Australosphenida

Australosphenida is a group of mammals that includes the modern monotremes and their fossil relatives. This group is characterized by a unique type of molar tooth that is different from the tribosphenic molars found in most other mammals. The name "Australosphenida" refers to the southern (Australo-) distribution and the sphenoid (-sphenida) type of molar.

Evolution and Distribution

The Australosphenida evolved in the Mesozoic era, during the time of the dinosaurs. They are known from fossils found in Australia, Antarctica, and South America, which were all part of the southern supercontinent Gondwana. The oldest known australosphenidan is Asfaltomylos, a fossil from the Middle Jurassic of Argentina.

Anatomy and Physiology

Australosphenidans have a unique type of molar tooth known as australosphenidan molar. This type of molar has three main cusps arranged in a triangle, which is different from the linear arrangement of cusps in the tribosphenic molars of most other mammals. The australosphenidan molar is thought to have evolved independently from the tribosphenic molar.

Classification

The Australosphenida includes the modern monotremes, such as the platypus and the echidna, as well as a number of extinct groups. These extinct groups include the Ausktribosphenidae, the Henosferidae, and the Steropodontidae. The exact relationships between these groups are still a matter of scientific debate.

See Also

• Evolution of mammals
• Monotreme
• Molar (tooth)