This paper dates the El Chango Lagerstatte (Chiapas, SE Mexico), known for its exceptionally wellpreserved marine fishes, plant remains, crustaceans, insects and mollusks, based on ammonite biostratigraphy. The age of the El Chango fauna is imprecisely assigned to either the late Albian or Cenomanian. We document here an ammonite assemblage comprising Graysonites and ?Metengonoceras. Graysonites, which gives a lowermost early Cenomanian age, Cintalapa Formation pro parte. We noticed that the terminology of lithostratigraphic units in the studied area is confusing and varies considerably depending on the author, therefore proposing the use of Cintalapa Formation for the El Chango section and the Sierra Madre as a group that includes it.