Intraspecific variation in ostracods: Sohniella spinosa, an example from the early Paleozoic.

SALAS, MARIA JOSE

Brasilia

Simposio; ISO- International Symposium on Ostracoda; 2009

The intraspecific variation in ornate ostracods has been largely discussed among marine and non-marine forms. However, these analyses are mainly focused in Recent or Cenozoic species. An example of polymorphism in Palaeozoic faunas (Ordovician, Sandbian) of the Argentine Precordillera is described. Sohniella spinosa (Metacopa) is one of the most frequent ostracod of the Las Aguaditas Formation in terms of numerical abundance. It occurs along the middle and upper levels of the formation, associated with fine-bedded, hemipelagic mudstones interpreted as an upper to middle slope environment. Its carapace is elongate with a straight hinge line. A rounded node occupies the obtuse anterior cardinal angle. The preadductorial node is oval and situated in front of the mid-length and slightly above the mid-height of the valves. A curved posterodorsal horn more or less slender is directed posteriorly. An additional posteroventral horn is present. Two morphotypes of Sohniella spinosa are recognized: 1) smooth forms and 2) tuberculate forms, which in addition of the sharp differences in their external ornament, also differ in the size range (smooth forms are smaller), and in the position of the posterodorsal horn (closer to the posterior margin in smooth forms). Both morphotypes appear together in all the levels, although the smooth forms are always the most abundant (more than 65 %). The development of tubercles could be interpreted as a possible physiological response of animals to the environment; however, a complex array of factors might be responsible for these variations. The later are difficult to resolve even in recent species. The recognition of such intraspecific variability in Palaeozoic ostracodes has significant taxonomic implications at specific and generic levels and largely prevents overestimation of diversity.