Year

2011

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences

Abstract

There are three integral components of this thesis. One part, published as an article (Appendix 1), contains the formal species description of two new bivalves of the family Corbulidae. The taxon Lentidium dalyfluvialis is a freshwater species whose known distribution is restricted to the Daly River in the Northern Territory, northern Australia. The other taxon, Lentidium origolacus, was the dominant mollusc in the Gulf of Carpentaria, also in northern Australia, in lower sea-levels than those of the present (~80–12 ka), during which the environment was largely non-marine. This species still occurs in the modern Gulf of Carpentaria, with its known current distribution being the region’s south-eastern estuaries. No non-marine corbulids have previously been described from Australia.

This study also presents a molecular phylogenetic study of the family Corbulidae, using the ribosomal genes 18S rRNA, 28S rRNA, and a dataset merging the two. This analysis distinguishes, for the first time, a non-marine clade of corbulids (of which the two abovementioned species form a part) comprising the genera Lentidium and Potamocorbula, as well as the genus Erodona, the latter currently, and inappropriately, belonging to the family Erodonidae. These findings have implications for the taxonomy of the family; it is suggested herein that the subfamilies Lentidiinae and Potamocorbulinae, in addition to the family Erodonidae, are taxonomically redundant, and that these three genera ought to be placed within the same subfamily (alternatively family, although this only briefly discussed in this study). Additionally, shell morphological comparisons between these non-marine taxa and selected marine species of the same family consolidate the hypothesis that this non-marine clade is a distinct group of Corbulidae sensu lato. These molecular/morphological results are also important in that this non-marine group comprises species that are ecologically important and, in certain environments, extremely abundant. In the palaeo-record this has been shown for instance in the Gulf of Carpentaria with L. origolacus, whereas other species have proven highly invasive in the modern environment. The recognition of such an ecologically important group may thus provide direction in future studies applicable to palaeocological- and palaeoenvironmental reconstruction, invasive species ecology- and management, and further phylogenetic investigation of bivalves.

Another integral component of this study is the palaeoenvironmental reconstruction of the Gulf of Carpentaria throughout the Last Interglacial period (~130 ka), based on the molluscan fauna obtained from six sediment cores, MD-28 to MD-33, from the central gulf, with a focus on core MD-32. A chronological account of molluscs obtained from these cores provides a palaeohistory of the gulf, which is accompanied by cluster analysis of MD-32; this analysis defines seven assemblage zones, three of which are decidedly nonmarine, one that is arguably non-marine, and three zones that are marine in species composition. Ultimately, a trace-element analysis of shells of L. origolacus provides an additional independent interpretation of the non-marine environmental conditions in the gulf. These results, based on Ba/Ca, Sr/Ca, Mg/Ca, Fe/Ca, Mn/Ca, U/Ca and Na/Ca, show that these environments fluctuated considerably with regards to for example continental input, oxygen levels and salinity levels in the non-marine lake systems. These analyses, comprising palaeoecology and geochemistry, are the first to investigate molluscs from the Gulf of Carpentaria in order to elucidate the palaeoenvironments of the region.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.