An Ordovician (Whiterockian) marine community from Sonora, Mexico: Paleoecology and paleobiogeographic affinity with North America and Argentina

Abstract

A Lower and Middle Ordovician sequence, containing an abundant biota of cyanobacteria, bryozoans, brachiopods, gastropods, cephalopods, trilobites, and echinoderms, is exposed at Rancho Las Norias in the central part of the state of Sonora, Mexico. This sequence that consists of wackestone, packstone, and grainstone is deposited in a shallow water marine environment within the photic zone, as indicated by the presence of cyanobacteria such as Nuia sibirica. A paleoecological study was performed on one fossiliferous level S1 at the middle part of the sequence in this study, where 113 individuals of 11 taxa were analyzed, which allows to reconstruct the paleocommunity structure of this calcareous layer. The paleocommunity consists mainly of brachiopods, gastropods, and trilobites. The applied paleoecological indices (Dominance and Evenness of Simpson, Diversity of Shannon-Weaver, Diversity of Menhinick, and Total Richness of Chao) indicate a benthic marine community constituted mainly by sessile-suspension feeders, and brachiopods (Orthidiella cf. O. longwelli) are the dominant forms. A Jaccard Similarity Index was also applied, where a similarity of ∼100% was obtained between Canada and Mexico, ∼80% between the USA and Mexico, and ∼50% between Argentina and Mexico, thus confirming the Ordovician paleogeographic affinity of these localities with Laurentia. The Ordovician of northwestern Mexico corresponds to the southernmost deposits of the North American craton and is an essential link in understanding the relationship between North and South America.