The Lower Pliocene marine gastropods of Santa Maria Island, Azores: Taxonomy and palaeobiogeographic implications

Abstract

In this work, all the Lower Pliocene gastropod assemblages of Santa Maria Island are revised. These all form part of the Touril Complex. Seventy-seven species are identified representing 61 genera. Two species are described as new: Bittium miradouroense nov. sp. and Erato mayeri nov. sp. The name Tritonium secans Bronn in Reiss 1862 is considered a junior subjective synonym of Monoplex comptus (A. Adams, 1855). Pleurotoma perturrita Bronn in Reiss 1862 is considered a junior subjective synonym of Crassopleura maravignae (Bivona, 1838). However, due to the difficulties in collecting from these deposits, this is likely to be a considerable underestimate of the original diversity of local Early Pliocene gastropod faunas. The assemblage reflects a fully tropical with mean annual sea surface temperatures (SSTs) estimated about 3.7°C to 6.3°C higher than the present-day 20.6°C, and with mean monthly SSTs ranging from 20°C to 28°C, with six months with mean SSTs over 24°C. The assemblages all represent relatively shallow water, fully saline marine environments. At genus level the assemblage is typical of that seen in the tropical Mediterranean-West African Palaeobiogeographical Province during the early Pliocene (MPPMU1; roughly equivalent to the Zanclean and earliest Piacenzian). At species level, 35% of the species taxa found in Santa Maria are also found in the Mediterranean during MPPMU1. There is a much weaker association with the northern warm temperate Pliocene Boreal-Celtic Province, with only 17% of species occurring in both. Moreover, most of those are ubiquitous European species with both wide geographic and stratigraphic distributions. The assemblage is relatively endemic (29%) suggesting that the Azorean subprovince/ecoregion, which is recognised today, was already in place in the Lower Pliocene. A small number of tonnoidean species found in Santa Maria are species known to have had an amphiatlantic distribution during the Pliocene, and for Distorsio mcgintyi Emerson & Puffer, 1953, a well-known and widely distributed tropical American Atlantic species from the Early Miocene to present-day, its presence in Santa Maria marks the most easterly range expansion for the species, as today is not found in the Azores.