Santanichthys diasii is a species of extinct fish that existed around 115 million years ago during the Albian epoch. S. diasii is regarded as the basal-most characiform, and is the earliest known member of Otophysi. It appears as a small fish, similar in appearance to a modern-day herring little more than 30 millimeters in length. Its most striking characteristic is the presence of a Weberian apparatus, which makes it the most primitive known member of the order Characiformes, the order in which modern-day tetras (including piranhas) are classified in. Santanichthys has been unearthed from numerous locations throughout Brazil, in rocks dating to the Cretaceous Period. Its presence in this strata is seen as an indicator for the biogeography and evolution of its order.