Allotheria was a branch of successful Mesozoic

Mesozoic

The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

 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...

s. The most important characteristic was the presence of lower molariform

Molar (tooth)

Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....

 teeth equipped with two longitudinal rows of cusps. Allotheria includes Multituberculata

Multituberculata

The Multituberculata were a group of rodent-like mammals that existed for approximately one hundred and twenty million years—the longest fossil history of any mammal lineage—but were eventually outcompeted by rodents, becoming extinct during the early Oligocene. At least 200 species are...

, probably Haramiyida

Haramiyida

Haramiyidans seem to be the earliest known herbivores amongst basal mammals, assuming they are mammals. Their teeth, which are by far the most common remains, resemble those of the multituberculates...

, and possibly the enigmatic Gondwanatheria

Gondwanatheria

Gondwanatheria is an extinct group of mammals that lived during the Upper Cretaceous through the Eocene in the Southern Hemisphere, including Antarctica...

.

Allotheres also had a narrow pelvis, indicating that they gave birth to tiny helpless young like marsupials do.

Interpretations

When he first identified Allotheria in 1880, Othniel Marsh regarded this group as an order within Marsupialia. But in 1997, McKenna and Bell classified Allotheria as an infraclass.

Further reading

Zofia Kielan-Jaworowska, Richard L. Cifelli, and Zhe-Xi Luo, Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure (New York: Columbia University Press, 2004), 249.