FORASIEPI Analia Marta
Comment on Independent Origins of Middle Ear Bones in Monotremes and Therians (II)
ROUGIER, G. W.; FORASIEPI, A. M.; MARTINELLI, A. G.
Lugar: USA; Año: 2005 vol. 309 p. 1492 - 1492
Rich et al. (2005; Science 307: 910-914) argue that the malleus has been independently freed from the dentary in monotremes and in therians based on observations made on new specimens of Teinolophos, an extinct relative of the Australian modern platypus. Nevertheless, our recent study of all available Teinolophos specimens suggests that this claim is not supported. Rich et al. acknowledge that Teinolophos lacks facets indicating the presence or location of the prearticular-articular. Consequently, there is no direct evidence for the middle ear bones, making their report a discussion of indirect evidence. It is the position and relation of the angular/ectotympanic, which is not a middle ear bone per se, together with the trough on the posteromedial side of the dentary, that are the core of the Rich et al. argument. The identified facet for the purported angular/ectotympanic is not a facet; it is merely the floor of the large mandibular canal that would transmit the hypertrophied trigeminal system, as in the platypus. The deep, well-defined medial trough, with a prominent medial flange characteristic of mammaliaforms with known postdentary bones is lacking in Teinolophos; individual facets and ridges inside the trough are also absent. Following the platypus model, the likely occupant of the shallow medial groove in Teinolophos would be V3, the mandibular division of the trigeminal nerve. Rich et al. recognize, as do we, that other Mesozoic forms may question the monophyletic origin of the mammalian middle ear, but the uncertain phylogenetic position of these taxa renders the case inconclusive. Finally, the topology of the tree selected by Rich et al. does not support their claim, because upon optimization, the character free middle ear bones is equivocal at the base of Monotremata and present at the root of Mammalia.