Taxonomy of Ordovician ostracodes from western Newfoundland, Anticosti Island and the St. Lawrence Lowlands of Québec

Pitman, David John (1991) Taxonomy of Ordovician ostracodes from western Newfoundland, Anticosti Island and the St. Lawrence Lowlands of Québec. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The results of an investigation of Middle and Upper Ordovician ostracodes from southeastern Canada are reported. Sixty-two samples were collected from the Table Head and Long Point groups, western Newfoundland, the Jolliet Group, Anticosti Island, and the Trenton Group, St. Lawrence Lowlands. These yielded over 2600 moderately to well-preserved ostracodes. The Carter Collection, an existing collection of ostracodes from the Lowlands, was also examined. During the research, a series of difficulties were encountered in obtaining the faunas, interpreting their taxonomy, and understanding their paleoenvironmental implications. -- In the sections studied, Ordovician ostracodes occurred sporadically and were often rare or absent. All the collections were obtained by washing soft shales or by vibra-tool preparation of indurate limestones; selection of samples that can be readily processed is essential. Ordovician ostracode taxonomy is often confusing; many species are narrowly defined and allow little morphological variation. In this study, the taxonomy of a number of species is revised and a number of existing species are re-interpreted as synonyms. Fifty-four species representing forty genera are described and illustrated. No new species are established, because the current species are either already described in the literature, or could only be identified at the generic level because of a lack of well-preserved material. -- Although faunal composition, abundance and diversity arc directly influenced by the paleoenvironment, it is difficult to develop ostracode-based criteria for the elucidation of Ordovician paleoenvironments. Observations made during this study indicate that sea-level changes and water depth were strong controls on the nature of Ordovician ostracode faunas. In the Lourdes Formation, western Newfoundland and the Ellis Bay Formation, Anticosti Island, marked increases in faunal diversity and the rates of speciation can be directly correlated with transgressive events that increased the shelf area and the variety of niches available. Water depth, through its control of a variety of interdependent ecological factors, such as water temperature, food supply and substrate, was also an important influence on the type of ostracode community present. A series of depth-related communities are tentatively recognised. High- energy, nearshore environments tend to have a high-abundance, low-diversity fauna consisting of large, robust, smooth ostracodes; speciation rates are low and the community structure is relatively stable. Stable, open shelf, carbonate environments tend to have high-diversity faunas, consisting of a wide variety of ornamented forms such as Eurychilina subradiata and Foramenella phippsi; speciation rates are high and the community structure frequently changes. Outer shelf to shelf-slope environments tend to have a low-diversity, low-abundance fauna consisting of drepanellids such as Warthinia nodosa and smooth-elongate ostracodes like Phelobythocypris cylindrica; speciation rates are low and the community structure is stable. As depth increases and the shelf-slope transition is crossed, faunal abundance and diversity diminish rapidly and in situ ostracode faunas disappear altogether. -- Effective use of Ordovician ostracodes as biostratigraphic indicators, depends on their ability to cross facies and provincial boundaries. At present it is not possible to make more than crude correlations, because the faunas are not well enough known. A few of the species, such as Eurychilina subradiata and Tetradella quadrilirata, are wide ranging and have obvious potential for correlating Ordovician shelf sediments. Many other species cannot be used for inter- or intra-regional correlation, because they are locally endemic, occurring in some areas, but not in others.

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