Early cambrian small shelly fossils from northwest mexico: Biostratigraphic implications for laurentia

Abstract

The early Cambrian record of Small Shelly Fossils (or SSFs) of Laurentia has been relatively poorly studied despite their major importance for understanding the Cambrian explosion. They represent key biostratigraphic tools for the subdivision and correlation of the Terreneuvian and Cambrian Series 2 at regional and global scales. This study is aimed at improving our knowledge of SSF stratigraphic ranges of Laurentia by focusing on the Puerto Blanco Formation of the Cerro Rajón section, near Caborca in northwestern Sonora, Mexico. Four SSF assemblages have been recovered that include sclerites of chancelloriids (Archiasterella hirundo, A. charma, A. cf. A. pentactina, Allonnia erromenosa, A. tetrathallis and Chancelloria spp.), hyolithelminths (Hyolithellus spp.), hyoliths (Cupitheca cf. C. mira, Petasotheca sp., Hyolithid sp. and Parkula bounites), micromolluscs (Mackinnonia corrugata, Xianfengella sp., Pelagiella sp. and Pojetaia sp.), brachiopods (Eoobolus sp. and Rajonia ornata), sclerites of the lobopod Microdictyon multicavus, one unidentified bradoriid and an indeterminate fossil. Distribution of the SSFs that extend below the first trilobite and archaeocyathan demonstrates that most of the Puerto Blanco Formation is Cambrian Age 3 to 4. The SSFs provide additional clues to correlate Ediacaran-Cambrian sedimentary successions in Sonora with the southern Great Basin (USA). The described assemblages are also compared with SSF assemblages of other localities of the American Cordillera and southeastern Laurentia. At the global scale, they support faunal connections with Australia and possibly China.