Ontogeny, structure and moultin of Parabolina frequens argentina (Kayser) (Trilobita, Olenidae) from the Furongian of northwestern Argentina

摘要

Parabolina frequens argentina (Kayser) (Trilobita, Olenidae) is a guide fossil occurring in different assemblages and lithologies of the upper Cambrian (Furongian) of northwestern Argentina. The black and dark grey shales of the Lampazar Formation at sierra de Cajas and the Santa Rosita Formation (lower part) at Iruya and Alfarcito (Jujuy and Salta provinces) contain numerous well-preserved specimens at different stages of development. The juveniles represent anaprotaspis, metaprotaspis and degrees 0-11 meraspis. The latter exhibit a high variability related with the pattern of glabellar furrows and lobes, the sagittal length of the thoracic segments, the splay and length of the genal and thoracic spines, the relative width of the axis, and the relative development of the axial tubercles. As in other species of Parabolina, the meraspid stages of P. frequens argentina do not form well-defined instar groupings. Besides, in the adult specimens the length of the anterior cephalic border and the preglabellar field are also variable characters. The hypostome is characterized by having almost completely reduced anterior wings; it lies in close association with the ventral doublure (conterminant condition). Although some authors suggested that P. frequens argentina and P.frequensfrequens (Barrande) could be synonyms, these taxa differ in having distinctive pygidia and hypostomes. Many exoskeletal configurations can be interpreted as exuviae. From a functional point of view, it is likely that the genal and macropleural spines on the 8th thoracic segment supported the body of the trilobite when resting on the sea floor. The exoskeleton can be reconstructed in two alternative postures, the "alert" and "relaxed" attitudes.