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2 Acta Geologica Polonica

2 2008

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Mesozoic hybodont sharks from Asia and their relationships to the genus Ptychodus

Cuny G.

Acta Geologica Polonica

2008

Vol. 58, no. 2

211-216

The affinities of the selachian genus Ptychodus is discussed and Hemiptychodus is here re-introduced as a subgenus characterized by unusual tooth ornamentation pattern. An overview of Asian hybodont genera showing tooth ornamentations pattern similar to that of Ptychodus is given. Lacking a triple-layered enameloid, Ptychodus may be a hybodont shark or a batoid, but both hypotheses are not without problems. Based only on dental characters, hybodont affinities for this genus are favoured here, although the evidence to do so is indeed weak.

Carcharopsis prototypus and the adaptations of single crystallite enameloid in cutting dentitions

Duffin C., Cuny G.

Acta Geologica Polonica

2008

Vol. 58, no. 2

181-184

Carcharopsis is a Palaeozoic shark comprising an enameloid-covered cutting dentition comprising serrated teeth. The enameloid ultrastructure of the teeth of C. prototypus is examined for the first time and consists of a 35 [my]m thick layer of tightly compacted apatite crystallites with a maximum individual length of 0.1 [my]m. The crystallites are randomly-oriented toward the base of the enameloid layer, but assume a roughly parallel orientation higher up in the enameloid, with their long axes arranged normal to the tooth surface. The enameloid of typical hybodonts comprises a compact outer crystallite layer and a looser, bundled inner layer. The Cretaceous hybodonts, Priohybodus arambourgi and Thaiodus ruchae, by contrast, have a compact, less differentiated enameloid very similar to that of C. prototypus. This suggests that crystallite compaction is a corollary of the evolution of serrated dentitions possessing single crystallite enameloids.