|Skull of Oenosaurus in ventral view, showing the formidable tooth plates.|
Preparation and careful examination and comparison of the skull confirmed this identification. Oenosaurus really represented a rhynchocephalian, though with a dentition as it has never been described before in a tetrapod. The dentition consists of massive tooth plates, which, under closer inspection, seem to be made up of hundreds or thousands of fused individual teeth, with small internal cavities and concentric arrangements of dentine layers around them. That's what it looked like when examining the tooth plates under the microscope, and our first interpretation was that these plates indeed represented simply fused individual teeth, possibly including several tooth generations, as in the tooth batteries of some ornithischian dinosaurs. Given that rhynchocephalians usually do not show tooth replacement, this would have been weird enough, but then we made a computer tomography of the tooth plates. The results showed no evidence of replacement teeth, but apparently continously growing dentine tubules that were fused into a single structure towards the surface and sometimes even showed branching patterns. A literture survey revealed that similar structures are basically only found in chimeran chondrichthyans and lungfishes, where this tooth tissue is called osteodentine or petrodentine.
|Right mandible of Oenosaurus in lateral view.|
Rhynchocephalians are an ancient lineage of lepidosaurian reptiles, the group that modern lizards and snakes also belong to. However, in contrast to the latter, which are currently represented by several thousand species, only two species of rhynchocephalians survived to the present day, both in the genus Sphenodon. This genus, commonly named the Tuatara, is currently restricted to s few islands off the coast of New Zealand, where these animals have found their last refuge. Since Sphenodon belongs to such an ancient lineage and also shows some rather primitive looking features, it is often considered a living fossil, and was consequently used frequently in studies relating to allegedly ancestral conditions for modern lizards, also in recent times.
|Photo of the rather sympathetic looking Sphenodon, the only recent rhynchocephalian (courtesy Helmut Tischlinger).|
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