Early evolutionary history of Monalaria (Mollusca: Gastropoda: Struthiolariidae) from the Palaeogene of New Zealand

Abstract

The endemic and short-lived struthiolariid gastropod, Monalaria, is one of the most widespread marine molluscs of upper lower to middle Eocene shelf deposits in New Zealand. A new shallow shelf species, Monalaria poliveta n. sp., described herein from the Five Forks Glauconitic Sands Member of the Kauru Formation dated as Mangaorapan to Heretaungan in age (late early to early middle Eocene, c. 55-50 Ma), is the oldest recorded member of the genus.

Monalaria is represented by only three Eocene species, M. poliveta n. sp., M. gracilis Finlay & Marwick, 1937 (Porangan? Stage, middle middle Eocene), and M. concinna (Suter, 1917) (Bortonian Stage, late middle Eocene), after which time it became extinct along with several other molluscan groups. These taxa show a strong affinity with the latest Cretaceous struthiolariid, Conchothyra marshalli (Trechmann, 1917), from Haumurian Campanian?-Maastrichtian) deposits of the South Island. Conchothyra marshalli is the most likely candidate to be the progenitor of the Monalaria stock, despite a gap of approximately 10 Ma in the fossil record of these two biostratigraphically significant groups.