This is just one of the many pit vipers that inhabit South America. It is currently particularly prevalent in Argentina and Paraguay, though it was historically mostly found in Brazil - this may be a result of more people living in the Paraguayan and Argentinian territories than Brazilian, however.
This is a significant species of pit viper primarily for its venom being the source of several base peptides used in ACE inhibitors, used for congestive heart failure cases. Though its bites cause death only ~1% of the time, these snakes are considered dangerous as they are very prevalent, and cause death years after the bite more frequently than other snakes. Their hemotoxic venom affects the heart and vessels in such a way that they are sometimes unable to deal with the stresses of aging.
Bilder-atlas zur wissenschaftlich popularen. Leopold Fitzinger, 1867.