Reigitherium bunodontum is a mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

 that lived during the Late Cretaceous

Late Cretaceous

The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...

 (Late Campanian

Campanian

The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...

-Maastrichtian

Maastrichtian

The Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...

). Fossils of it have been found in the Los Alamitos

Los Alamitos Formation

The Los Alamitos Formation is a geological formation in Rio Negro, Argentina whose strata date back to the Late Cretaceous. Dinosaur remains are among the fossils that have been recovered from the formation.-Dinosaurs:-Mammaliaforms:-References:...

 and the La Colonia

La Colonia Formation

The La Colonia Formation is a geological formation in Argentina whose strata date back to the Late Cretaceous. Dinosaur remains are among the fossils that have been recovered from the formation....

 Formations of Argentina

Argentina

Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...

.

Reigitherium was initially classified in the order Dryolestida

Dryolestida

Dryolestida is an extinct order of mammals known from the Jurassic to Tertiary. It has been suggested that these mammals are either the possible ancestors of therian mammals or an offshoot from the same evolutionary line. It is also believed that they developed a fully mammalian jaw and also had...

. In 2000 Pascual et al. found it to be a Docodont. Later authors found it to be neither a Dryolestoid nor a Docodont, and found it to be of uncertain placement within Mammalia. In 2011, Rougier et al. found it to be a Dryolestoid again, within Meridiolestida, a clade composed of Gondwanan Dryolestoids.It "differs from other Mesozoic mammals including docodontans in that the lower molars decrease in length and notably increase in width posteriorly."