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Ambondro mahabo
''Ambondro mahabo'' is a mammal from the middle Jurassic (about 167 million years ago) of Madagascar. The only species of the genus ''Ambondro'', it is known from a fragmentary lower jaw with three teeth, interpreted as the last premolar and the first two molars. The premolar consists of a central cusp with one or two smaller cusps and a cingulum (shelf) on the inner, or lingual, side of the tooth. The molars also have such a lingual cingulum. They consist of two groups of cusps: a trigonid of three cusps at the front and a talonid with a main cusp, a smaller cusp, and a crest at the back. Features of the talonid suggest that ''Ambondro'' had tribosphenic molars, the basic arrangement of molar features also present in marsupial and placental mammals. It is the oldest known mammal with putatively tribosphenic teeth; at the time of its discovery it antedated the second oldest example by about 25 million years.
Upon its description in 1999, ''Ambondro'' was interpreted as a primitive relative of Tribosphenida (marsupials, placentals, and their extinct tribosphenic-toothed relatives). In 2001, however, an alternative suggestion was published that united it with the Cretaceous Australian ''Ausktribosphenos'' and the monotremes (the echidnas, the platypus, and their extinct relatives) into the clade Australosphenida, which would have acquired tribosphenic molars independently from marsupials and placentals. The Jurassic Argentinean ''Asfaltomylos'' and ''Henosferus'' and the Cretaceous Australian ''Bishops'' were later added to Australosphenida, and new work on wear in australosphenidan teeth has called into question whether these animals, including ''Ambondro'', did have tribosphenic teeth. Other paleontologists have challenged this concept of Australosphenida, and instead proposed that ''Ambondro'' is not closely related to ''Ausktribosphenos'' plus monotremes, or that monotremes are not australosphenidans and that the remaining australosphenidans are related to placentals.
==Discovery and context==
''Ambondro mahabo'' was described by a team led by John Flynn in a 1999 paper in ''Nature''. The scientific name derives from the village of Ambondromahabo, close to which the fossil was found. It is known from the Bathonian (middle Jurassic, about 167 million years ago) of the Mahajanga Basin in northwestern Madagascar, in the Isalo III unit, the youngest of the three rock layers that make up the Isalo "Group". This unit has also yielded crocodyliform and plesiosaur teeth and remains of the sauropod ''Lapparentosaurus''.〔Flynn et al., 1999, pp. 57–58〕
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