Multituberculata

Evolution

Multituberculates first appear in the fossil record during the Jurassic period, and then survived and even dominated for over one hundred million years, longer than any other order of mammaliforms, including placental mammals. The earliest known multituberculates are from the Middle Jurassic (Bathonian ~166-168 million years ago) of England and Russia, including Hahnotherium and Kermackodon from the Forest Marble Formation of England, and Tashtykia and Tagaria from the Itat Formation of Russia. These forms are only known from isolated teeth, which bear close similarity to those of euharamyidans, which they are suspected to be closely related to. During the Late Jurassic and Early Cretaceous, basal multituberculates, collectively grouped into the paraphyletic "Plagiaulacida", were abundant and widespread across Laurasia (including Europe, Asia and North America). During the Aptian stage of the Early Cretaceous, the advanced subgroup Cimolodonta appeared in North America, characterised by a reduced number of lower premolars, with a blade-like lower fourth premolar. By the early Late Cretaceous (Cenomanian) Cimolodonta had replaced all other multituberculate lineages.

During the Late Cretaceous, multituberculates experienced an adaptive radiation, corresponding with a shift towards herbivory. Multituberculates reached their peak diversity during the early Paleocene, shortly after the Cretaceous–Paleogene extinction event, but declined from the mid Paleocene onwards, likely due to competition with placental mammals such as rodents and ungulates. The group finally became extinct in the Late Eocene.

There are some isolated records of multituberculates from the Southern Hemisphere, including the cimolodontan Corriebaatar from the Early Cretaceous of Australia, and fragmentary remains from the Late Cretaceous Maevarano Formation of Madagascar. The family Ferugliotheriidae from the Late Cretaceous of South America, traditionally considered gondwanatherians, may actually be cimolodontan multituberculates.

During the Late Cretaceous and Paleocene the multituberculates radiated into a wide variety of morphotypes, including the squirrel-like arboreal ptilodonts. The peculiar shape of their last lower premolar is their most outstanding feature. These teeth were larger and more elongated than the other cheek-teeth and had an occlusive surface forming a serrated slicing blade. Though it can be assumed that this was used for crushing seeds and nuts, it is believed that most small multituberculates also supplemented their diet with insects, worms, and fruits. Tooth marks attributed to multituberculates are known on Champsosaurus fossils, indicating that at least some of these mammals were scavengers. A ptilodont that thrived in North America was Ptilodus. Thanks to the well-preserved Ptilodus specimens found in the Bighorn Basin, Wyoming, we know that these multituberculates were able to abduct and adduct their big toes, and thus that their foot mobility was similar to that of modern squirrels, which descend trees head first.

Another group of multituberculates, the taeniolabids, were heavier and more massively built, indicating that they lived a fully terrestrial life. The largest specimens weighed probably as much as 22 kg (49 lb), making them comparable in size to large rodents like the modern beaver.

Classification

Multituberculata is generally placed within Allotheria alongside Euharamiyida, a clade of mammals known from the Middle Jurassic to Early Cretaceous of Asia and possibly Europe that possess several morphological similarities with multituberculates.

Gondwanatheria is a monophyletic group of allotherians that was diverse in the Late Cretaceous of South America, India, Madagascar and possibly Africa and occurs onwards into the Paleogene of South America and Antarctica. Their placement within Allotheria is highly controversial, with some phylogenies recovering the group as deeply nested within multituberculates, while others recover them as a distinct branch of allotherians separate from multituberculates.

In their 2001 study, Kielan-Jaworowska and Hurum found that most multituberculates could be referred to two suborders: "Plagiaulacida" and Cimolodonta. The exception is the genus Arginbaatar, which shares characteristics with both groups.

"Plagiaulacida" is paraphyletic, representing the more primitive evolutionary grade. Its members are the more basal Multituberculata. Chronologically, they ranged from perhaps the Middle Jurassic until the mid-Cretaceous. This group is further subdivided into three informal groupings: the allodontid line, the paulchoffatiid line, and the plagiaulacid line.

Cimolodonta is, apparently, a natural (monophyletic) suborder. This includes the more derived Multituberculata, which have been identified from the lower Cretaceous to the Eocene. The superfamilies Djadochtatherioidea, Taeniolabidoidea, Ptilodontoidea are recognized, as is the Paracimexomys group. Additionally, there are the families Cimolomyidae, Boffiidae, Eucosmodontidae, Kogaionidae, Microcosmodontidae and the two genera Uzbekbaatar and Viridomys. More precise placement of these types awaits further discoveries and analysis.

Taxonomy

Phylogeny

After Chimento et al. 2015:

Cladogram after Carvalho et al. 2025:

Palaeobiology

Behaviour

Multituberculates are some of the earliest mammals to display complex social behaviours. One species, Filikomys, from the Late Cretaceous of North America, engaged in multi-generational group nesting and burrowing.

Life history

Osteohistological evidence in the form of differential proportions of bone tissue types indicative of length of lactation periods suggests that multituberculates had life history strategies more closely resembling those of placental mammals than those of marsupials.

Extinction

The extinction of multituberculates has been a topic of controversy for several decades. After at least 88 million years of dominance over most mammalian assemblies, multituberculates reached the peak of their diversity in the early Palaeocene, before gradually declining across the final stages of the epoch and the Eocene, finally disappearing in the early Oligocene.

The last multituberculate species, Ectypodus childei, went extinct near the end of the Eocene in North America. It is unclear why this particular species persisted for so long when all of its counterparts succumbed to replacement by rodents.

Traditionally, the extinction of multituberculates has been linked to the rise of rodents (and, to a lesser degree, earlier placental competitors like hyopsodonts and Plesiadapiformes), which supposedly competitively excluded multituberculates from most mammalian faunas. Adams et al. (2019) argued in favor of this hypothesis as rodents have a higher bite force than multituberculates, which would have given them access to harder, drier seeds which were becoming more abundant and had a greater range of food compared to multituberculates. The authors also argued rodents had was longer gestation periods and larger neonates which gave them a competitive advantage over multituberculates. In addition, they believed the diversity of new predators such as owls, creodonts, and carnivorans, also played a role in their extinction.

However, the idea that multituberculates were competitively replaced by rodents and other placentals has been criticised by several authors. For one thing, it relies on the assumption that these mammals are "inferior" to more derived placentals, and ignores the fact that rodents and multituberculates had co-existed for at least 15 million years. According to some researchers, multituberculate "decline" is shaped by sharp extinction events, most notably after the Tiffanian, where a sudden drop in diversity occurs. Finally, the youngest known multituberculates do not exemplify patterns of competitive exclusion; the Oligocene Ectypodus is a rather generalistic species, rather than a specialist. This suggests that multituberculates simply could not cope with climatic and vegetation changes, as well as the rise of new predatory eutherians, such as miacids. However, rodents probably still played a role in their decline.

More recent studies show a mixed effect. Multituberculate faunas in North America and Europe do indeed decline in correlation to the introduction of rodents in these areas. However, Asian multituberculate faunas co-existed with rodents with minimal extinction events, implying that competition was not the main cause for the extinction of Asiatic multituberculates. As a whole, it seems that Asian multituberculates, unlike North American and European species, never recovered from the KT event, which allowed the evolution and propagation of rodents in the first place. A recent study seems to indeed indicate that eutherians recovered more quickly from the KT event than multituberculates. Conversely, another study has shown that placental radiation did not start significantly until after the decline of multituberculates.

However, competitive replacement among North American species has been called into question by Benjamin John Burger in his 2025 study. He suggests the extinction of multituberculates in North America was correlated to the decline of boreal forests that were dominated by dawn redwoods and Chinese swamp cypress. The analysis suggested that multituberculates avoided pine and spruce-dominated forests despite having similar geographic distributions to those trees. However, Burger argues competitive replacements by seed-eating passerine birds such as songbirds, and several mammalian groups such as paromomyids, may have played a role in the extinction of multituberculates, although this requires more testing.