Tuesday, June 19, 2007

Jaw biomechanics of Smilodon fatalis

Biomechanically, Smilodon fatalis is an interesting animal. Where most extant felids have big strong canines, S. fatalis had long flat ones.

This implies that Smilodon would not have been capable of the same kind of precision biting that modern cats employ. Struggling with prey to deliver a fatal bite to the nape of the neck and dislocating the cervical vertebrae would result in the canines contacting with bone. The long flattened canines of Smilodon probably would not have withstood that kind of load.

The canines of Smilodon don't look like stabbing teeth like in modern cats, instead they look more like slicing teeth - like in Komodo dragons.

To the left is a tiger skull for comparison. The tiger has a lower longer face, larger eyes, and above all has pronounced zygomatic arches. Smilodon on the other hand had smaller zygomatic arches. This reduced the amount of available space for the temporalis muscle thus reducing the overall muscle force.

The tiger also has huge coronoid processes enabling a higher mechanical advantage, whereas Smilodon had very small coronoid processes.

These anatomical features would imply that Smilodon did not have a strong bite compared with modern cats of similar size.

Perhaps, this is a byproduct of having extremely long canines. In order for Smilodon to clear its lower jaw of the canines, it would have needed a large gape. Large gape results in an increased stretch of jaw muscles. This reduces tensile stresses in muscles thus less effective force. For this, Smilodon perhaps had to rely on another way of driving its canines into its prey.

Akersten (1985) hypothesised that rather than relying solely on a jaw-driven bite, Smilodon would have used the cervical musculature to push down the whole head and drive the sabre-teeth into the prey. Instead of biting into the neck, Smilodon may have bitten into the soft abdominal region, drove its canines in deep, then sliced out a chunk of flesh. This leaves a gaping wound and the prey would go into shock and die from loss of blood.

A masters student last year tested this hypothesis. She'd reconstructed the temporalis and masseter muscles along with the cervical muscles to see the effect on bite force with and without the presence of neck muslces. Depending on the reconstructed sizes of the cervical muscles, the neck-driven bite forces can range between 46-88% of the jaw-driven bite forces. So on its own, the neck-driven bite force is no better than the jaw-driven bite force. However, when the two are combined, then bite force is substantially higher, obviously.

References:
Akersten, W. A. 1985. Canine function in Smilodon (Mammalia; Felidae; Machairodontinae). Contributions in Science 356: 1-22.

Sweet, E. J. H. 2006. Biomechanical analysis of the biting performance in the sabretooth cat Smilodon fatalis. Unpublished MSc thesis. University of Bristol.

Friday, June 1, 2007

Megalosaur

This is some kind of a megalosaur I drew some time ago. I think it's supposed to be a Dubreuillosaurus, but I didn't really bother with any anatomical accuracy so I just call it a megalosaur.

Anyway, megalosaurs are interesting. They're usually agreed to be basal tetanuran theropods and sister-taxon to spinosaurs, together forming the Spinosauroidea. But the intrarelationship among the Megalosauridae is a mess. In short no one really knows which dinosaur is more closely related to the other. This is inevitable to a certain extent due to the fragmentary nature of many megalosaur specimens but also because of the historical treatment of 'megalosaurs' being kind of a wast basket for theropod fossils of uncertain affinities.