The name is Tupi, and it has two possible meanings. One derivation gives *itá* (stone), *porã* (beautiful), and *y* (river) - *river of beautiful stones*. The other substitutes *berab* (brilliant) for *porã*, giving *river of brilliant stones*. Either works; both describe what the Tupi people observed before geologists named the same thing in Latin-derived technical terms. The city of Itaboraí, the basin that lies beneath it, and the rock formation that fills that basin all share the same name. They are 34 kilometers northeast of Rio de Janeiro in southeastern Brazil, and the basin itself is tiny - a half-graben stretching only 1 square kilometer - the smallest of the rift basins that string across the Continental Rift of Southern Brazil from the Paraná Basin to the Santos Basin.

Between 1933 and 1984, a local cement company exploited the limestones of the Itaboraí Basin for building material. As they quarried, workers noticed that the rock contained bones. The site turned out to be a series of fissure fillings - vertical cracks in the limestone that, millions of years ago, had trapped animals as they fell or were washed into them, then filled with sediment that preserved their remains. The paleontological value became obvious, but not before the quarrying had done its damage. When the cement operation ended in 1984, the basin began to fill with water, and it is now effectively flooded, making further collection difficult. What had already been extracted in fifty-one years of commercial mining became the basis for decades of paleontological research. In 1995 the site was designated a paleontological park - the Parque Paleontológico de São José de Itaboraí - and in 1996 the IUCN named it a fossil site of potential World Heritage Value.

For a long time, researchers thought the Itaboraí Formation dated to the Paleocene. In 2014, a study by Woodburne and colleagues placed it more precisely in the Early Eocene, between 53 and 50 million years ago. The formation spans polarity chron 23, a recognizable magnetic interval. Basalts that overlie the fossil-bearing limestones have been dated to 52.6 million years ago, consistent with that placement. This timing matters because the Itaboraí fossils formed during the Early Eocene Climatic Optimum - a period just after the Paleocene-Eocene Thermal Maximum, when global temperatures briefly spiked and ecosystems reorganized. The formation preserves a snapshot of South American mammal life during one of the warmest periods of the Cenozoic. At that time, South America was still biogeographically connected to Antarctica, and - across the widening South Atlantic - to Africa. Mammals could still cross between these continents in ways that would become impossible as the land masses drifted apart.

When paleontologists need to compare fossil faunas across continents, they use land mammal ages - regional time units defined by characteristic mammal assemblages. The Itaboraí Formation is the defining unit for the Itaboraian South American Land Mammal Age, which means that this one small basin sets the reference point for a span of evolutionary time across an entire continent. The fauna that defines it is diverse. At the family level, 44 percent of the fossils are mammals, 23 percent mollusks, 14 percent reptiles (lizards, turtles, crocodyliforms), 7 percent birds, 5 percent amphibians, and 7 percent plants. Fish are notably absent. The mammals include marsupials and related metatherians - the lineage that still dominates Australia today - and a variety of extinct groups. *Lamegoia conodonta*, the largest "condylarth" at Itaboraí, approximates the size of a wolf. *Ricardocifellia protocenica* is the smallest, and also the most abundant. *Protolipterna ellipsodontoides* is the most common litoptern, an extinct South American hoofed mammal.

Several genera and species were named after the formation itself, making Itaboraí a prefix attached to an entire menagerie of extinct species: the marsupials *Itaboraidelphys camposi* and *Carolopaulacoutoia itaboraiensis*; the birds *Itaboravis elaphrocnemoides* and *Eutreptodactylus itaboraiensis*; the snake *Itaboraiophis depressus*; the caiman *Eocaiman itaboraiensis*; and the gastropods *Itaborahia lamegoi*, *Biomphalaria itaboraiensis*, and *Gastrocopta itaboraiensis*. Fossil birds from this period are rare everywhere in the world - bird bones are pneumatized, hollow, and fragile, and they usually weather away before fossilization. Only three bird species have been described from the Itaboraí Basin so far, but one of them, *Diogenornis fragilis*, is a probable ratite ancestor preserved in multiple good specimens. The Itaboraí records are the oldest for the gastropod genera *Austrodiscus*, *Brachypodella*, *Bulimulus*, *Cecilioides*, *Cyclodontina*, *Eoborus*, *Gastrocopta*, *Leiostracus*, *Plagiodontes*, and *Temesa* - and the oldest records for the snail families Orthalicidae, Gastrocoptidae, Ferussaciidae, and Strophocheilidae. A flooded limestone quarry near Rio de Janeiro has, in other words, given the world a disproportionate share of what we know about early Cenozoic terrestrial life in the Southern Hemisphere.