The Miocene fishes of Capo di Fiume, Central Italy: Stratigraphy, taphonomy and paleoecology

The Capo di Fiume site is located at the north-eastern margin of the Monte Porrara, a few km south of the town of Palena, in the Chieti province. The section consists of paralic to open marine mud-dominated Messinian deposits overlying a relatively thick terra rossa horizon, testifying a prolonged subaerial exposure. Fissile marls and laminated argillites overlie the terra rossa horizon. These deposits record the passage from palustrine to brackish (freshwater marshes, tidal creeks, swamp, estuarine bay) paleobiotopes. Marine deposits overlie freshwater to brackish marls; these deposits consist of not less than six cycles of alternated dark grey calcareous marls and finely laminated ichthyolitiferous diatomitic marls. The diatomitic marls consist of a dense series of clastic-biogenic (diatomitic) couplets. Several types of laminae have been identified based on chromatic characters, thickness, lateral extension, biological content and other morphological features. The macrofossil content of the diatomitic marls is relatively rich and includes well-preserved articulated skeletal remains of teleost fishes primarily represented by clupeids of the genus Spratelloides, a single articulated skeleton assigned to the ochotonid Prolagus cf. apricenicus, bird feathers, rare decapods crustaceans, insects, and bivalves and plant remains. Foraminiferans are rather scarce exclusively represented by ammoniids, bolivinids and rare dwarfed globigerinids. Fish skeletons are mainly associated with thick continuous laminae formed by massive accumulation of Thalassionema nitzschioides and/or Coscinodiscus sp. The preservation of fish skeletal remains seems to be primarily due to the action of the bacterial and fungal film that proliferated on the mucilage of the diatom flocs, promoting the rapid phosphatization of the bones and the mineralization of organic components. Several hundreds of fish specimens have been analyzed, among which not less than 22 taxa belonging to 14 families have been identified. The dussumierine clupeid Spratelloides lemoinei sharply dominates the assemblage, represented by more than 75% of the recognized specimens. The comparative analysis of the ecological categories indicates that the assemblage primarily consists of demersal neritic (Boops roulei, Callionymus sp., Capros sp., Diplodus cf. oranensis, Diplodus sp., Epinephelus sp., Halobatrachus cf. didactylus, ?Mioblennius sp., Pagrus sp., Syngnathus sp.) and coastal epipelagic taxa (Alosa elongata, Etrumeus boulei, Sardina pilchardus, Spratelloides lemoinei, Trachurus sp.), with a diverse contingent of migratory pelagic and oceanic species(Diaphus edwardsi, Lestidiops sphekodes, Maurolicus cf. muelleri, Merluccius sp., Myctophum columnae, Paralepis albyi); the Nile perch is the only freshwater/marginally paralic taxon recognized in the assemblage. The paleoecological analysis of the fish assemblage suggests that the deposition of the diatomitic marls occurred not far from the coastline in a moderately shallow basin surrounded by rocky or coral reefs and seagrass beds and influenced by the open sea. The presence of the Nile perch unequivocally indicates that a river system also contributed to the physiography of the basin. The heterogeneous composition of the assemblage, which is characterized by the co-occurrence of coastal and opportunistic pelagic taxa, is consistent with the unstable eutrophic conditions indicated by the monospecific or oligospecific accumulations of Coscinodiscus spp. and/or Thalassionema nitzschioides that formed the biogenic substrate of the fish bearing diatomitic laminae.