Systematic, morphometric and palaeobiogeographic study of Blainia gregaria Walcott, 1916 (Trilobite, Ptychopariida): Middle Cambrian of the Precordillera of western Argentina

Abstract

The ptychoparioid trilobite Blainia gregaria Walcott, 1916 is described for the first time from the late middle Cambrian (Series 3) of the Argentine Precordillera. It occurs commonly in the upper part of La Laja Formation, a carbonate platform succession that crops out in the Precordillera Oriental of San Juan where it reaches a thickness of some 600 m. Sections were sampled on cerro Tres Marías in Sierra de Marquesado and in quebrada de Zonda at the northern termination of Sierra Chica de Zonda. More than 1200 specimens were recovered from 22 stratigraphic levels over a thickness of 155 m, located in the uppermost 15 m of the Soldano Member, the approximately 100 m thick Rivadavia Member, and in the lower 40 m of the Juan Pobre Member. Our biometric study dealt with 205 cranidia and 365 pygidia from cerro Tres Marías. Measured dimensions of a subset of this collection provide bivariate datasets that in some characters show broad scatter plots and relatively low linear correlation coefficients, indicative of rather wide intraspecific morphological variation. Six morphotypes (three cranidial and three pygidial), recognized mainly on the basis of shape, occur together in the same collections. This evidence for an unusually high degree of variability leads to a proposed synonymy of some 30 previously named species in North America belonging to two genera,Blainia Walcott, 1916 and Glyphaspis Poulsen, 1927, the latter considered a junior synonym. The biogeographic distribution of B. gregaria, as redefined here, shows that it was an endemic Laurentian species that inhabited the inner parts of the carbonate platform that rimmed the craton and mantled portions of its interior. Its presence in western Argentina is further evidence for the very close faunal relationship between the Precordilleran terrane, known as Cuyania, and Laurentia during the Cambrian.