Multituberculates represent an ancient and successful lineage of mammals that thrived for an immense span of geological time. These creatures emerged during the Mesozoic Era, coexisting with the dinosaurs for millions of years. Their long history makes them one of the most enduring mammalian groups known from the fossil record. They offer a glimpse into early mammalian evolution and adaptation, showcasing a unique evolutionary path.
What Were Multituberculates?
Multituberculates were a diverse group of extinct mammals, superficially rodent-like in appearance. Their name, “multi-tubercles,” refers to the multiple cusps on their molar-like cheek teeth. These cusps were arranged in several longitudinal rows, allowing for efficient shearing and grinding of plant material, an adaptation resembling those found in modern rodents.
Beyond their unique dentition, multituberculates exhibited a range of body forms, with sizes varying from that of a small mouse to a beaver, with the largest species, Taeniolabis, reaching approximately 22 kg. Many species possessed features like prehensile tails, suggesting an arboreal, tree-climbing lifestyle. Their specialized ankle joints also indicated climbing abilities, suggesting varied locomotion. Some multituberculates, like Ptilodus, were adapted for climbing trees, while others, such as Lambdopsalis, showed adaptations for burrowing, similar to modern prairie dogs.
A Timeline of Success
Multituberculates first appeared in the fossil record during the Middle Jurassic period, 166-168 million years ago, making them one of the earliest known mammalian lineages. Early examples have been found in places like England and Russia. Their diversification expanded significantly during the Cretaceous period, when they radiated into a wide array of forms across various ecosystems.
This group survived the Cretaceous-Paleogene (K-Pg) extinction event about 66 million years ago, which caused the demise of non-avian dinosaurs. Following this mass extinction, multituberculates flourished, reaching their peak diversity in the early Paleocene epoch. They were among the most common mammals during this time, indicating a strong recovery and adaptive success. Despite their early Paleocene success, their numbers began to decline from the mid-Paleocene onwards, eventually leading to their disappearance from the fossil record in the late Eocene, around 50 million years ago.
Lifestyle and Diet
Multituberculates occupied a wide array of ecological roles. Fossil evidence indicates that different species adopted varied lifestyles, including terrestrial (ground-dwelling), fossorial (burrowing), and arboreal (tree-climbing) habits. This adaptability allowed them to fill diverse niches within ancient ecosystems.
Their dental anatomy suggests that most multituberculates were primarily herbivores or omnivores. Their molars were well-suited for processing plant material such as seeds, nuts, and fruits. Some smaller multituberculates also supplemented their diet with insects and worms. While many lineages became herbivorous, some, particularly those with larger blade-like premolars, were more carnivorous or omnivorous, even scavenging on carrion.
The End of a Dynasty
Multituberculates concluded their existence in the late Eocene. The leading hypothesis for their extinction centers on competitive displacement by the newly diversifying placental mammals, particularly true rodents. Rodents, which began to diversify rapidly in the Paleocene, outcompeted multituberculates for resources and ecological niches.
Rodents possessed certain advantages, such as faster reproduction rates and more efficient gnawing incisors, which gave them an edge in processing food resources, especially hard nuts. While some research suggests their decline occurred in some regions before rodent arrival, the overall pattern points to a competitive interaction. The decline in multituberculate diversity notably correlated with an increase in rodent diversity, particularly in North America, suggesting that as rodents spread, they gradually replaced the long-established multituberculate populations.