ONTOGENY OF A NEW SPECIES OF PARAPARCHITES (OSTRACODA) FROM THE LOWER PERMIAN SPEISER SHALE IN KANSAS

Abstract

Both ontogeny and sexual dimorphism of the Ostracoda are important for understanding their evolution. The purpose of this study is to evaluate the ontogeny and possible sexual dimorphism of the ostracode species Paraparchites punctatus n. sp. by assessing size, shape, and ornamentation at the stages of its growth for which we were able to obtain specimens: adults and five or six instars. Brooks's rule suggests that with each molt crustaceans double their volume (Brooks, 1886; Teissier, 1960; see also Przibram, 1931). The degree of adherence to Brooks's rule during ontogeny bears on the importance of heterochrony in evolution. The new species follows Brooks's rule rather closely. Another aspect of morphology investigated herein is sexual dimorphism: although many ostracodes have sexually dimorphic valves and carapaces, this is not true of all species of Paraparchites Ulrich and Bassler, 1906, and some other major groups (Scott, 1959; Moore, 1961). These ostracodes were chosen for study because they are particularly abundant in the Speiser Shale, numbering in the thousands from a liter of shale or mudstone. Many of them are preserved as valves rather than as complete carapaces, enabling study of the morphology of their interiors. The Speiser Shale has attracted the attention of numerous geologists and paleontologists because of its unique vertebrate fauna, its mixture of marine and continental deposits, and its relationship to cyclothemic stratigraphy (Lane, 1964; Heckel, 1977; Schultze, 1985; Cunningham, 1990; West, 1990; Watabe, 2001; Hembree, 2002). The rock at the study site includes both marine and nonmarine vertebrate and invertebrate fossils preserved in different parts of the 3-m-thick unit (Schultze, 1985). The fauna from various levels of the Speiser Shale, especially in the lower part, which has a rich fauna of vertebrate fossils, includes osteichthyans (Palaeoniscoidea, Platysomoidea, Dipnoi), …