New dental remains of Monotrematum sudamericanum. Its bearing on Monotreme (or Prototheria) relationships and the Gondwanan mammalian radiation

PASCUAL, R.; GOIN, F.; BALARINO, L.; UDRIZAR SAUTHIER, D. E.

La Plata

Jornada; XV Jornadas Argentinas de Mastozoología; 2000

Facultad de Ciencias Naturales y Museo- SAREM

Two new upper and lover dental remains of the platypus Monotrernaturn sudamericanum (Monotremata,Ornithorhynchidae) coming from the same locality and level than the holotype (Chubut province, Hansen Member of the Salamanca Formation, Early: Paleocene) are described, and analyzed, within the new context (Pascual´s "other history") provided by recent new findings of Gondwanan mammals. These two molars unequivocally show the size and features distinguishing M. sudamericanum. The present Gondwanan record suggest that monotremes differentiated during the earliest stages of mammalian radiation and that at least by the Early Cretaceous the basic ornithorhynchid state of evolution had been achieved. The dental features of Steropodon galmani, the oldest platypus so far know, compared with those of the extinct and living forms, suggest that -like many other non-tribosphenic mammals- they passed through a "pseudotribosphenic" pattern and a reversed occlusal pattern, the latter inferred only in Steropodon galmani but present in the remaining Omithorhynchidae. According to the evidence, this pattern departed from a cusp-on line form, like that of Morganucodon, by expansion and elevation of the cingular cusps (lingual on the lowers, and labial on the uppers), and connection by ridges of two of them to one of the principal ones, as was postulated for the Laurasian Docodon by Jenkins, and for the Gondwanan Reigitheriun by us. As we postulated, this process could have occurred in the gondwanatherian Ferugliotherium , as well as in many non-Holotherian mammals, i.e. that the reversed triangle occlusal pattern -and an pseudotrisbosphenic pattern could have been achieved without the reversed rotation in upper an lower molars -in some cases (e.g., Steropodon galmani) without developing a tribosphenic occlusion. It is suggestive that both the Prototheria-Theria dichotomy and the Edentata-Epitheria dichotomy - t w of the most transcendental events occurred in mammalian phylogeny- had a similar pattern: mammals having t m different sets of primitive reptilian features diverged by the acquisition of some advanced characters that led them to initiate radiations that totally changed the perspective of mammalian evolution.