Early Cambrian sponge spicules from the Cerro Clemente and Cerro Rajón, Sonora, México

Acid maceration and thin section analysis of archaeocyathan limestones of the Cerro Clemente and Cerro Rajón, Sonora, México, have yielded some of the most ancient sponge fossils reported from the North American Cordillera. The sponge fossils are from Unit 3 of the Puerto Blanco Formation. The fossils include one of the earliest known specimens of Kiwetinokia WALCOTT. These new data indicate that diverse archaeocyaths and sponges occur in close association within Lower Cambrian limestones of Sonora. The new specimen of Kiwetinokia sp. provides evidence that geometrically complex spicules in Cambrian sponges evolved by the fusion of simpler spicule types. These are among the earliest sponges known to have lived in reef habitats.