Ever since Baryonyx walkeri was announced to the scientific community in 1986, spinosaurid dinosaurs have often been compared to crocodylians (at least as far as the construction of their skulls are concerned), the similarity going so far as to result in one African spinosaurid receiving the name Suchomimus ("crocodile mimic"). In the latest issue of the Journal of Vertebrate Paleontology, however, Rayfield et al. put such comparisons to the test to see if spinosaurid dinosaurs had bites that were functionally similar to living crocodylians.
The problem with spinosaurid skulls is that they display a mosaic of characters that show similarities to both crocodylians and other theropod dinosaurs. The presence of a secondary palate, a "rostral terminal rosette" (or a rounded, cup-like region at the tip of the snout), a less blade-like and more rounded tooth structure, and reduced antorbital fenestrae all show some convergence with crododylians. Such adaptations are typical strength-increasing changes in the skull, the closing of the antorbital fenestrae and the development of a secondary palate both making the skulls of living crocodylians more resistant from torsional stresses during feeding. In order to test whether the same might be true in spinosaurid dinosaurs, the team created computer models of the snouts of Baryonyx, an American alligator, a gharial, and a "generalized" (or "stylized," as they sometimes call it) theropod, the crocodylians being chosen as they represented extremes for crocodylian skull morphology (alligators have a broad, flat snout and gharials have a very narrow, tubular snout). The researchers also state that they did not recreate entire-skull models due to time constraints and the fact that the back of the skull of Baryonyx was incomplete, so the results are based entirely upon snout morphology.
What the researchers found when they carried out their tests was that the alligator and generalized theropod were functionally closer to each other than either was to the gharial and Baryonyx in terms of stress resistance and distribution, the gharial and Baryonyx also being closer to each other than either was to the alligator or theropod. In the case of the generalized theropod and alligator, for instance, torsional stresses became greater as their tooth rows diverged from the midline (or moving backwards from the front of the jaw as the jaws make a V shape). The gharial and Baryonyx, by contrast, had minimized torsional stress by having a narrow jaw where stress was spread out more evenly. Likewise, the secondary palate provides alligators strength against torsional stress while in the gharial and Baryonyx the secondary palate provided resistance against bending, so the function of a secondary palate as a reinforcing feature of the skull can have different functions depending on skull shape. The study also did something else that was interesting; they gave the alligator and gharial skulls antorbital fenestrae at one point, and it seems that such large holes in a gharial skull definitely weakened it (the alligator skull didn't seem to experience much of a difference). This suggests that the reduction and closing up of antorbital fenestrae was likely a consequence of a narrow snout and the stress load distribution that comes with it.
The researchers do pose an interesting question, though; if it is advantageous to develop a secondary palate and to close up antorbital fenestrae in relatively narrow skulls, why didn't large theropods do so? There could be other aspects of theropod skulls that could deal with stresses involving bite forces, especially since the study focused on stressed in the rostral region and did not take the rest of the skull/musculature into account. Also, theropod skulls are compressed laterally rather than dorso-ventrally, so such an arrangement may have its own strengths and weaknesses that will merit further research. As far as dealing with stress loads in the rostral part of the skull, though, Baryonyx seems to be functionally convergent with gharials, although further study (especially of other crocodilians and spinosaurids) is required.
References;
Rayfield, E.J.; Milner, A.C.; Xuan, V.B.; Young, P.G. (2007) "Functional Morphology of Spinosaur 'Crocodile-Mimic' Dinosaurs." Journal of Vertebrate Paleontology, Vol. 27(4), pp. 892-901
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Definately interesting. It's good that somebody finally tested the crocodilian comparison, as modern analogues are not always apt. If you have this paper, Brian, please send it my way. I can also bug Julia and Anne-Marie.
(If you're reading this, ladies, please send it my way!) :-)
This is all quite interesting, in that the perception ( at least in my mind ) has been that the terminal rosette in spinosaurs is a derived "expansion" of the element, when in actuality the real change is in relative compression of the post terminal jaws themselves. In other words, the shape, size (scaled) and function of the premaxillae in spinosaurs, gharials and any other crocodilians are all relatively the same. It's only the compression that occurs in the balance of the feeding mechanism in spinosaurs and gharials,that redistributes the stresses according to requirement of specialization in prey capture. If narrow jaws are advantageous to accomodate to a largely piscivorous diet (gharials, presumably spinosaurs), lateral expansion of the jaws in other crocodilians (with a broader range of consumption), and the added kinesis in jaw joints of theropods are an allowance to grapple with larger, more ungainly prey items that require more leverage, finesse and control in efficiency of capture.
Just breezed the paper (thanks, Brian!), and it's quite good. I wonder, too, why other theropods didn't evolve a secondary palate, although like Mike said, it could have to do with flexibility in the skull. Certainly in an animal like Carnotaurus, who has an incredibly flexible skull, a secondary palate might hinder prey capture.