In their paper, Flynn and colleagues described
Ambondro as the oldest mammal with
tribosphenic molars—the basic molar type of
metatherian (
marsupials and their extinct relatives) and
eutherian (
placentals and their extinct relatives) mammals, characterized by the protocone cusp on the upper molars contacting the talonid basin on the lower molars in chewing. The discovery of
Ambondro was thought to extend the known temporal range of tribosphenic mammals 25 million years further into the past. Consequently, Flynn and colleagues argued against the prevailing view that tribosphenic mammals originated on the northern continents (
Laurasia), and instead proposed that their origin lies in the south (
Gondwana). They cited the retention of a distal metacristid and an "open" trigonid as characters separating
Ambondro from more modern tribosphenidans. They characterized Australosphenida by the shared presence of a cingulum on the outer front corner of the lower molars, a short and broad talonid, a relatively low trigonid, and a triangulated last lower premolar. Also in 2001, Denise Sigogneau-Russell and colleagues in their description of the earliest Laurasian tribosphenic mammal,
Tribactonodon, agreed with the relationship between
Ausktribosphenos and monotremes, but argued that
Ambondro was closer to Laurasian tribosphenidans than to
Ausktribosphenos and monotremes. As evidence against the integrity of Australosphenida, they cited the presence of lingual cingula in various non-australosphenidan mammals; the presence of two cusps in the anterior cingulum in
Ambondro as well as some boreosphenidans; the different appearance of the premolar in
Ambondro (flat) and
Ausktribosphenos (squared); and the contrast between the talonids of
Ambondro (with a well-developed hypoconid on the labial side) and
Ausktribosphenos (squared). The next year, Luo and colleagues published a more thorough analysis confirming their previous conclusion and adding the Cretaceous Australian
Bishops to Australosphenida. They mentioned the condition of the hypoconulid, which is inclined forward, rather than backward as in boreosphenidans, as an additional australosphenidan character and noted that
Ausktribosphenos and monotremes were united, to the exclusion of
Ambondro, by the presence of a V-shaped notch in the distal metacristid. In the same year,
Asfaltomylos was described from the Jurassic of Argentina as another australosphenidan. In contrast to
Ambondro, this animal lacked a distal metacristid and did not have as well-developed a lingual cingulum. However, in 2003
Michael Woodburne and colleagues revised the
phylogenetic analysis published by Luo and colleagues, making several changes to the data, particularly in the monotremes. Their results (Figure 3, bottom) challenged the division between Australosphenida and Boreosphenida, as proposed by Luo et al. Instead, they excluded monotremes from Australosphenida and placed the remaining australosphenidans close to Eutheria, with
Ambondro most closely related to
Asfaltomylos. In 2007, Guillermo Rougier and colleagues described another australosphenidan,
Henosferus, from the Jurassic of Argentina; they argued against a relationship between Eutheria and Australosphenida (Figure 3, top), but were ambivalent about the placement of monotremes within Australosphenida. Based in part on Martin and Rauhut's earlier work on wear facets in australosphenidans, they questioned the presence of a true functional protocone on the upper molars of non-monotreme australosphenidans—none of which are known from upper teeth—and consequently suggested that australosphenidans may not, after all, have had truly tribosphenic teeth. ==Notes==