Sigogneau-Russell, D., Bonaparte, J. F., Frank. R. M. & Escribano. V. 1991 01 15: Ultrastructure of dental hard tissues of Gondwanatherium and Sudamerica (Mammalia, Gondwanathcria). Lethaia. Vol. 24, pp. 27–38. Oslo. ISSN 0024–1164.

The ultrastructural study of small dental fragments of the South American mammals Gondwanarherium Bonaparte, 1986 (late Cretaceous) and Sudamerica Scillato-Yané & Pascual. 1985 (middlc Paleocene). tentatively classified in the Paratheria, his shown the same structural characteristics. In the first genus one notes the presence of a thick, well differentiated, prismatic enamel with prisms of pattcrn I of Boyde and interprismatic substance. In places, lines of apposition are visible. The most internal zone of the enamel is very rich in tubules, which become rarer in the peripheral enamel. The dentinc presents numerous parallel tubules, sometimes continuing into the enamel. The presence of enamel over the whole height of the tooth fragment indicates that the hypsodonty here would be due almost entirely to lengthening of the crown. While structurally resembling the specimen of Gondwunarherium, the dental fragment of Sudamerica differs in its enamel locally constituted of open prisms. In spite of the further differences in lobation and enamel folding, the similar kind of hypsodonty met with in both genera studied tends to support their grouping in the order Gondwanatheria Moncs, 1987. an order which may well be situated outside the Tribosphenida. On the contrary, the abscnce. in thc fragments under study. of the dental specializations seen in typical Xenarthra, such as enamel rcduction. suggest that the morphologic resemblance observed between these genera of the Gondwanatheria and Xenarthra might be a phenomenon of convergence rather than of phyletic relationship. However. diversity within Gondwanathcria, and our poor knowledge of the carliest stages of the complex superorder Xenarthra, do not permit at the moment any definitive statement as to their phylogenetic relationships. Teeth, ultrastructure, Gondwanarheria. Xenarthra.