Morphological and histological study of the hyperostoses of Lepidopus albyi (Sauvage, 1870), a fossil Trichiuridae from the Tortonian (Upper Miocene) of Piedmont (Italy)
Two fossil specimens of the cutlassfish Lepidopus albyi, from the Tortonian (Upper Miocene) of Piedmont (Italy) showed hyperostotic pterygiophores. The comparative morpho-histological study of this material with the fossil L. proargenteus and the living Trichiurus lepturus showed that the swollen bones are constituted of spongy bone in the three species. This hyperostosis results from a double process: an increase of the periosteal osteogenesis combined to an important resorption that creates numerous vascular cavities. The factors that control this hyperostotic phenomenon in scabbardfishes were probably the same in the Tortonian time as today. However, they remain hypothetical, even if an epigenetic hydro-mechanical constraints or genetic factors seem the more likely. In contrast, a physiological response to survival in an extreme biocenosis with salt excess should be rejected. Whatever the causal factors involved, the hyperostotic phenomenon is an old one in this teleost family, that is at least 8.8 My old.