The mammal, probably belonging to the Polydolopimorphia, is the oldest known representative of marsupials or any therian mammal in the Southern Hemisphere.
Etymology
The genus was named Cocatherium ("Coca Beast") after "Coca", the nickname of San Martín of Estancia San Ramón, who assisted the researchers of the Museo de La Plata in working in the region where the fossils were found. The species epithetlefipanum honors Lefipán, one of the most prominent native Mapuche inhabitants of the region where the Lefipán Formation was deposited.[2]
Description
Cocatherium was described on the basis of the holotype fossil LIEB-PV 1001, an isolated right lower molar with a total length 2.90 millimetres (0.114 in), a trigonid width of 2.20 millimetres (0.087 in) and a talonid width of 2.27 millimetres (0.089 in). The molar has a well-developed wear pattern, contains all the features characteristic of a therian mammal and indicates it belonged to a bunodont marsupial of relatively large size. The mammal is the oldest known representative of marsupials or any therian mammal in the Southern Hemisphere.[3] The presence of the mammal predates the TiupampanSouth American land mammal age.[4]
The Lefipán Formation ranges in age from the late Maastrichtian to the early Danian and preserves the Cretaceous-Paleogene boundary, though the exact layer is disturbed by bioturbation.[8] The Danian section of the formation shows a remarkably rapid recovery from the extinction event,[9] with aquatic plant taxa virtually undisturbed and other flora reappearing quickly.[10] A layer in the formation, known as the Turitella bed, is considered closest to the onset of the Paleogene and the fossil of Cocatherium, as well as the bivalveMeretrix chalcedonica, was found approximately 5 metres (16 ft) above the estimated K/Pg boundary.[11]
The formation has provided several macro- and microfossils of flora, which has led researchers to conclude the rapid recovery and low extinction rate of several genera and families of plants.[12]Hemipteran insect predation on leaves is noted at levels slightly above the K/Pg boundary.[13] While Cocatherium is the only mammal found in the formation, shark,[3] and ray teeth of Hypolophodon patagoniensis,[14] occur in the Danian section and are indicative of a shallow marine to littoral (coastal) environment.[15]