Pseudomulceodens: A Mississippian Rostroconch from Mexico

This finding of Pseudomulceodens in Mississippian-age rocks of the Santiago Formation provides the first evidence of the molluscan class Rostroconchia in Mexico. Elsewhere in North American Mississippian rocks the class occurs in Arkansas (Hoare et al., 1982, 1988); Illinois (Weller, 1916); Indiana (Beede, 1906); Iowa (White and Whitfield, 1862); Michigan (Winchell, 1870); Montana and Nevada (Pojeta and Runnegar, 1976); Ohio (Hyde, 1953; Hoare, 1990); and Oklahoma (Branson, 1958). Pseudomulceodens cancellatus (Hyde, 1953) confirms the Osagean age (Early Mississippian) of the Santiago Formation, and reinforces the conclusion that there is paleobiogeographic similarity between faunas of the Nochixtlan region of Oaxaca, Mexico and the midcontinent of the USA during Early Carboniferous time. It seems likely that there was a close connection between southwestern Mexico and the Mid-Continent Paleoprovince, located in the east and central regions of the United States. Prior to this study the rostroconch genera Aphelakardia and Pseudomulceodens were placed in the family Hippocardiidae; they are here transferred to the family Conocardiidae.