Os carcharhiniformes (cCondrichthyes, Neoselachii) da Bacia de Alvalade (Portugal)

The uppermost Miocene, Esbarrondadoiro Formation (Alvalade basin, Portugal) yielded more than 10 thousand Selachian teeth at Santa Margarida, Esbarrondadoiro and Vale de Zebro outcrops. Forty-five taxa were identified belonging to the orders Hexanchiformes, Squaliformes, Lamniformes, Carcharhiniformes, Torpediniformes and Myliobatiformes. The Carcharhiniformes make up about 40% of the selachian fauna that has been identified in the studied area. The families Scyliorhinidae, Triakidae, Hemigaleidae, Carcharhinidae and Sphyrnidae, and fifteen species are recognized. The distribution of the Carcharhiniformes by the Santa Margarida, Esbarrondadoiro and Vale de Zebro localities is shown (Table 1). The quantitative taxa distribution among these localities is very unequal. The predominant forms are Premontreia (Oxyscyllium) cf. dachiardi, Mustelus sp., Paragaleus antunesi, and especially Carcharhinus cf. perezi. The rarity of the tiger-shark, Galeocerdo, is interesting. Its decline in european seas was then obvious. It represents the last occurrence in the Miocene of Portugal. The different distribution of the Carcharhiniformes (as well as that of the other orders) by the three sites points out to distinct environments in the corresponding areas: Esbarrondadoiro indicates relatively deeper, rather still waters; Santa Margarida represents a very littoral area and rough waters; while Vale de Zebro was a (probably inner) part of a gulf with muddy bottoms. Stenotherm, termophilous forms that were common or plentiful before are absent. The near absence of Galeocerdo gives a similar indication, as well as the total lack of termophilous crocodilians that were common until the Middle Miocene and still were represented in the Tortonian. At the Alvalade Basin localities there are several “temperate” elements that still are present in Portuguese waters. However the high frequence of Carcharhinus, a genus that is scarce at present off Portugal, indicates waters somewhat warmer than today’s but less warm than during Tortonian times. This is corroborated by the large number of Dasyatis species and the rarity of Raja. We may conclude that moderately warm (less warm than during Tortonian, but warmer than today’s) waters predominated in the Alvalade Basin close by the end of Miocene. The scarcity of pelagic forms and especially of the larger pelagic predators points out to a quite narrow gulf and not to an open Atlantic front. The faunal differences between the Tortonian near Lisbon and the Messinian of the Alvalade Basin also indicate a rather confined gulf here in contrast with the open sea environments recognized for the Tortonian near Lisbon. Among the modernity signs of the Alvalade Basin selachian fauna in comparison with earlier Miocene ones are the highly developed characters of Galeocerdo and Carcharhinus.