The increase of bony mass in a small Cyprinodontidae from the Messinian deposit of Monte Tondo (Ravenna, Italy); paleoecological implications. The ichthyofauna of the late Messinian succession of Monte Tondo (Ravenna, Italy) is examined. Forty Aphanius crassicaudus, one clupeid and five goby specimens are described. Morphometric and meristic characteristics are determined, and a pachyostosis index is presented for the small cyprinodontid (A. crassicaudus) specimens. Aphanius crassicaudus in the Monte Tondo assemblage is represented by the two well-known Messinian phenotypes (non-pachyostotic or normal and pachyostotic). The increase in bone mass in the pachyostotic specimens is generally homogeneously extended throughout the body, but in the Monte Tondo assemblage, one young specimen of A. crassicaudus exhibits a peculiar characteristic distribution of increasing bone mass in the skeleton from head to caudal fin. A. crassicaudus was a dominant constituent of Mediterranean estuarine/lagoon teleost assemblages during the Messinian. The composition and ecological structure of the ichthyic association indicates that the lagoon of the Monte Tondo basin was a brackish water body that was occasionally influenced by seawater influx.
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