A late Atokan regional encrinite (early late Moscovian, Middle Pennsylvanian) in the Sierra Agua Verde, Sonora state, NW Mexico

In the Sierra Agua Verde, central Sonora state, NW Mexico, the La Joya Formation exhibits an alternation (100 m thick) of calcareous siltstone and fossiliferous limestone with nodular cherts. This latter contains an abundant and diverse late Atokan (i.e. Podolskian =early late Moscovian, Middle Pennsylvanian) fossil assemblage composed of phylloid alizae, fusulinids, chaetetids, tabulate corals, gastropods, fenestellid bryozoans, spiriferid arid productid brachiopods, crinoids and conodonts. The crinoidal beds constitute a good example of a regional encrinite. They include several species of the parataxonomic stem form-genera cyclocaudex, Cyclocrista, Hcterosteleschus, Mooreanteris, Pentagonopterix, Preptoprennnan, Cycloscapus and Pentaridica. Their preservation indicates the combination of preburial decay on the sea floor and post-burial decay within the sediment. The high degree of silicification of the crinoids indicates that they were possibly associated with siliceous organisms (Porifera?), not preserved in the assemblages. The studied thanatocoenosis is typical of tropical shallow seas, and reveals strong biogeographical affinities with the assemblages of the midcontinental and southern regions of the USA. Particularly, the Atokan crinoids of central Sonora are similar to those from Kansas and Texas, confirming the close palaeogeographic connection of southern USA and northern Mexico during the Middle Pennsylvanian.