Systematic, morphometric and palaeobiogeographic study of Blainia gregaria Walcott, 1916 (Trilobita, 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.

Early Middle Cambrian Trilobites from La Laja Formation, Cerro El Molle, Precordillera of Western Argentina

“…Cambrian trilobites occur sporadically in the autochthonous platform succession (Bordonaro et al, 2008(Bordonaro et al, , 2013, but more commonly in large limestone olistostroliths in Ordovician shales (e.g., Bordonaro and Fojo, 2011;Tortello, 2011) which represent blocks of fossiliferous upper slope facies (Bordonaro, 2003). While the Laurentian aspect of the trilobite faunas is evident, relatively few species have been documented in sufficient detail to make firm biostratigraphic and biogeographic comparisons.…”

“…The Laurentian affinity of the Precordillera persisted through the Cambrian (Benedetto et al 1999;Bordonaro et al, 2008Bordonaro et al, , 2013 into the Middle Ordovician when Gondwanan taxa began to make their presence felt (Benedetto et al, 1999).…”

Early Middle Cambrian Trilobites from La Laja Formation, Cerro El Molle, Precordillera of Western Argentina

“…Cambrian trilobites occur sporadically in the autochthonous platform succession (Bordonaro et al, 2008(Bordonaro et al, , 2013, but more commonly in large limestone olistostroliths in Ordovician shales (e.g., Bordonaro and Fojo, 2011;Tortello, 2011) which represent blocks of fossiliferous upper slope facies (Bordonaro, 2003). While the Laurentian aspect of the trilobite faunas is evident, relatively few species have been documented in sufficient detail to make firm biostratigraphic and biogeographic comparisons.…”

Preface: Lower Palaeozoic fossils, biostratigraphy and events from western Gondwana

“…Bordonaro et al . () present a systematic and morphometric study of the Middle Cambrian trilobite Blainia gregaria described for the first time from the Argentine Precordillera, offering information about the palaeobiogeographic distribution of this species. This study indicates a wide intraspecific variation of the taxon and concludes that B. gregaria was an endemic Laurentian species that inhabited the inner part of the carbonate platform.…”

“…The authors summarize the geological and stratigraphical information about the rocks bearing archaeocyathan occurrences in South America, and discuss their palaeogeographic context in the Late Palaeozoic glaciations of Gondwana. Bordonaro et al (2013) present a systematic and morphometric study of the Middle Cambrian trilobite Blainia gregaria described for the first time from the Argentine Precordillera, offering information about the palaeobiogeographic distribution of this species. This study indicates a wide intraspecific variation of the taxon and concludes that B. gregaria was an endemic Laurentian species that inhabited the inner part of the carbonate platform.…”