A new species of Late Cretaceous South American mammal has been named and described. The omnivorous Orretherium tzen is only the second mammal from the Mesozoic known from Chile. The newly described O. tzen joins Magallanodon baikashkenke which was named in 2020. Orretherium has been described from a partial lower jawbone, which had 5 teeth in situ and a single isolated tooth found just a few metres away from the jaw fragment. It is thought to have been about the size of a modern skunk, although it was only distantly related to modern mammals.

Orretherium Life Reconstruction
Orretherium lived some 74-72 million years ago in South America. It shared its habitat with numerous dinosaurs including titanosaurs (seen in the background).  Picture credit: Universidad de Chile.

The Mammal Quarry

The fossils were found in exposures of the Dorotea Formation (late Campanian to early Maastrichtian faunal stages of the Late Cretaceous), on a small hill nicknamed “the mammal quarry”, reflecting the significance of the site in terms of Late Cretaceous mammalian fossil finds. Although the isolated tooth that helped describe this species was found close to the jaw fragment, the researchers cannot unambiguously refer this tooth to the same individual animal although it is highly probable taking in account their compatible size, wear and close proximity.

Orretherium fossil study.
Map (inset) showing the fossil find location, a reconstruction of the head of Orretherium along with a photograph of the jawbone and a computer enhanced image of the fossil.  Picture credit: Universidad de Chile.

Classified as a member of the Meridiolestida

Orretherium has been classified as a member of the Meridiolestida, an extinct group of mammals known from South America and Antarctica.

Co-author of the research paper published in the journal Scientific Reports, Sergio Soto-Acuña (University of Chile), commented:

“This mammal is a primitive lineage of the group of meridiolestids, very successful at the end of the Age of dinosaurs in South America. The jaw found has five teeth in place that indicate omnivorous habits, it probably fed on plants and insects”.

Looking for Late Cretaceous Mammals
Field work being carried out at “the mammal quarry” as scientists from the University of Chile in collaboration with researchers from Universidade Federal do Rio Grande do Sul (Brazil), Museo de La Plata (Argentina) and other South American academic institutions collaborated to produce the scientific paper describing Orretherium.  Picture credit: Universidad de Chile.

Everything Dinosaur acknowledges the assistance of a media release from the Universidad de Chile in the compilation of this article.

The scientific paper: “New cladotherian mammal from southern Chile and the evolution of mesungulatid meridiolestidans at the dusk of the Mesozoic era” by Agustín G. Martinelli, Sergio Soto-Acuña, Francisco J. Goin, Jonatan Kaluza, J. Enrique Bostelmann, Pedro H. M. Fonseca, Marcelo A. Reguero, Marcelo Leppe and Alexander O. Vargas published in Scientific Reports.

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