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Fossils of therian mammals, which include the marsupials as well as the eutherians, may be recognized and distinguished by the number, morphology, and replacement pattern of their teeth.
It is therefore puzzling that such adaptations appear to have a limited distribution within the Theria .
First, we point out that the loss of tooth replacement in marsupials is neither unusual among therian mammals nor highly correlated with the extended period of nipple attachment in any obvious manner.
The reproductive system of the therians (marsupial and placental mammals) is a complex variation on the egg-layer's basic tubular structure.
In this way the ancestor of therian mammals came to possess a single generation of replacement teeth, which erupt when the jaw reaches a size large enough to accommodate them.
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