging its hooked front claws, backed up by those extremely powerful arms, and shredding its prey's insides with its hind claws at the same time!  Check out the video below of my cat, Petrie, who was happy to demonstrate this behavior on my foot for you all.  Thanks, Petrie.  (Don't worry.  My foot is still intact from the incident.)



But what about those amazing teeth?  Smilodon's teeth are actually the subject of much debate.  At first glance it seems like a no-brainer that they were simply an extreme stabbing/biting adaptation.  But it's not that simple.  You see, Smilodon's teeth were actually pretty delicate and prone to breaking if put under too much pressure.  So in life, if a Smilodon was just chomping as hard as it could into anything, it was bound to get hurt.  Remember, mammals don't constantly regenerate teeth like reptile do, either.  So where crocodiles and dinosaurs could afford to be less careful with what they bite, Smilodon can't.  With this in mind, a next logical guess would be that maybe Smilodon's long canines were for sexual display?  Plenty of other animals, living and extinct, have unusually shaped or large body parts, including teeth, for this reason.  The only problem with this hypothesis is the fact that we have literally hundreds of Smilodon specimens on the fossil record, and all of the adults have the same long teeth.  If they were really for sexual display, it is much more likely that they would differ between sexes, likely longer in males than in females.  Because this is not what the data shows us, we are probably safer going back to guessing that Smilodon was using those teeth for something other than looking cool, like dealing with prey, but it probably had to be much more meticulous about how it went about it.  Going back to those unusually powerful front limbs, a strong current hypothesis is that Smilodon used its arms to hold still struggling prey so it could use its canines to carefully stab the perfect spot, likely the jugular vein or the trachea(wind pipe).  The arms would have helped ensure that the teeth weren't damaged by the prey when it was still alive.

Smilodon skeleton on display at the Mesalands Dinosaur Museum in Tucumcari, New Mexico.

Smilodon's jaws can open wider than those of any other cat in order to make clearance for the saber teeth.  Despite this, at first glance, even with some clearance, those canines would get in the way if Smilodon tried to bite a piece of food off a carcass with the front of its mouth.  Looking at house cats eat bite-sized kibble can't help us here as a reference.  For a better clue we need to look at cats that have to bite off chunks of food from larger carcasses.  What you will notice is that when eating off large carcasses, cats actually don't use the front of their mouths that often to bite off smaller pieces.  They use the sides of their mouths, armed with large, sharp teeth, called carnassals, to cut meat like scissors.  This may have been how Smilodon got around its massive canines to feed itself, as well.  Check out the clips of exotic cats eating in this video below.  You will notice that most of the time, they are biting with the sides of their jaws to process food.  (at one point in the video they show a random Binturong, which is not a cat, but a civet.  Cute, but not as relevant.)




La Brea, California, is home to tar pits that are tens of thousands of years old, and is where the majority of Smilodon specimens on the fossil record have been found.  During the Pleistocene, animals would occasionally get trapped and die in these tar pits.  While these unfortunate animals were likely giving off distress calls, they would have attracted meat eating-animals to come and eat them, including Smilodon.  Those meat-eaters in turn would get stuck, themselves, and also die, attracting even more meat-eaters and...well you get the idea.  Hundreds of Smilodons have been unearthed out of these tar pits, as well of lots of other kinds of ice age animals, especially predators.  In fact, most of the remains that are found in these tar pits are of meat-eaters because of stuck animals acting as bait.  Even today, animals are still getting stuck and dying in the La Brea tar pits.

Thanks to all these wonderful specimens, paleontologists were able to learn a lot about Smilodon, including how it grew.  Like most mammals, Smilodon had a set of baby teeth, which would later fall out and be replaced by bigger, adult teeth.  The cool thing about Smilodon, however, is that the adult teeth didn't initially cause the baby teeth to fall out.  In fact, when it came to the famous saber canines, there was a period of about a year in a Smilodon's life where the adult canines were growing in next to the baby teeth.  The specimens that show this are extremely rare, even from La Brea, which suggests that juvenile Smilodons at that age probably weren't going out to hunt, and likely were still having food brought to them by parents.

Underside of juvenile Smilodon skulls.  The left one shows the baby canines alongside the growing adult canines.  These skulls were unearthed at the La Brea Tarpits, and are on display at the George C. Page Museum in California.

That is all for this week!  As always feel free to comment below or on the Prehistoric Beast of the Week facebook page!


References

Antón, M. (2013). Sabertooth (1st ed.). Bloomington: Indiana University Press.

Feranec, R. C. (2004). "Isotopic evidence of saber-tooth development, growth rate, and diet from the adult canine of Smilodon fatalis from Rancho La Brea". Palaeogeography, Palaeoclimatology, Palaeoecology206 (3–4): 303–310. 

Kurtén, B.; Werdelin, L. (1990). "Relationships between North and South American Smilodon". Journal of Vertebrate Paleontology10 (2): 158–169. 

Meachen-Samuels, J. A.; Van Valkenburgh, B. (2010). "Radiographs reveal exceptional forelimb strength in the sabertooth cat, Smilodon fatalis"PLoS ONE5 (7): e11412. 


Mihlbachler, M. C.; Wysocki, M. A.; Feranec, R. S.; Tseng, Z. J.; Bjornsson, C. S. (2015-07-01). "Using a novel absolute ontogenetic age determination technique to calculate the timing of tooth eruption in the saber-toothed cat, Smilodon fatalis"PLoS ONE10 (7): e0129847. 

"Vegetarian Cat?" Vegetarian Cat? – Dr. Sophia Yin. N.p., n.d. Web. 14 June 2017.

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