Morphological study of the 'conchostracans' (Crustacea - Spinicaudata) from the Jurassic-Cretaceous Cañadón Calcáreo Formation Chubut, Argentina

ZACARÍAS, I.A.; GALLEGO, O.F.; CABALERI, N.; SHEN, Y.B.

Gral. Roca Río Negro

Congreso; 11° Congreso de la Asociación Paleontológica Argentina; 2016

Asociación Paleontológica Argentina

The Cañadón Calcáreo Formation (Cañadón Asfalto Basin) crops out in the eastern margin of the middle valley of Chubut River in the central region of the Extrandean Patagonia. Thissequence of Upper Jurassic-Cretaceous discussed age shares a particular association of´conchostracans´ (spinicaudatans) composed of species of the families Antronestheriidae and Jilinestheriidae. The latter family has Cretaceous affinities between South America and Asia, in particular the presence of genus Nemestheria Zhang and Chen, 1964, recorded for the Cenomanian of China. New studies of specimens of the genus Nemestheria from Cañadón Calcáreo Formation (7394 CTES-PZ) allow us to present in this communication the morphological characteristics of the carapace undertaken by scanning electron microscopy. The specimen has a shell of small size (length 4.8 mm - 2.7 mm height) with an oval outline, straight dorsal margin, and the umbo with subterminal position; with about 22 lines of evenly spaced growth bands in the ventral and narrow closer to the dorsal region. The SEM images showed that the radial lirae ornamentation is long (0.12 mm) interspersed with short lirae (0.05 mm) at the bottom of the bands ornamentation appears as a crosslinked radial lirae. This analysis provides new information that enables complete the diagnostic features of the genus. The presence of this taxon in South America provides additional evidence on the already defined relations between the South American forms and Jurassic-Cretaceous species of China, and expands the evolutionary scenario and distribution of this group during the Mesozoic globally.