ONTOGENY, STRUCTURE AND MOULTING OF PARABOLINA FREQUENS ARGENTINA (KAYSER) (TRILOBITA, OLENIDAE) FROM THE FURONGIAN OF NORTHWESTERN ARGENTINA

M. Franco Tortello, Euan N. K. Clarkson

Abstract


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. frequens frequens (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

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