M. sudamericanum is known only from two lower and one upper platypus teeth. It is the only known non-Australasian ornithorhynchid. The main difference, apart from continent and age, is its size: the teeth of Monotrematum are around twice as large as other similar species. These fossils presently reside in the collections of Museo de La Plata and Museo Paleontológico Egidio Feruglio, both in Argentina.
According to Pascual, "The preserved enamel in the central region shows that the crown pattern is almost identical to that of Obdurodon: it is composed of two V-shaped lobes, the anterior of which is wider, separated from the posterior one by a valley that connects the lingual and buccal sides of the crown separating the anterior and posterior lobes."
- Monotrematum at Fossilworks.org
- Pascual, et al. "First discovery of monotremes in South America". Nature 356 (1992), Pages 704-706 (Monotrematum).
- Pascual, R., F. J. Goin, L. Balarino, and D. E. U. Sauthier. 2002. New data on the Paleocene monotreme Monotrematum sudamericanum, and the convergent evolution of triangulate molars. Acta Palaeontologica Polonica 47:487–492.