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The recent  find of a new complete  specimen of Psephoderma  alpinum  Meyer, 1858 in the Norian  Calcare  di Zorzino (Zorzino Limestone), in northern  Italy, adds  to our knowledge  of the osteology  of this species.  The new  specimen  is the largest  so far collected, reaching  180 cm in length. Allometric  growth of  the limbs  during  ontogeny is demonstrated.  A new  interpretation  of its palaeoecology  and mode of life is given on the bases  of  both functional morphology and palaeoenvironmental  observation. The streamlined  keeled carapace is interpreted as having  a hydrodynamic rather than  a defensive function.  The long, stiff tail and, probably, the dorsal  caudal plate served for balance. The gap in the carapace, above  the sacral  region, enabled Psephoderma  to swim at least  as well as trionychid  turtles, though  no particular  swimming  adaptations of the pelvic girdle and of the femur are observed.  Invertebrates  and a rich fish fauna  were found  together with these placodonts,  allowing  the restoration  of the oxic part of the basins environment as a normal marine  one. The surface  waters were influenced by tides that possibly  generated  rarher  strong  local currents close  to the mouth of tidal channels  crossing  the platform.  This provided for a rich nectonic  and benthic life in the more superficial waters: Psephoderma  fed on bissate molluscs,  such  as Modiolus and Isognomon, dwelling  on shallow  oxic bottoms at the basins  edge. Due to  water stratification,  a long-lasting  anoxic  environment developed in the deepest part of the basins  themselves, allowing fine preservation  (Konservat-Lagerstaetten)  of allochthonous  organisms.