Marine, benthic ostracod ecology, taphonomy and evolution-studies from the south west Pacific 南西太平洋の海生底生貝形虫の生態・タフォノミー進化

博士論文

授与名簿のタイトル: Ecology, taphonomy and evolution of marine benthic ostracods in the South West Pacific (南西太平洋の海生底生貝形虫の生態・タフォノミーと進化)

Results of over twenty five years of ostracod (Crustacea)research in the south west Pacific, especially the New Zealand region are documented. These studies have focussed on three themes which may be broadly categorised as: a) systematics, b) ecology/taphonomy and c) evolution. With respect to the systematics of Ostracoda, forty nine new species, one new subgenus and five new genera have been described. From Lower Miocene shelf sediments of the Middle Waipara District of north Canterbury I have described as new (Swanson 1969), the foilowing taxa: Cytherella elongata, Cyhterell hemipuncta, Cytherelloidea willetti, Bairdia canterburyensis, Callistocythere hanaii, Munseyella rectangulata, Hemicythere hornibrooki, Waiparacythereis joanae, Waipracythereis caudata, Waparacythereis decora, Urocythereis? opima, Quadracythere claremontensis, Cytheridea symmetrica, Neocytherideis mediata and Cytheropteron mosaica. The new genus Waiparacythereis described at that time was subsequently found by me living in littoral sediments at many localities throughout the New Zealand region. My description of the soft anatomy of that animal published in 1982, confirmed the assignment of those taxa to the subfamily Hemicytherinae. Involvement with a University of Otago led to an investigation of the benthos of the Otago Shelf and resulted in more precise determinations of distribution of the ostracod biocoenose and the description of nineteen new species, one subgenus and one new genus. The species described by me as new in New Zealand oceanographic Memoir 78 were: Anchistrocheles araforma, 'Bythocythere' bulba, Pseudocythere(Plenocythere) fragilis, Munseyella brevis, 'Munseyella' arca, Munseyella modesta, 'Munseyella' tumida, Kotoracythere formosa, Callistocythere neoplana, Callistocythere obtusa, Hemicythere munida, Bradeya opima, Hermanites andrewsi, Hermanites briggsi, Jacobella papanuiensis, 'Cythereis' incerta, Ponticocythereis decora, Semicytherura arteria and Semixestoleberis taiaroaensis. The hemicytherinid genus Jacobella and bythocytherinid subgenus Pseudocythere (Plenocythere) were new to science. Benthic samples from the most southern of New Zealand's three main islands were also examined for ostracods and from that assemblage, five new species and one new genus were described(Swanson,1980): Tanella dedeckkeri, Kangarina unispinosa, 'Loxoconcha' tubmani, Microxestoleberis triangulata and the new monospecfic, bythocytherinid genus Microceratina with M. quadrata as the type species. Involvement in a project examining late Pleistocene carbonate lake deposits from alpine Seuth Island resulted in the discovery of a new freshwater ostracod species of the genus Paralimnocythere. Subsequently, living representatives of the new species Paralimnocythere vulgaris were recovered by me from a small pond near Lake Tekapo, South Island. This material enabled a detailed description of both the carapace and soft anatomy of that species to be made (McKenzie and Swanson, 1981). The ostracod assemblage of the Australasian region remains poorly known with many new and interesting taxa yet to be described. Co-operative projects examining ostracods from Cenozoic sediments of South Island and recent material from offshore sites in the Tasman Sea have, to date, resulted in the description of two new ostracod genera and six new species (Ayress & Swanson, 1991). One new genus Bisulcocythere was based on descriptions of five species B. novaezealandiae, B. eocenica, B. micropuncta, B. compressa and B. campbelli, the other Clinocythereis is at present represented only by the type species C. australis. In 1989 after a detailed examination of carapace morphology, soft anatomy and ontogeny, I became the first researcher to publish details of a new living species of punciid ostracod Manawa staceyi. With respect to the distribution and ecology of the extant, marine podocopid ostracod fauna, signficant tracts of the shelf and slope off Otago and Westland have been examined and the hydrologic/sedimentologic character of some niches defined (Swanson, 1979 and 1993 unpub. thesis). Such data is of considerable value for continuing studies of the deep-sea record of latest Quaternary climatic events, and of ostracod shell-chemistry as a paleothermometric tool. Syndiagentic alterations of both shallow and deep water, benthic, podocopid ostracods have been identified establishing a) a more vigorous routine for sample selection for shell chemistry and b) a new hypothesis relating to the evolution of carapace architecture (Swanson & van der Lingen, 1994; Swanson, in press). It is probable that the need to evolve a dissolutionresistant exoskeleton in an environment in which alkalinity fluctuated significantly has been a key determinant with respect to ostracod evolution and, the invasion of the psychrosphere by that group. A statistical measure of ostracod carapace corrosion (Cl) is not only useful for paleothermometric studies, but also provides useful proxy data for fluctuating levels of paleoproductivity in surface waters (Swanson & van der Lingen, 1994; in press). The discovery of living representatives of the enigmatic punciid ostracod Manawa in very shallow waters of northernmost New Zealand is of considerable scientific interest (Swanson, 1985a and 1987). Results of this research have established (a) that the gait of Manawa staceyi is found in no other extant ostracod and (b), that condition (valves fully extended horizontally) is plesiomorphic for ostracods - an observatibn confirmed by the fact that open-gapedness is recorded commonly in Cambrian palaeocopid assemblages (Swanson, 1989a). I have also concluded the existence of a domeshaped valve on some larval stages and the subsequent development of bivalvedness provides a window to the evolution of both the hinge and sensory systems in 'modern' podocpid ostracods (Swanson, 1989a and 1990a). With respect to the soft anatomy, the trunk of Manawa has ten segments (herein considered plesiomorphic for ostracods), but reported homologies with Cytherella are now considered superficial (Swanson, 1990b). Determination of the exact position of attachment (vis. thoracid segmentation) of the copulatory apparatus of the playtcopids and punciids is of considerable importance with respect to the phylogeny of Crustacea in general and Ostracoda in particular. However, my studies have shown that such a result is most likely to be achieved by a histological examination of specimens rather than observation of external morphology. Unexpectedly, the latter approach did establish a new technique for differentiation of various species of Cytherella using differential interference contrast and SEM examination of the adult hemipene (Swanson, 1991). The fact that Manawa carries three cephalic limb pairs and four on the thorax is at odds with prevailing views - the 'normal' configuration being four cephalic, two to the thorax and one to either the thorax or the cephalon. However, this apparent contradiction is explained by observations made on one specimen of Cambrian phosphatocopine 'ostracod' in which fou 'thoracic'indentations were observed on the interior surface of the carapace (Jones & McKenzie, 1980). Thus, in the living punciid ostracod we are able to observe the last remaining renmant of a group of ostracods whose decline was initiated over 500 million years ago.