Multituberculatea

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The order Multituberculata, informally also known as 'multis', is a diverse lineage of Mesozoic to early Cenozoic mammals that occupied a rodent-like ecological niche.

[edit] Overview

They appear first in the late Jurassic and are last known from the early Oligocene. The late Triassic haramiyids were sometimes considered as early multituberculates. More complete fossils have recently shown that haramiyids are a very different group of early mammals. But even without haramiyids, multituberculates existed for a time span of about 100 million years, the undisputed record for an order of mammals. Multituberculates do not belong to any of the groups of mammals living today: the primitive egg-laying monotremes and the more advanced marsupials and placentals, both also known as therians. The relationships of the multituberculates to these groups are still debated. Multituberculates have been considered as either a group that branched off even before the monotremes, as close relatives of the monotremes or as sister group of the therian mammals multituberculates were clearly very mammal-like, both in details of the internal anatomy, like the structure of the middle ear with the three auditory ossicles, and in external appearance, like the recently demonstrated possession of hair (see below). The anatomy of the pelvis suggests that multituberculates did not lay eggs like monotremes but gave birth to very small, immature young like marsupials.

In the late Cretaceous multituberculates were widespread and diverse in the northern hemisphere, making up more than half of the mammal species of typical faunas. Although some lineages became extinct during the faunal turnover at the end of the Cretaceous, multituberculates managed very successfully to cross the K/T boundary and reached their peak of diversity during the Paleocene. They were an important component of nearly all Paleocene faunas of Europe and North America, and of some late Paleocene faunas of Asia. Multituberculates also were most diverse in size during the Paleocene, ranging from the size of a very small mouse to that of a beaver.