The porbeagle shark, Lamna nasus (Elasmobranchii: Lamniformes), from the late Pliocene of the central Mediterranean Basin

摘要

The porbeagle shark, Lamna nasus, is an extant species of mackerel shark characterized by a tuna-shaped body, high swimming velocities, regional endothermy, and a predilection for subpolar and cold-temperate waters below 18 degrees C. Porbeagle sharks inhabit a broad variety of mid-and high-latitude coastal and open-ocean environments, ranging across the northern Atlantic Ocean and in a continuous band in the Southern Hemisphere. We report on a fossil tooth, identified as belonging to Lamna nasus, discovered in late Pliocene (i.e., Piacenzian) upper bathyal mudstones exposed at Castelnuovo Berardenga Scalo (Siena Basin, Tuscany, Italy), a site which has also provided a well-diversified deep-water elasmobranch assemblage rich in forms such as Chlamydoselachus lawleyi and Centrophorus granulosus. To our knowledge, this specimen represents the first fossil record of L. nasus in the Mediterranean Basin; as such, it expands our knowledge on the late Pliocene central Mediterranean biodiversity and contributes to the still fragmentary fossil history of the genus Lamna. Our finding suggests that, at least at times, seawater temperatures suitable for cold-temperate coastalpelagic organisms were present along the coasts of Tuscany during deposition of the mudstones exposed at Castelnuovo Berardenga Scalo. Since these deposits are believed to postdate the so-called Mid Piacenzian Warm Period (3.264-3.025 Ma), the occurrence of a tooth L. nasus could be regarded as an early marker of the long-lasting cooling trend which affected the Mediterranean waters starting from around 3 Ma. The porbeagle shark, therefore, might have been one of the earliest Atlantic cold-water fish taxa to enter the Mediterranean realm in Plio-Pleistocene times.