Scleritomes of Chancelloria eros and sclerits from the middle Cambrian (Series 3, Stage 5) of central Sonora, Mexico
Fragments of scleritomes of Chancelloria eros from the middle Cambrian of Sonora, Mexico are reported for the first time. Chancelloriid scleritomes partially articulated and isolated sclerites belonging to Allonnia tintinopsis and Archiasterella sp. are widely distributed in reddish calcilutite of the El Gavilán Formation, in the Sahuaral section, central Sonora. The faunal association is represented by chancelloriids, spicules of reticulosan sponges, linguliform brachiopods, hyoliths and trilobites. The El Gavilán Formation is an open shelf sequence of 65 m thick, composed of calcilutite interbedded with thin-bedded marly limestone. Trilobite association allowed assigning the open shelf deposit to the middle Cambrian, Altiocculus Subzone, Ehmaniella Zone (Series 3, Stage 5). Chancelloriid genera of the El Sahuaral section may be compared with the biota of San Jose de Gracia, in central Sonora, México, and also with other mid-Cambrian shelf deposits around Laurentia as Burgess Shale in Canada and formations from Utah, USA. Cambrian rocks of Sonora were part of the southern-western border of Laurentia craton, which was located near the paleo-equator during Cambrian times.