STL Science Center

STL Science Center

12 June 2013

Jaws and Families

©Scott Anselmo, right maxilla
Suchomimus was a well equipped dinosaur. As a smaller spinosaur it was still equipped with a dorsal ridge (sail, hump, flap, etc), though the anatomical feature on Suchomimus was clearly much smaller than on on Spinosaurus. The dorsal ridges, neural spines, of the vertebrae that formed this proposed feature were short but evident and were confined to the lumbar-sacral vertebrae. That simply means the ridge was above the hips of Suchomimus. Problematic points with this ridge are that the skeleton shown to posses this is thought to be a subadult, meaning that perhaps a radiation of growth during and after this stage of life may have elongated the ridge extensively (thus making Suchomimus a young Spinosaurus and nothing more); however, this idea has not been published or extensively researched as far as I can tell. The more likely synonymization of Suchomimus has always been with regard to the genus Baryonyx.

The hand claws and much of the general shape of the animals are what have tied them closely together. However, Sereno et al (1998) detailed characters that were unique to Suchomimus and separated the two genera. One such character was the number of teeth. Baryonyx possessed 96 teeth in its jaws while Suchomimus houses approximately 130. Our mouths are not designed anything like these giant fish seines. Despite the image of spinosaur mouths as tooth lined nets do not forget that some of the diet of Suchomimus and others in the family tree probably came from carrion as well as fresh caught fish. What is the notch for in the jaw though?

©Scott Anselmo,mandible
That notch in the jaw was far from vacant. In fact, a number of the largest teeth in the mandible fit snugly into the groove of the maxilla at that point. The teeth in the maxilla are correspondingly small where the mandible teeth are oversized; Suchomimus would have a horrendous dental problem if there were large teeth at the top and bottom where the notch is. The notch is not just a random feature of the bone either. The anterior portion of the notch consists of the posterior ventral edge of the premaxilla and the posterior edge of the notch is made from the anterior most section of the maxilla itself. Essentially that means that it is the point at which two bones of the jaw meet but do not form an even edge the entire length of the jaw; the bulbous indentation of the notch is found mainly in the premaxilla however. We see this in other spinosaurs but also in a more basal family; the Dilophosauridae. Are the two families more closely related than we think at present or is there an evolutionary jolt that caused two very separate families to develop similar jaw anatomy (though not overall skull shape)?

Reference for the day:
Sereno, P.C.; Beck, A.L.; Dutheil, D.B.; Gado, B.; Larsson, H.C.E.; Lyon, G.H.; Marcot, J.D.; Rauhut, O.W.M.; Sadleir, R.W.; Sidor, C.A.; Varricchio, D.D.; Wilson, G.P; and Wilson, J.A. (1998). "A long-snouted predatory dinosaur from Africa and the evolution of spinosaurids". Science 282 (5392): 1298–1302

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