page_title: Ambondro mahabo
section_title: Description
hierarchical_section_title: Ambondro mahabo / Description
caption_attribution_description: English: Labeled diagram of a generalized lower molar of an australosphenidan mammal. Drawn after Flynn et al. (1999, Nature 401, p. 58, fig. 2) and Rougier et al. (2007, American Museum Novitates 3566, p. 13). Abbreviations: ant, anterior; pos, posterior; ci, lingual cingulum; pa, paraconid; pr, protoconid; me, metaconid; hy, hypoconid; hl, hypoconulid; ec, entocristid; tb, talonid basin.
context_page_description: Ambondro mahabo is a mammal from the Middle Jurassic Isalo III Formation of Madagascar. The only described 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 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.
context_section_description: Ambondro was described on the basis of a fragmentary right mandible (lower jaw) with three teeth in it (Figure 1), interpreted as the last premolar (p-last) and the first two molars (m1 and m2). It is in the collection of the University of Antananarivo as specimen UA 10602. Relative to other primitive mammals, it is small. Each of the teeth has a prominent cingulum (shelf) on the inner (lingual) side. The p-last has a strong central cusp. There is a cuspule (small cusp) on the back of the tooth and probably another on the inner front corner. This tooth resembles the molars of symmetrodonts, a group of primitive mammals, but the back cusp is smaller than the metaconid of symmetrodonts.
  The front half of the m1 and m2 consists of the trigonid, a group of three cusps forming a triangle: the paraconid at the front on the inner side, protoconid in the middle on the outer (labial) side, and metaconid at the back on the inner side (see Figure 2). The three cusps form a right angle with each other at the protoconid, so that the trigonid is described as "open". The paraconid is higher than the metaconid. At the front margin, a cingulum is present that is divided into two small cusps. Unlike in various early tribosphenic mammals and close relatives, there is no additional cuspule behind the metaconid. At the back of the trigonid, the crest known as the distal metacristid is located relatively close to the outer side of the tooth and is continuous with another crest, the cristid obliqua, which is in turn connected to the back of the tooth.
  The talonid, another group of cusps, makes up the back of the tooth. It is wider than long and contains a well-developed cusp, the hypoconid, on the outer side and a depression, the talonid basin, in the middle. The cristid obliqua connects to the hypoconid. The smaller hypoconulid cusp is present towards the inner side of the tooth, and the hypoconid and hypoconulid are connected by a cutting edge which is suggestive of the presence of a metacone cusp on the upper molars. Further towards the inner side, a crest, the entocristid, rims the talonid basin; on m1, it is swollen and on m2, it contains two small cuspules, but a distinct entoconid cusp is absent. This entocristid is continuous with the lingual cingulum.
  Wear facets are areas of a tooth that show evidence of contact with a tooth in the opposing jaw when the teeth are brought together (known as occlusion). Flynn and colleagues identified two wear facets at the front and back margins of the talonid basin; they argue that these wear facets suggest the presence of a protocone (another cusp on the outer side of the tooth) on the upper molars. In a 2005 paper on Asfaltomylos, a related primitive mammal from Argentina, Thomas Martin and Oliver Rauhut disputed the presence of these wear facets within the talonid basin in Ambondro and instead identified wear facets on the cusps and crests surrounding the basin. They proposed that wear in the australosphenidan talonid occurs mainly on the rims, not in the talonid basin itself, and that australosphenidans may not have had a functional protocone.
caption: Figure 2. Generalized australosphenidan lower molar seen from above, illustrating major features. Abbreviations: ant, anterior (towards the front); pos, posterior (towards the back); ci, cingulum; pa, paraconid; pr, protoconid; me, metaconid; dm, distal metacristid; co, cristid obliqua; hy, hypoconid; hl, hypoconulid; ec, entocristid; tb, talonid basin.
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