A new conchostracan genus from Triassic Potrerillos Formation, Argentina

SHEN, YAN-BIN, GALLEGO, O.F. & ZAVATTIERI, A.M.

ACTA GEOLOGICA LEOPOLDENSIA

Universidade do Rio do Sinos - UNISINOS

Lugar: Sao Paulo; Año: 2001 vol. 24 p. 227 - 236

0102-1249

Several conchostracan samples were collected from the uppermost Middle Triassic to lowermost Upper Triassic Potrerillos Formation on the eastern side of the ''El Mastil'' hill, at the north of the El Challao locality, Mendoza province, (Argentina). The new genus and species, Challaolimnadiopsis mendozaensis, are erected in the present paper and ascribed to Family Pemphilimnadiopseidae Tasch. It is characterized by the growth lines recurved on the postero-dorsal margin of the val ve; two nades are separated by one fine ridge on the umbonal regían, and by the ornamentation with median polygonal reticulation on the growth bands in anterior and middle parts of the valve which gradually transite into radial lines in the ventral and posterior parts. Based on the recurvate growth lines of the val ve it appears to be attributed to Family Paleolimnadiopseidae. However, this new taxa seems to be similar to ''vertexideans'', especially in the presence of nades and ridge on the umbonal region. This kind of combination of umbonal spinosity and recurvate growth lines has also been recorded in the genus Qingxiestheria from the Upper Devonian Wutong Formation in China, and in Fa/sisca from Upper Permian-Lower Triassic of Tonguska, Russia. So they should belonged to the Family Pemphilimnadiopseidae. According to soft parts living genera Limnadia, Eulimadia and Limnadiopsis are referred to the Family Limnadidae. The first two forms have a large urnbo but few growth lines that are not recurved near the dorsal margin. Generically, the growth lines of ''Limnadiopsis'', including the fossil forros, are not recurved at the dorsal margin in the early and rniddle developmental stages, but appear sinuate posterior curvatures in the late stage. Paleolimnadiopsidean conchostracans were probably originated from paleolimnadideans during the early Devonian. So the finding of the new taxa is important for understanding of evolution relationship among paleolimnadiopsideans, paleolimnadideans and vertexideans. Pemphilimnadiopseidae might be a transitional group between Paleolimnadiopseidae and Vertexiidae.