ALTERATIONS IN LATE CRETACEOUS DINOSAUR EGGSHELLS FROM RÍO NEGRO, ARGENTINA. Abnormal dinosaur eggshells are described from the Allen Formation (Late Cretaceous) at Salitral de Santa Rosa (Río Negro Province). Two different types of abnormalities were identified, i.e., (1) eggshells with altered microstructure and heterogeneous ornamentation, among which can be distinguished (1a) eggshells without rosettes (in megaloolithids and faveoloolithids), and (1b) eggshells with rosettes (in faveoloolithids); and (2) multilayered eggshells (only in megaloolithids). In the first group (1a) the eggshells display a greater range of diameters than the normal ones and the surface nodules are overgrown by recrystallization. Thin sections show alterations of the normal eggshell structure of the units. In the second group (1b) eggshells present groups of flower-shaped nodules (or rossettes) on certain sectors of their surface. In thin sections it is possible to observe that the outline of the eggshell units is completely irregular, while the pore channels are corroded. Two identical layers were identified in the double eggshell (2), the externalmost of which is the thinnest and lacks growth nuclei. Cathodoluminescence analysis established that double eggshells and eggshells with heterogeneous ornamentation with rossettes have a pathological origin, while eggshells with heterogeneous ornamentation without rossettes are only strongly altered diagenetically.