CHARACTERIZING JURASSIC SPINICAUDATANS OF ANTARCTICA: SYSTEMATIC AND PALEOECOLOGICAL IMPLICATIONS

Spinicaudatans (“Conchostracans”) are small arthropods that are completely surrounded by a bivalved carapace. Conchostracans typically reside in shallow temporary ponds of alkaline fresh to brackish water no larger than an acre. Associated fauna within these ponds includes ostracodes, fish, insect nymphs, and plants, all of which could have been sustained by microbial mats.

Prior investigators have primarily used morphometric data on carapace shape and size to classify conchostracan species. These studies, however, typically neglected ontogenetic changes in patterns of ornamentation preserved within the growth lines of the carapace. Therefore, the previous interpretations of species definitions require reexamination and possible reinterpretation.

This study focuses on Jurassic conchostracans from the Transantarctic Mountains collected from three separate localities: Storm Peak, Blizzard Heights, and Carapace Nunatak. The fossils were preserved in lacustrine sedimentary interbeds within the Jurassic Kirkpatrick Basalt. These siliciclastic mudrocks preserve the microstructure of the conchostracan carapaces in detail. Conchostracan fossils were systematically analyzed and identified to species. Length, height, and curvature measurements of the carapace were recorded, and special attention was paid to the ornamentation (granular and linear) and patterns of ornamentation found within the growth lines on the carapace.

Previous systematic interpretations of the Kirkpatrick Basalt taxa, including Tasch (1987), have significantly oversplit the fauna. Reconsideration of the fossils preserved in the Kirkpatrick Basalt interbeds has resulted in recognition of fewer taxa, which are now well supported by morphological criteria including carapace microstructure. In addition, size differences in the Carapace Nunatak conchostracans occur due to environmental constraints, possibly temperature.