Letrero Formation
Stratigraphic range: Late Miocene ( Mayoan- Montehermosan) ~11.6–5.3  Ma PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geological formation
Lithology
Primary	Siltstone
Other	Sandstone
Location
Coordinates	3°18′S 79°06′W / 3.3°S 79.1°W / -3.3; -79.1
Approximate paleocoordinates	3°42′S 77°24′W / 3.7°S 77.4°W / -3.7; -77.4
Region	Azuay Province
Country	Ecuador
Extent	Sierra Region
Letrero Formation (Ecuador)

The Letrero Formation is a Late Miocene ( Mayoan to Montehermosan in the SALMA classification) geologic formation in south-central Ecuador. The formation comprises lacustrine sediments with strong fluvial clastic input and contains siltstones and fine-grained sandstones. ^[1]

The formation has provided fossils of:

• Anadasypus aequatorianus ^[2]
• Caviomorpha indet.
• Litopterna indet.
• Toxodontia indet.

• List of fossiliferous stratigraphic units in Ecuador

• Carlini, Alfredo A.; Castro, Mariela C.; Madden, Richard H.; Scillato Yané, Gustavo J. (2013), "A new species of Dasypodidae (Xenarthra: Cingulata) from the late Miocene of northwestern South America: implications in the Dasypodini phylogeny and diversity", Historical Biology: An International Journal of Paleobiology, 26 (6): 728–736, retrieved 2019-02-09