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SummaryThe Upper Jurassic–Lower Cretaceous strato-tectonic belts of the southern Andes and South Georgia, 2000 km apart, can be correlated and explained as the products of an island-arc–back-arc system. From the Pacific Ocean to the Atlantic, these belts, which exhibit structural and metamorphic differences, are: (1) a pyroclastic belt developed on an ensialic volcanic arc; (2) a back-arc flysch sequence underlain in the southern Andes by a basic complex with oceanic affinities; this was intruded into continental crust as a result of sea-floor spreading which created a marginal basin; (3) a slate sequence deposited on a continental shelf. The pyroclastic and marginal basin belts and the adjacent part of the continental shelf were folded and uplifted during the early Upper Cretaceous, whereas the foreland part of the continental shelf assemblage underwent deformation during the early Tertiary.

Theropod dinosaurs were the dominant predators in most Mesozoic era terrestrial ecosystems. Early theropod evolution is currently interpreted as the diversification of various carnivorous and cursorial taxa, whereas the acquisition of herbivorism, together with the secondary loss of cursorial adaptations, occurred much later among advanced coelurosaurian theropods. A new, bizarre herbivorous basal tetanuran from the Upper Jurassic of Chile challenges this conception. The new dinosaur was discovered at Aysén, a fossil locality in the Upper Jurassic Toqui Formation of southern Chile (General Carrera Lake). The site yielded abundant and exquisitely preserved three-dimensional skeletons of small archosaurs. Several articulated individuals of Chilesaurus at different ontogenetic stages have been collected, as well as less abundant basal crocodyliforms, and fragmentary remains of sauropod dinosaurs (diplodocids and titanosaurians).