The Gatuncillo Formation (Tgo)^[1] is a geologic formation in central Panama. The formation is exposed in the Panama Canal Zone and surrounding areas. It preserves fossils dating back to the Middle to Late Eocene period.^[2]

The Gatuncillo Formation is the oldest sedimentary unit of the Panama Canal Zone, unconformably resting on Cretaceous basement. The formation was defined as Gatuncillo Shale by Thompson in 1944, who named it after the Gatuncillo River. The estimated thickness ranges from 150 to 800 metres (490 to 2,620 ft).^[3]

Though the Gatuncillo consists chiefly of mudstone and siltstone, it includes bentonitic mudstone, sandstone, and limestone, and at the base a conglomerate of variable thickness.^[4]

• Zamia nelliae^[5]
• Podocnemididae indet.^[6]

• List of fossiliferous stratigraphic units in Panama

• Cadena, E.; Bourque, J.R.; Rincón, A.F.; Bloch, J.I.; Jaramillo, C.A.; MacFadden, B.J. (2012), "New turtles (Chelonia) from the late Eocene through Late Miocene of the Panama Canal Basin", Journal of Paleontology, 86 (3): 539–557, Bibcode:2012JPal...86..539C, doi:10.1666/11-106.1, retrieved 2019-02-09
• Erdei, Boglárka; Calonje, Michael; Hendy, Austin; Espinoza, Nicolas (2018), "A review of the Cenozoic fossil record of the genus Zamia L. (Zamiaceae, Cycadales) with recognition of a new species from the late Eocene of Panama - evolution and biogeographic inferences" (PDF), Bulletin of Geosciences, 93 (2): 185–204, doi:10.3140/bull.geosci.1671, retrieved 2019-02-09
• Stewart, R.H.; Stewart, J.L.; Woodring, W.P. (1980), Geologic Map of the Panama Canal and Vicinity (PDF), USGS, p. 1, retrieved 2019-02-09
• Woodring, W.P (1957), Geology and Paleontology of Canal Zone and Adjoining Parts of Panama - 306A Geology and description of Tertiary mollusks (Gastropods: Trochidae to Turritellidae) (PDF), USGS, pp. 1–186, retrieved 2019-02-09