Arrival of Semicassis pyrum (Lamarck) and other tonnacean gastropods in the southern ocean during Pleistocene time

Abstract

Xenophalium Iredale, 1927 and Xenogalea Iredale, 1927, are synonymised with Semicassis Mörch, 1852. Semicassis pyrum (Lamarck) forma hamiltoni Powell in early Nukumaruan (earliest Pleistocene) sandstone in southern Wairarapa district is the earliest record of the genus in New Zealand.

Semicassis pyrum and the closely related S. labiata (Perry) (first record late Castlecliffian) are extremely similar to the European Miocene and Pliocene S. laevigata (Defrance), suggesting that smooth forms of Semicassis evolved in Europe, migrated through the Atlantic to South Africa, and were dispersed around the Southern Ocean in the circumpolar current during earliest Pleistocene time.

Smooth forms of Semicassis occur in the Southern Ocean and in Japan but not in the intervening central Pacific; they and several other tonnaceans (Charonia lampas, Septa exarata, Septa parthenopea, Bursa (Tritonoranella) ranelloides, and possibly Tonna variegata) with similar distributions in space and time are Pleistocene migrants replaced by other species in the Central Pacific. Several other widespread Southern Ocean Tonnacea (Argobuccinum pustulosum, Fusitriton cancellatus, Ranella olearia) have a Northern Hemisphere Tertiary record but did not reach New Zealand until latest Pleistocene time, or later, and are unknown in Japan. Thus lengthening of their larval lives by sea cooling enabled several teleplanic tonnaceans to make major changes in their distributions during Pleistocene glaciations. Species entering the Southern Ocean at Africa arrived in New Zealand several glaciations earlier than did species entering it at South America.