Article: Discohelix (Archaeogastropoda, Euomphalacea) as an index fossil in the Tethyan Jurassic

Morphological features, growth, mode of life, and stratigraphical distribution are examined in more than 300 extremely well-preserved specimens of Discohelix from the Sicilian Lower and Middle Jurassic. In some species parabolic ribs similar to those in ammonites are formed owing to resorbtion during brief discontinuities in growth. Deviated peristomes recurring at an angle of more or less 72° and resulting in a pentagonal outline of the shell, are developed in two species for which the new subgenus Pentagonodiscus (type species D.(P.) angusta n. sp.) is established. Ten species of Discohelix sensu stricto are described, five of which are new: D. dictyota, D. conica, D. centricosta, D. costata, and D. levis. The short stratigraphical ranges of the representatives of the genus are proved here for the first time and may be useful for the division of cephalopod-free series in the Tethyan Jurassic.