Notes on fossil chitons. 4. Polyplacophora from the Pliocene of Altavilla (NW Sicily)

This paper discusses the chiton fauna (Mollusca, Polyplacophora) from some deposits at Altavilla Milicia (NW Sicily) presumably older than the late Upper Pliocene. The study material consists of 550 valves from 5 samples (ORTO1-2 and KYLA1-3a) of the “Sabbie di Altavilla” Formation. This is the first comprehensive account of fossil chitons from the Pliocene of Sicily. We identified 14 species: Leptochiton cancellatus, L. algesirensis, Leptochiton sp., Hanleya hanleyi, Ischnochiton rissoi, I. anserinus, I. martinelli, Stenosemus dolii, Callochiton septemvalvis, Chiton sp., Lepidochitona caprearum, Acanthochitona fascicularis, A. crinita and Craspedochiton altavillensis. Two species (S. dolii and C. altavillensis) represent 77.3 % of the total amount of the valves. The finding of I. anserinus and I. martinelli is particularly important since these rare, scarcely known species, were reported only from few Pliocene localities in Italy and Spain. Nine species are still living in the Mediterranean, of which seven also occur off the Atlantic coasts of Europe. Two species are known only from the Mediterranean Pliocene (Ischnochiton anserinus and I. martinelli), and only Craspedochiton altavillensis has a Miocene to Pleistocene distribution. The still living Leptochiton algesirensis dates back to late Paleogene (Oligocene), but is so far not recorded from the Miocene. Eight species are new for Altavilla: Leptochiton cancellatus, Hanleya hanleyi, Ischnochiton rissoi, I. martinelli, Callochiton septemvalvis, Lepidochitona caprearum, Acanthochitona fascicularis and A. crinita. The analysis of the Altavilla samples suggests shallow water depositional palaeoenvironments, possibly from lower infralittoral to circalittoral depth, with the exception of the oldest investigated layer (sample ORTO1) for which a lower circalittoral-epibathyal environment is supposed. The occurrence of the extant bathyal species Stenosemus dolii in ORTO2 and KYLA1-3a suggests that this species had wider eco-bathymetric requirements during the Pliocene, as no evidence of reworking has been observed in these samples and in their respective layers.