Fig. 2

Comparison of fossil and living biota at Bahia Blanca as seen by Charles Darwin early in 1832. a The extinct Edentate megafauna (including such elements as glyptodonts and giant ground sloths) is today represented by distantly allied armadillos and sloth species. On the other hand, one of the small fossil “Rodentia” seemed to Darwin to be an extinct species congeneric with the living “Agouti” (Patagonian cavy). By merging fossil mammals from two different localities and interpreting them as coeval, Darwin was showing that some mammalian genera faced extinction where others did not. In further contrast, the species-level identity of South American fossil and living mollusks at Bahia Blanca showed persistence of invertebrate species throughout the entire interval—a pattern Darwin had first observed at Santiago in the Cap de Verde Islands. b The fossil caviomorph rodent as depicted in the Zoology of the Voyage of the Beagle in 1840, interpreted by Darwin as an extinct species directly related to the living cavy. c The living Patagonian cavy (“mara”)