Large shell injuries in Middle Ordovician Orthocerida (Nautiloidea, Cephalopoda)

Abstract

Sublethal injuries are described from six fragments of orthocerids, which belong to Orthoceras regulare Schlotheim, 1820, Orthoceras scabridum Angelin, 1880, Nilssonoceras nilssoni (Boll 1857) and Plagiostomoceras laevigatum (Boll 1857) from the Baltic Orthoceratite Limestone (Arenig-Llanvirn, Middle Ordovician) of Sweden, and of northern Germany. The injuries represent shell breakages with an exceptionally large absolute size. The largest observed injury measures more than 60 mm from the aperture to its distal rim. Injuries of that magnitude have previously never been described from Ordovician molluscs. All breakages represent aperture peelings, exclusively affecting the body-chamber of the living animal. The predators which are responsible for these injuries were most probably nautiloids or eurypterids. A calculation of the relative dimension of the shell loss which resulted in the breakage shows that these breaks were small in comparison with the maximum tolerated shell loss in Recent Nautilus or Mesozoic ammonoids.