The systematic, paleoecologic and paleobiogeographic analysis of the Plio-Pleistocene Mediterranean elasmobranch fauna herein discussed provides new relevant data concerning the diversity and disparity of this important marine vertebrate group, in relation to the main climatic, oceanographic and geological evolutionary events. Moreover, This study is part of a wider project in order to understand the main Neogene historical and evolutionary processes involved in the establishment of the present Mediterranean fish fauna.
The database used, comprehensive of all the Plio-Pleistocene shark teeth fauna of the Mediterranean area, was compiled from a generic and species revision of historical elasmobranch teeth collections, from a study of the new findings collected the middle Pliocene sections of Rio Merli, Rio dei Ronchi, Rio Cugno and Rio Co di Sasso, and from the lower-middle Pleistocene sections of Fiumefreddo, Grammichele, Archi and Vallone Catrica, as well as from a critical analysis of as many publications as possible figuring and describing Plio-Pleistocene Mediterranean elasmobranch faunas. The qualitative and quantitative analysis of this database provided a general paleoecological and paleobiogeographical pattern of the Mediterranean elasmobranch fauna through the Plio-Pleistocene.
A total of 27 species, included into 19 genera, 13 families and 8 orders, have been described. The main systematic results concerned the revision of several teeth, once included in the fossil taxon C. egertoni, from the lower-middle Pliocene of Italy, assigned to the carcharhinid species C. brachyurus, C. falciformis, C. leucas, C. longimanus, C. obscurus, C. perezi, C. plumbeus. Such revision provided the high diversity and disparity of the genus Carcharhinus starting from the lower-middle Pliocene in the Mediterranean, and supported the hypothesis of C. egertoni as a “wasted-basket species”. Moreover, according to the recent diagnosed proposed by Purdy et al. (2001), the shark teeth herein documented and once included in the fossil taxon Isurus hastalis have been assigned to the wide-toothed mako shark Isurus xiphodon. The wide geographic distribution of this fossil taxon during the lower-middle Pliocene (upper Pliocene ?) in the Mediterranean was consistent with the strong presence of marine mammals, mainly pinnipeds and/or small cetaceans, on which I. xiphodon fed. Finally, six large teeth from the Pliocene of Italy have been assigned to the giant-toothed white shark Carcarodon megalodon. This voracious shark was considered extinct between the Upper Miocene-lower Pliocene from the Mediterranean. Therefore, the teeth documented might extend the fossil record of this shark at least during the lower Pliocene into the basin, even if more detailed studies needed to confirm this record.
The data provided by the systematic study carried out in this work integrated with those from the critical analysis of the main publications have allowed to recognise a total of 73 elasmobranch species, of which 14 in open nomenclature for the identification at specific level, included in 51 genera, 26 families, and 11 orders. In particular, Carcharhiniformes, representing the 30,6% of the total fauna, was the most diversified shark order of Mediterranean elasmobranch fauna. Moreover, Lamniformes (16,6%), Squaliformes (16,6%) and Myliobatiformes (15,3) provided a relevant part of the elasmobranch fauna diversity. All the other elasmobranch orders, Hexanchiformes (5,5%), Echinorhiniformes (1,4%), Squatiniformes (4,2%), Pristiophoriformes (2,8%), Orectolobiformes (1,4%), Rajiformes (4,2%), and Torpediniformes (1,4%), were poorly represented and mostly subordinated to these four large groups.
From a stratigraphic point of view, the teeth recorded in the lower Pleistocene section of Fiumefreddo assigned to the living Apristurus aff. laurussoni represent the first fossil record for the genus Apristurus, while the teeth assigned to Chlamydoselachus anguineus, from the same lower Pleistocene section, represent the first fossil record for this living species.
The qualitative and quantitative analysis of the database provided a progressive decrease in diversity and disparity of the elasmobranch fossil record across the Plio-Pleistocene. Moreover, two different evolutionary trend were recognised for the Plio-Pleistocene elasmobranch fauna. A first evolutionary trend involved all the epipelagic and mesopelagic groups of the elasmobranch fauna, characterized however by the less structural variations, mainly related to the progressive disappearance of Miocene relict taxa, such as Isurus xiphodon, Parotodus benedeni or Dipturus olisiponensis, as well as extra-Mediterranean tropical and subtropical species, such as Galeocerdo cuvier, Carcharhinus leucas or C. perezi, showing, starting from the middle Pliocene, a faunal composition very similar to the present. By contrast, a second evolutionary trend involved all those upper mesopelagic and/or bathypelagic shark families, such as Centrophoridae or Dalatiidae, as well as many scyliorinid species. These elasmobranch groups provided the most relevant structural changes across all the Plio-Pleistocene, through several periods of diversification and crisis, probably supported by changes in the deep waters hydrographic conditions of the Mediterranean, as well as by major connection with the near Atlantic Ocean.