For more than 120 years, the coastal exposures of the Santa Cruz Formation havebeen fertile ground for recovery of vertebrates from the late Early Miocene(~18 to 16 million years ago, Ma). As long ago as the 1840s, CaptainBartholomew Sulivan collected fossils from this region and sent them to CharlesDarwin, who passed them to Richard Owen. Carlos Ameghino undertook severalexplorations of the region starting in the late 1880s. Carlos' specimens weredescribed by his brother Florentino, who believed that many of the species weremore ancient than now understood and represented the ancestors of many Holarcticmammalian orders. Ameghino's novel claims prompted William B. Scott to organizefossil collecting expeditions in the Santa Cruz beds led by John B. Hatcher. Thefossils were described in a series of exhaustive monographs with the conclusionthat the fauna was much younger than Ameghino thought. Several brief expeditionstook place during the twentieth century, led by researchers from differentinstitutions. Since 2003, we have undertaken the collection of over 1600specimens, including large series of relatively complete skeletons. In thisedited volume we have gathered together a group of researchers to study thecoastal Santa Cruz Formation and its associated flora and fauna to provide apaleobiological reconstruction of the Santacrucian vertebrate community and toplace it in its biotic and physical environment.