Megaichnus igen. nov.: Giant Paleoburrows Attributed to Extinct Cenozoic Mammals from South America

Abstract

In the last ten years, more than 1,500 large burrows have been discovered in southern and southeastern Brazil, dug in rocks that include weathered granitic and basaltic rocks, sandstones, and other consolidated sediments. Their presence in geological units of Plio-Pleistocene age suggests that large extinct mammals produced these structures. The internal walls exhibit scratches and grooves left by the animals that inhabited these structures. The burrows are straight or slightly sinuous tunnels that measure up to tens of meters in length. One smaller type measures up to 1.5 meter in diameter, and the larger type can reach 2 meters in height and 4 meters in width, suggesting that such structures have been produced by at least two kinds of organisms. This contribution proposes a classification for these ichnofossils under the generic designation Megaichnus igen. nov., consisting of two ichnospecies identified so far: M. major and M. minor ispp. nov. Although the exact identity of the producers of the burrows is yet unknown, the dimensions and morphology point to ground sloths and giant armadillos.

Fotogrametria de um icnofóssil escavado por preguiças-gigantes (Megaichnus major)

“…Dentre os túneis abertos, a maior altura encontrada foi 2,15 m (túnel D-E) e a menor foi 0,40 m (túnel F) (Figura 2). As medidas maiores estão de acordo com os registros atribuídos à icnoespécie Megaichnus major (Carmo et al, 2011;Frank et al, 2011aFrank et al, , 2015Ruchkys et al, 2014;Buchmann et al, 2016;Lopes et al, 2017). M. major possui seis ramificações (túneis A, B, C, D, E e F) que partem do salão principal.…”

“…Os túneis E e F apresentam a direção concordante ao rumo das fraturas do arenito (Figuras 2 e 3). Buchmann et al (2009Buchmann et al ( , 2016 e Lopes et al (2017) sugerem que o animal, durante a escavação, aproveitava-se de falhas estruturais na rocha para produzir os túneis.…”

“…As paleotocas do icnogênero Megaichnus são estruturas de bioerosão na forma de túneis e salões escavados durante o Plioceno e Pleistoceno por mamíferos placentários gigantes extintos (Buchmann et al, 2009(Buchmann et al, , 2016Lopes et al, 2017). Os organismos escavadores são pertencentes à ordem Cingulata (gliptodontes, tatus gigantes e tatus atuais) e subordem Folivora (preguiças terrestres e arborícolas), superordem Xenarthra, um dos clados mais representativos de mamíferos placentários da América do Sul durante o Neógeno e o Quaternário (Scillato-Yané et al, 2005;Lopes et al, 2017).…”

“…As dimensões dos túneis e salões, assim como os traços de escavação e locais de repouso encontrados nos Megaichnus, permitem classificar como Megaichnus major, associada a escavação por preguiças-gigantes (família Mylodontidae), e Megaichnus minor associada a escavação por tatus-gigantes (família Dasypodidae) (Frank et al, 2015;Buchmann et al, 2016;Lopes et al, 2017).…”

The Santa Vitória Alloformation: an update on a Pleistocene fossil-rich unit in Southern Brazil

“…At first, the fossil-bearing sediments were interpreted as belonging to the Graxaim Formation, and regarded as of Pliocene-middle Pleistocene age (Delaney 1965), later regarded as of Pliocene age based on its position overlying Miocene marine deposits (Closs 1970). That formation, however, is not only lithologically distinct from the fossil-bearing units, but also unfossiliferous so far, except for giant vertebrate burrows (Buchmann et al 2009b, Lopes et al 2017. The absence of fossils in the Graxaim Formation led Paula Couto and Souza Cunha (1965) to recognize the fossil-bearing deposits as a distinct unit of Upper Pleistocene age, based on its mammal in fossil assemblage and biostratigraphic correlation with deposits in Argentina.…”