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2 Acta Geologica Polonica

2 2008

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When the "primitive" shark Tribodus (Hybodontiformes) meets the "modern" ray Pseudohypolophus (Rajiformes) : the unique co-occurrence of these two durophagous Cretaceous selachians in Charentes (SWFrance)

Vullo R., Neraudeau D.

Acta Geologica Polonica

2008

Vol. 58, no. 2

249-255

The palaeoecology and palaeobiogeography of two Cretaceous selachian genera, Tribodus BRITO & FERREIRA, 1989 and Pseudohypolophus CAPPETTA & CASE, 1975, are briefly discussed. These two similar-sized taxa developed an analogous pavement-like grinding dentition, characterized by massive teeth with a rhomboidal to hexagonal occlusal surface. Although both genera appear to have been euryhaline forms, the hybodont Tribodus occurred in fresh/brackish water habitats (e.g. deltas) to shallow marine lagoons, whereas the ray Pseudohypolophus lived in brackish water to coastal marine environments. Palaeobiogeographically, their global distribution displays two distinct but adjoined areas, with Tribodus being present in the northern part of Gondwana (Brazil and North Africa), and Pseudohypolophus occurring on both sides of the North Atlantic (North America and Western Europe). However, the two genera coexisted during Cenomanian times within a small overlap zone, localized in western France. A trophic competition may have arisen from this situation between these two selachians belonging to the same trophic guild. This peculiar situation is well documented within the Cenomanian transgressive series of Charentes (SW France), where a turnover between the two forms is observed (replacement of Tribodus by Pseudohypolophus).

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.