Multituberculata

Multituberculata, often called multituberculates, were a unique group of extinct mammals known for the special shape of their teeth, which had many small bumps or "tubercles." These creatures lived over 130 million years ago, first showing up in the Middle Jurassic period and becoming most common during the Late Cretaceous and Paleocene. Sadly, they began to disappear in the mid-Paleocene and were completely gone by the late Eocene.

With more than 200 known species, multituberculates were the most diverse group of mammals during the age of dinosaurs. They came in many sizes, from as small as a mouse to as large as a beaver, and lived in many different ways. Some stayed in burrows underground, others climbed trees like squirrels, and a few hopped around like jerboas.

Scientists usually place multituberculates outside the two main groups of living mammals—placentals and marsupials—and instead consider them closer to placentals than to monotremes. They are thought to be closely related to groups like Euharamiyida and Gondwanatheria, all part of a larger group called Allotheria. These ancient mammals help us understand how different life can be on Earth long ago.

Description

The multituberculates looked a bit like rodents, with their teeth having several rows of tiny bumps called tubercles. Unlike rodents, whose teeth never stop growing, multituberculates' teeth were replaced in a pattern more common in other mammals. They had a special large tooth called a plagiaulacoid, which is not found in most other mammals.

Multituberculates chewed food differently from rodents. Their jaws moved front to back instead of side to side, which gave them a unique way of grinding food. Scientists think they might have had long pregnancies, similar to some modern mammals, and they gave birth to very small, underdeveloped babies. Some multituberculates also had teeth that grew long, extending above the gumline. Recent studies show that these animals had fairly complex brains.

Evolution

Multituberculates first appeared in the fossil record during the Jurassic period and survived for over one hundred million years, longer than any other group of early mammals. The earliest known multituberculates lived during the Middle Jurassic, around 166-168 million years ago, in places like England and Russia. During the Late Jurassic and Early Cretaceous, these small mammals were common across Europe, Asia, and North America.

Multituberculates reached their peak diversity during the early Paleocene, shortly after a major event that changed Earth's life. They had many different shapes and sizes, from squirrel-like forms that could climb trees to larger, heavy-built types that lived on the ground. Their special teeth, with a long, blade-like premolar, helped them eat seeds, nuts, insects, and fruits. Some even ate scraps from other animals. One well-known multituberculate was Ptilodus, which lived in North America and had feet similar to modern squirrels.

Classification

Multituberculata is grouped within Allotheria along with Euharamiyida, a group of mammals from the Middle Jurassic to Early Cretaceous of Asia and possibly Europe that share some features with multituberculates.

Gondwanatheria is a group of allotherians that lived during the Late Cretaceous in South America, India, Madagascar, and possibly Africa, continuing into the Paleogene in South America and Antarctica. Scientists debate whether Gondwanatheria belongs within multituberculates or forms its own branch of allotherians.

In a 2001 study, researchers Kielan-Jaworowska and Hurum identified two main suborders of multituberculates: "Plagiaulacida" and Cimolodonta, except for the genus Arginbaatar, which has traits from both groups.

"Plagiaulacida" is considered an early stage of evolution and includes the most basic multituberculates, existing from perhaps the Middle Jurassic to the mid-Cretaceous.

Cimolodonta appears to be a natural group and includes more advanced multituberculates found from the lower Cretaceous to the Eocene. This group contains several superfamilies and families, such as Djadochtatherioidea, Taeniolabidoidea, Ptilodontoidea, Cimolomyidae, and others. Their exact relationships need more research.

Paleoecology

Multituberculates were some of the earliest mammals to show complex social behaviors, like living in groups and building burrows together. One species, Filikomys, lived in North America during the Late Cretaceous and built nests that lasted across many generations.

Multituberculates existed for over 130 million years but eventually went extinct. They were most diverse during the early Palaeocene but slowly declined and disappeared by the early Oligocene. Scientists have many ideas about why they died out. Some think it was because rodents became better at eating tough seeds and had advantages in raising their young. Others suggest that changes in climate, plants, and new predators played a big role. In Asia, multituberculates lived alongside rodents without going extinct, showing that competition might not have been the only reason they disappeared.