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Smilodon gracilis
Topic Started: Jan 8 2012, 03:21 PM (11,597 Views)
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Smilodon gracilis

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Temporal range: Early Pleistocene to Late Pleistocene

Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Family: Felidae
Subfamily: Machairodontinae
Genus: Smilodon
Species: Smilodon gracilis

Smilodon gracilis was a saber-toothed cat of the Pliocene and Pleistocene.
Smilodon gracilis, 2.5 million-500,000 years ago; the smallest and earliest species (estimated to have been only 55 to 100 kg (120 to 220 lb)) was the successor of Megantereon in North America, from which it probably evolved. The other Smilodon species probably derived from this species.

Physical Description
Smilodon gracilis was the smallest species of its genus. It had a height of 1-1.2 m. (3.28-3.94 feet). The saber-like teeth were about 17.8 cm. (7 inches) long.
The legs of Smilodon gracilis were powerful and muscular. The hind limbs had strong adductor muscles. They may have stabilized the body of Smilodon gracilis while it was taking down prey.
Smilodon gracilis had a proportionally short lumbar in its back. The hind limbs were shorter than the front ones. Smilodon gracilis and the other species of its genus do not seem to have been especially fast runners. They probably hunted prey by stealth. Smilodon was able to open its jaws 120 degrees. It may have killed prey animals by biting into their necks.

Robert S. Feranec wrote an article titled THE GROWTH RATE AND PERIOD OF THE
. It was published in 2002. The abstract is :

Trophic structure and interconnectedness have important implications for diversity and stability in ecosystems. It is generally difficult to determine trophic structure and the specific prey of a particular predator in ancient ecosystems. However, variation in the stable carbon isotope ratio in tooth enamel typically reflects differences in diet, and can be used to determine if a carnivore preferred prey that predominantly ate C3 or C4 plants. Using this technique, it is possible to determine what taxa, or at least exclude particular taxa, on which a predator fed. This study investigates the rate of growth and growth period of the adult canine in Smilodon gracilis from the Leisey Shell Pit 1A locality in Hillsborough County Florida, and determines whether it shows a preference for preying upon C3 or C4-feeders in the hope of determining its specific prey by analyzing stable carbon and oxygen isotope ratios found in tooth enamel. The oxygen isotope results show that this individual of S. gracilis had a mean d18O value of 0.2‰, and a range from -0.7‰ to 1.3‰. The oxygen isotope pattern indicates that nearly one year was sampled, showing about 60 mm of enamel growth over the year, which results in a growth rate of about 5 mm/month. The total length of enamel in the S. gracilis canine was about 80 mm suggesting a 16-month growth period if enamel growth rate remained constant. The carbon isotope results show that this individual had a mean d13C value of -8.8‰, and a range from -9.1‰ to -8.3‰. The carbon isotope values suggest consumption of animals that depended on a C3 diet. Analysis of the herbivores from the Leisey Shell Pit 1A locality suggest that Hemiauchenia and Platygonus may have been included in the diet of this individual of Smilodon, while Equus and Mammuthus were probably excluded from its diet. Also, because the carbon isotope values of S. gracilis were more negative than many of the individual herbivores at the Leisey Shell Pit 1A locality, the results might be reflecting prey capture in a closed environment with the more negative numbers indicating a canopy effect. This study shows that determining trophic relationships and interconnectedness between organisms within a particular ancient ecosystem is possible

Per Christiansen and John M. Harris wrote an article titled Body size of Smilodon (Mammalia: Felidae). It was published in 2005 in the Journal of Morphology, Volume 266, Issue 3, pp. 369-384. The authors compare the sizes of the three species of Smilodon. The abstract is :


The body masses of the three large saber-toothed machairodontines, Smilodon gracilis, S. fatalis, and S. populator, were estimated on the basis of 36 osteological variables from the appendicular skeleton of extant felids. A new model is introduced that takes the reliability of the predictor equations into account, since mass estimates are more reliable when computed from multiple variables per bone. At a body mass range of 55–100 kg, S. gracilis was comparable in size to extant jaguars, and S. fatalis was found to be somewhat lighter than previously assumed, with a body mass range of 160–280 kg, similar to that of the largest extant felid, the Siberian tiger. Smilodon populator was substantially heavier and larger than any extant felid, with a body mass range of 220–360 kg. Particularly large specimens of S. populator almost certainly exceeded 400 kg in body mass. The differences from previous estimates are most likely caused by differences in the databases used for mass estimation.

The holotype was found at Port Kennedy Cave in Montgomery County, Pennsylvania. It dates to the Irvingtonian interval (1.8 million - 300,000 years ago) of the Pleistocene. Other Irvingtonian remains have been discovered in the Palm Spring Formation (Huesos/Tapiado Member) at Vallecito Creek (CU 56) in California. Some were unearthed in the Bermont Formation at the Crystal River
Power Plant in Citrus County, Florida. Irvingtonian fossil remains have also been excavated at the UTEP 97 site in Dona Ana County, New Mexico. Pliocene remains were found at the Bass Point Waterway 1 site in Sarasota County, Florida. They date to the Blancan interval (4.9 - 1.8 million years ago).

Informative Article : The sabercat Smilodon gracilis from Florida and a discussion of its relationships (Mammalia, Felidae, Smilodontini)
Edited by Taipan, Mar 1 2012, 09:11 PM.
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Saber-toothed Cats Wrestled Prey with Powerful Arms

By Charles Q. Choi, LiveScience Contributor
posted: 02 July 2010 04:59 pm ET

Saber-toothed cats might be most famous for their oversized fangs, but scientists now find the feisty felines had another exceptional feature — powerful arms stronger than those of any cat alive today.

Commonly known as the "saber-toothed tiger," the extinct cat Smilodon fatalis roamed the Americas until roughly 10,000 years ago, preying on "megafauna" — large animals such as mammoths, bison, camels and mastodons. Their specialization on such giant creatures might have doomed these hunters when their Ice Age prey died off.

The most recognizable features of the saber-toothed cat — giant, dagger-like canines — were also perhaps its most puzzling. The fangs would have been excellent at inflicting deadly slashing bites to its prey's throat, but their size and shape would also have made them highly vulnerable to fracturing compared with modern cats. That led researchers to wonder how the fangs developed in the first place.

Cat teeth

"Cats living today have canines that are round in cross-section, so they can withstand forces in all directions," said researcher Julie Meachen-Samuels, a paleontologist at the National Evolutionary Synthesis Center in Durham, N.C. "If the prey is struggling it doesn't matter which way it's pulling — their teeth are unlikely to break."

In contrast, the long canines of saber-toothed cats were oval in cross-section, or thinner side-to-side, making them relatively fragile. This suggested that saber-toothed cats must have killed prey differently from other cats. In fact, research published in 2007 suggested Smilodon had a wimpy bite.

That's where the powerful arms come in. These predators might have pinned victims down with their heavily muscled forelimbs to protect their teeth from fracturing as they bit struggling prey, Meachen-Samuels said.

Feline wrestling match

In an arm-wrestling match of sorts, the researchers compared saber-tooth arms with those of other cats. To do so, they X-rayed the arm and leg bones of fossils recovered from the La Brea Tar Pits in Los Angeles. They also analyzed the limb bones of 28 cat species living today — ranging in size from the 6-pound (2.7-kilogram) margay to the 600-pound (272-kg) tiger — as well as the extinct American lion, the largest cat with conical teeth that ever lived. These measurements helped the researchers estimate bone length, rigidity and strength for each species.

Species with longer limbs generally had stronger bones. However, while saber-tooth leg bones fell within the normal range, their arm bones were exceptionally thick for their length. Not only that, their arms also had thicker cortical bone — the dense outer layer that makes bones strong and stiff.

"When I looked at Smilodon, I knew they were thicker on the outside than other cats, but I was really shocked at how much thicker they were on the inside as well," Meachen-Samuels told LiveScience.

The thicker cortical bone seen with the saber-toothed cats makes sense if the arms were under greater stress than normally expected for cats their size, Meachen-Samuels explained. Just as lifting weights improves bone density over time, so too may the repeated strain of grappling with prey have resulted in thicker and stronger arm bones in saber-toothed cats.

"As muscles pull on bones, bones respond by getting stronger," Meachen-Samuels said. "Because saber-toothed cats had thicker arm bones, we think they must have used their forelimbs more than other cats did."

The researchers would like to next look at other saber-toothed cats, as well as other saber-toothed predators that once existed. "There aren't a lot of arm bones to X-ray for other saber-toothed predators, but it'd be interesting to see if there were convergent processes with their arms as well," Meachen-Samuels said.

Meachen-Samuels and her colleague Blaire Van Valkenburgh detailed their findings online July 2 in PLoS ONE.

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Here's a pretty comprehensive PDF describing the fossil remains of S. Gracilis and how it compared to living cats and other species of sabertoothed cats + the differences between S. Gracilis and the other Smilodon species.

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How a Smilodon sabretooth cat closed its gaping mouth

By Victoria Gill
Science and nature reporter, BBC News
Page last updated at 12:14 GMT, Friday, 25 February 2011

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The sabretoothed cats killed prey with a deep stabbing bite to the throat

The mystery of how prehistoric sabretoothed cats coped with their oversized teeth has been solved in part by a new analysis of the cats' jaws.

To impale prey with these impressive weapons, the famous sabretoothed cats must have opened their mouths wider than any modern big cat, but it was unclear if their jaw muscles were strong enough to do it.

Now the new analysis reveals that the cats' jaw muscles evolved into a specialised pattern, which allowed them to open their mouths so wide.

Details are reported in the Zoological Journal of the Linnean Society.

Weak bite?

Per Christiansen, from Aalborg University in Denmark, led the study. He took a novel approach to studying the extinct predators by creating a complex model of how their jaws moved.

Beautifully preserved skeletons of the most recently extinct sabretoothed cat, Smilodon, have been uncovered in tar pits in the US, offering the researcher plenty of fossilised remains to work with.

These fearsome-looking cats - the biggest of which would have hunted very large prey, including buffaloes, horses and extinct giant ground sloths - had a relatively weak bite force compared to their modern feline relatives, previous studies have revealed.

But this is not surprising, according to Dr Christiansen. He has found the cats must have had remarkable jaw muscles to close their mouths with any force at all.

"Smilodon could open its mouth wider than any modern cat," explained Dr Christiansen, "because if you have big teeth, you have to open your mouth at a very high angle to get anything in your mouth."

Fatal stabs

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Smilodon (right) was able to open its mouth wider than any modern cat

Over the years, scientists have debated what the now extinct cats used their enormous teeth for, with some even suggesting the teeth were ornaments that males used to attract mates.

But the consensus is now that the teeth, which measured up to 20cm, delivered final, fatal stabs to already subdued prey.

Bulkier and more muscular than most modern cats, Smilodon would have brought large animals down with its forelimbs.

"They pounced on their prey, wrestled it to the ground and twisted the neck with massive forelimbs," explained Dr Christiansen.

"They then delivered a quick, powerful and deep stabbing bite to the throat or upper neck. But, there were no minute-long massive asphyxiating throat-clamps like those used by modern big cats when they suffocate prey. The sabrecats simply didn't have the jaw muscles for that."

Opening their mouths sufficiently wide to kill and consume their hard-earned meal would have stretched and significantly weakened their jaw muscles.

Dr Christiansen created a model of the cats' jaws, carefully calculating how they must have moved, to work out how Smilodon evolved to compensate for this weakness.

His model revealed how the cats' jaw muscles were aligned to pull its jaws closed, very directly and efficiently.

But Smilodon would also have done something that every cat-owner can see a relic of in their own pet.

"When you put a piece of food on the floor for your cat, you'll see it bobs its head forward as it eats it," he explained.

"And we know that [Smilodon] probably closed its jaws by twisting its head downward and throwing its head forward.

"Its neck was longer than that of modern cats and its neck muscles would have been stronger."

'Killing ecology'

The study also looked back at the earliest and most primitive sabretoothed animals.

"Smilodon was outrageous in terms of its anatomy," said Dr Christiansen.

"It was the most [highly evolved and therefore] different from modern cats, so to understand this animal from a biological sense, you need to study more primitive animals to work out why they have become that way through evolution."

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Smilodon would have brought down prey with its powerful forelimbs

He examined hundreds of skulls, drawing an evolutionary map showing why sabretooths evolved such different jaw anatomy from modern cats.

"Killing ecology", he explained, was the driving factor - the evolutionary pressure to kill prey with a deep and efficient stab to the throat.

"The cat species became gradually more and more specialised, culminating with monsters such as Smilodon," said Dr Christiansen.

As sabretooths evolved longer canine teeth, their jaw muscles actually grew smaller, but the fibres became more vertically orientated and thus probably more efficient in closing the jaw.

Dr Christiansen explained: "At the same time, changes in the way the muscle fibres inserted on the lower jaw meant that the animals could stretch their muscles more - the fibres became re-orientated so as to allow a higher gape, necessary for gaping with huge fangs."

But their impressive appearance might well have been key to their demise; the sabre-toothed cats were dependent on large, relatively slow-moving prey.

"They had very powerful but heavy bodies and rather short, extremely powerful legs," said Dr Christiansen, "so they probably weren't very fast, and certainly nowhere near as fast and agile as leopard and tigers."

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Megafelis Fatalis
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The face of Miller's Machairodont

American paleontologist George Miller set forth a set of features not previously thought of in the soft tissues of machairodonts, specifically Smilodon.

The first change he suggested in the appearance of machairodonts were lower ears, or rather the illusion of lower ears due to the higher sagittal crest. This claim has been generally discarded due to its unique nature: no other modern carnivores have these low set ears for this reason, but alternatively, no other carnivore has such a sagittal crest. For this reason, the positioning of the pinna, or outer ear, along with fur color, are dependent upon the individual doing the reconstruction. Large or small, pointed or rounded, high or low, fossils do not record these characteristics, leaving them up to interpretation.

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Miller's first change: low set ears, as seen with the African honey badger, Mellivora.

Miller also suggested a pug-like nose. Aside from the pug and similar dogs, no modern carnivore exhibits a pug-nose. The relatively low distribution of the pug nose has resulted in it being generally ignored. Miller's rationale is based upon the retraction of Smilodon nasal bones. Criticism of Miller's theory compares the nasal bones of lions and tigers. Lions, when compared to tigers, also have strongly retracted nasal bones, but a lion's rhinarium, or external nose, is no more retracted than the tiger's. Thus the pug-nose of Smilodon proposed by Miller has little evidence in the physical structures of comparable animals.

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Miller's second change: a pug nose, as seen with a domestic dog, the pug, Canis lupus familaris.

The third idea proposed is the elongation of the lips by 50 percent. While his other hypotheses have been largely discarded, the last is used significantly in modern depictions. Miller argues that longer lips allows the greater elasticity needed for biting prey with a wider gape. Although this argument has been rebuked strongly within the scientific community, it remains supported nevertheless by artists. Scientific criticism points out that the lips of modern cats, especially larger species, display incredible elasticity and the usual lip length would stretch suitably, despite the larger degree of opening. Regardless, reconstructions of Smilodon, Machairodus, and other species are shown with long lips, often resembling the jowls of large dogs.

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Miller's third change: jowls, as seen with a domestic dog, the Boxer, Canis lupus familiaris.

From Wikipedia
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[ *  *  *  *  * ]
From blaze, posted in the Smilodon gracilis vs Dire wolf thread. Useful to have it hear I guess.

Robusticity of S. gracilis

Source: Christiansen & Harris (2005)
TaxonNMean HL (mm)Mean HC (mm)Mean HC/HL
S. populator3365.2147.540.4%
S. fatalis6361.4121.233.5%
S. gracilis4247.881.532.9%
P. onca322673.232.3%
P. leo3343.3107.831.4%
P. tigris altaica3361108.330.1%

*The mean values of S. populator were brought down by one specimen considerably smaller than the rest (HL = 334mm vs 375mm and 388mm), that's why the humerus length is not that impressive when compared to S. fatalis of which none of the included specimens had a humerus length below 350mm.

*The P. leo sample consisted of a male and 2 females while the P. t. altaica consisted of 3 males over 220kg.

So, it is indeed slightly more robust than the jaguar but the sample sizes used here are small though, still, this only deals with the thickness of the humeral shaft and ignores that smilodontines generally have considerably more well developed sites for muscle insertion.

In table 4 of Chistiansen & Harris (2005) one can see the individual estimates obtained in the paper, the average weight estimated for S. gracilis specimens had a range of 56-97kg, this is the reason why a range of 55-100kg is mentioned in the paper but this doesn't mean that the weights are equally distributed throughout that range, the 56kg specimen is the only one below 70kg, if you exclude it from the sample the average weight estimated for S. gracilis is 86kg with 3 of the 8 individuals weighting 75-85kg and another 4 between 90-100kg.

Another thing that has to be mentioned is that S. gracilis increased in size from the early Irvingtonian to the late Irvingtonian (Berta, 1987), the later appear to be at least 10% larger on average (check the tables here) and guess what, S. gracilis remains housed in the University of Florida, from which all of the specimens used by Christiansen and Harris (2005) come from, appear to be early Irvingtonian.
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