Laguna del Hunco Formation

The Laguna del Hunco Formation or Laguna del Hunco Tuff () is a localized Early Eocene (Itaboraian in the SALMA classification) fossiliferous geological formation of the Cañadón Asfalto Basin in central Patagonia, Argentina. The thick formation comprises tuffaceous mudstones and sandstones deposited in a crater lake environment and crops out at Laguna del Hunco in the northwestern Chubut Province.
The formation has been precisely dated to 52.22 ± 0.22 Ma on the basis of sanidine crystals in the tuffs of the formation. The Laguna del Hunco formation overlies the Barda Colorada Ignimbrite and is covered by the Sarmiento Group. The unit is renowned for the preservation of an extraordinarily rich fossil flora assemblage of mixed South American families and presently uniquely Australasian flora, among which the oldest "Eucalyptus" fossils found worldwide. The formation also has provided many fossil insects, including insect eggs, fossil fish of "Bachmannia chubutensis" and the frog "Shelania pascuali". Periodic bursts of gas in the volcanic crater lake are thought to have produced the sudden death and preservation of the floral and faunal assemblage.
Description.
The Laguna del Hunco Formation, named after Laguna del Hunco ("Lake of Reeds"), a desert pond in Chubut Province, is a localized sedimentary unit comprising tuffaceous sandstones and mudstones with primary and reworked ashfall layers. The formation was deposited in a crater lake environment. The approximately thick formation forms part of the Middle Chubut River Volcanic Pyroclastic Complex of the western Cañadón Asfalto Basin. This complex comprises a variety of volcaniclastic, intrusive, pyroclastic, and extrusive rocks, deposited over several million years. The complex is characterized by a great variety of volcanogenic bodies, such as ignimbrites, domes, lava flows, necks, intrusives, tuffs, and volcaniclastic deposits (of predominantly lacustrine origin), all of them frequently interbedded.
The Laguna del Hunco Formation was formerly included in the Huitrera Formation, overlies the Barda Colorada Ignimbrite, and is overlain by the Sarmiento Group. The formation has been dated using 40Ar/39Ar analysis on sanidine crystals of the ash beds to 52.22 ± 0.22 Ma, placing the deposits in the Early Eocene, or Itaboraian in the SALMA classification.
Paleontological significance.
The paleoflora of the formation is considered one of the most biodiverse Cenozoic fossil deposits worldwide. The biota is composed of extraordinarily rich assemblages of ferns, conifers, and flowering plants, many of which have not yet been formally described. The flora of the formation, studied since the 1920s, was formerly thought to be Miocene in age.
Three described species in the genus "Gymnostoma" of the family Casuarinaceae, and the species "Ceratopetalum edgardoromeroi" of the family Cunoniaceae are the only members of these families found outside of Australasia. The floral assemblage is thought to represent a lakeshore vegetation, deposited during the Early Eocene Climatic Optimum (EECO), with estimated average yearly temperatures of and annual rainfall of . Periodic gas bursts in the crater lake of Laguna del Hunco probably led to the sudden deaths of the flora and fauna found in the formation.
Eocene bird distribution.
the present distribution of Coraciiformes sensu lato in the world seems to be explained by the deterioration of warm climates at middle and high latitudes after the early Eocene climatic optimum, resulting in their extant pantropical distribution (e.g., Ref.8). The rich fossil plant assemblage at the Laguna del Hunco locality represents the environmental conditions in Patagonia during the early Eocene climatic optimum. Recent paleoenvironmental and floristic comparisons indicate that the closest modern analogs for the Laguna del Hunco flora are the Malesian lower-montane tropical, everwet rainforests15, where diverse extant “Coraciiformes” exist today. The Laguna del Hunco paleoenvironment resembles that reported for Holarctic stem-Coracii, such as Primobucconidae7–9 in being a frost-free, warm lakeshore environment, although they differ in their seasonality (seasonally dry vs. everwet). In fact, the age of the Laguna del Hunco biota, ca. 52.2 Ma13–15,19, is very similar to that of the Fossil Butte Member of the Green River Formation (51.66 ± 0.09 Ma), the source of Primobucco mcgrewi7,38.
Although Ueekenkcoracias is not the oldest stem-Coracii, due to the age of Septencoracias at ca. 54 Ma, the new taxon presented here provides important data for understanding the early biogeographic history of Coracii during the early Eocene. According to Claramunt and Cracraft34, modern ornithofaunas are the result of recurrent dispersal events using two main routes: one connecting South America with the Old World via North America and another one connecting South America with Australia and New Zealand through Antarctica. Those authors34 postulated that “Coraciiformes” (i.e., Coracii + Alcediniformes) colonized the Paleotropics from North American ancestors. 
Given the presence of many of these lineages in the early Paleogene of Europe, they inferred that Coraciiformes reached the western Palearctic through a North Atlantic corridor before ~ 52 Ma. Although the ‘North American Gateway’ hypothesis explains well the origins of Musophagiformes30 and Coliiformes29, it does not explain the
current fossil record of Coracii, due to the presence of Ueekenkcoracias as the basalmost stem-Coracii in the early Eocene of South America. The arrival of a stem-Coracii lineage to South America may have occurred from North America if this clade has the biogeographic origin postulated by Ref.34, which would also open a second possible dispersal route from 
North America to Africa (in addition to the European route34). Alternatively, the stem-Coracii may have arrived in South America from Africa if the latter continent is the biogeographic origin.
Fossil content.
Holotype. MPEF-PV 10991, incomplete right hind limb, preserved in two slabs as part and counterpart.
The following fossils have been described from the formation: