What use is a mandible for a sabertooth?
The combination of old fossils and new technologies is taking us closer and closer to understanding the adaptations of extinct animals, and in particular of sabertooths. One recent piece of research by Stephen Wroe and colleagues takes the fossils of the marsupial sabertooth Thylacosmilus from the collections of the Chicago Field Museum, and uses 3D scanning and FEA analyses to compare it with the more “familiar” felid sabertooth cat Smilodon. The Field Museum Thylacosmilus sample was recovered by the 1926 Marshall Field Expedition to Argentina, and it served paleontologist Elmer Riggs to publish an excellent description of the species in 1934. Many decades later, it remains the best and most complete sample of the species, and it was rightly chosen for this “rocket-science” study (you can see the paper following this link: http://www.plosone.org/article/info:doi/10.1371/journal.pone.0066888 ).
The results of this comparison reassuringly confirm something that more conventional studies of the marsupial sabertooth fossils already indicated: that this animal not only had converged with placental sabertooths from its isolation in Cenozoic South America, but it had in fact taken the adaptations for a sabertooth way of hunting several steps beyond what we see in the true sabertooth cats like Smilodon. Sabertoothed predators had a different kind of killing bite than modern cats, so that when jaws were open at full gape and jaw-closing muscles were too stretched to pull with force, it was the muscles of the neck that came into action, pulling the whole head down to help sink the fangs into the flesh of prey. As my illustration here shows, the neck vertebrae of Thylacosmilus were very large with huge processes for muscle insertion, making it quite adequate for such a function.
The new study shows that the skull of Thylacosmilus was well designed to stand the stresses caused by that kind of motion –pulling down the head to sink the canines – and less well prepared to deal with the stresses that would be generated if the canines had to pierce the flesh using mostly the forces generated by the jaw musculature (as it does in modern cats).
The authors conclude from their comparisons that the action of neck muscles was even more important in Thylacosmilus than it was in Smilodon, something that seems pretty evident from the results. But they go one step further and argue that the lower canines did not play any major part in the killing bite in Thylacosmilus. If that were the case, the mandible would still have its role in biting off meat from carcasses and slicing it with the premolars and molars, but it would hardly contribute to the actual killing of prey… may that be true?
Well, most sabertoothed predators have something in common: the area where the two halves of the mandible join under the incisor teeth, technically called the symphysis, is more vertical than in other predatory mammals, and it tends to develop vertical ridges along its antero-lateral edges. In some sabertooths, including Thylacosmilus, this shape is exaggerated and a downward, scabbard-like projection is created. In others, like Smilodon, the projection is nearly absent, but the shape of the symphysis remains unmistakably “sabertoothy”. It is tempting to see the projection as a sort of “protection” for the sabers, but in fact it is more likely related with the kind of forces that the symphysis has to withstand during the killing bite. In “normal” cats, those forces have a large lateral component, but in sabertooths vertical forces were overwhelmingly dominant, and that has likey shaped the anterior part of their mandibles. Paleontologist Bill Akersten in his elegant 1985 hypothesis of the ”Canine Shear-Bite” gave the lower canines and incisors a role in the killing bite, a role that at least partly accounted for the unmistakable shape of the sabertooth symphysis. While there are some studies of the form and function of the mandible in sabertooths out there, more analyses are clearly necessary. But as long as a sabertooth mammal retains a vertical, reinforced symphysis (as is the case in both Smilodon and Thylacosmilus in spite of the obvious differences), we have good reason to think that the mandible had an important role to play in the killing bite.
If you are interested to read more about this, look for the book “Sabertooth” in late October!
Here are a couple of references:
Akersten, W. A. 1985. Canine function in Smilodon (Mammalia; Felidae; Machairodontinae). Contributions in Science 356: 1-22.
Riggs, E.C. 1934: A new marsupial saber-tooth from the Pliocene of Argentina and its relationships to other South American predacious marsupials. Transactions of the American Philosophical Society 24, 1–32.