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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

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Abstract

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.

dinocrocuta

 

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.

http://ufdc.ufl.edu/UF00095816/00001/1j

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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.


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.


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.


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

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Robusticity of S. gracilis

Source: Christiansen & Harris (2005)
Taxon	N	Mean HL (mm)	Mean HC (mm)	Mean HC/HL
S. populator	3	365.2	147.5	40.4%
S. fatalis	6	361.4	121.2	33.5%
S. gracilis	4	247.8	81.5	32.9%
P. onca	3	226	73.2	32.3%
P. leo	3	343.3	107.8	31.4%
P. tigris altaica	3	361	108.3	30.1%

Notes:
*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.

Size
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.