Late Miocene gastropods from northern Borneo (Brunei Darussalam, Seria Formation)

摘要

We present the first systematic description of a Tortonian (late Miocene) gastropod assemblage from the Ambug Hill section in the Tutong District in Brunei Darussalam. The low-diversity assemblage comprises 62 species of which 37 are unknown from other Neogene faunas of the Indo-West Pacific Region (IWP), 23 species are formally described as new. Carnivorous, scavenging and detritus feeding species predominate and the ecological requirements of extant relatives suggest a mud-bottom environment of the inner shelf in several tens of meters water depth with reduced vegetation. The small size of most specimens, the frequent occurrence of subadult shells and small size of several species relative to congeneric species might point to suboptimal environmental conditions during the deposition of this part of the Seria Formation. The very low relations with Neogene gastropod faunas from Indonesia at species level might be explained by the biogeographic isolation between the faunas of the Java and Celebes seas and that from the South China Sea. A severe undersampling and rather spotty taxonomic descriptions of Neogene IWP-faunas and a lack of assemblages from identical depositional environments from Indonesia may also contribute to the seemingly endemic character of the Ambug Hill fauna.