The excavation of a stratified sequence of deposits spanning the Initial Late Formative period (250 BC–AD 120) at Iruhito, in the upper Desaguadero Valley of Bolivia, provides insight into this previously unrecognized, four-century period separating the well-documented Middle Formative (800–250 BC) from the Late Formative (~AD 120–590) period. By tracking subtle shifts in ceramic, architectural, lithic, and faunal data, we can explore tempos of change in social life during this dynamic time. These data lead us to suggest that, rather than being a “transitional” period or a “hiatus” in regional occupation, the Initial Late Formative period was a distinct mode of sociality characterized by the realignment and expansion of interaction networks, on the one hand, and rejection of the decorative aesthetics, monumentality, and public-oriented performances of earlier periods, on the other. We argue that the Late Formative period centers emerging after ~AD 120 intentionally cited architecture and aesthetics that were distant in time and space, constituting a sophisticated political strategy. Finally, these data suggest that the chronological schemata we use to build regional histories often obscure social variability.

The Late Formative period immediately precedes the emergence of Tiwanaku, one of the earliest South American states, yet it is one of the most poorly understood periods in the southern Lake Titicaca Basin (Bolivia). In this article, we refine the ceramic chronology of this period with large sets of dates from eight sites, focusing on temporal inflection points in decorated ceramic styles. These points, estimated here by Bayesian models, index specific moments of change: (1) cal AD 120 (60–170, 95% probability): the first deposition of Kalasasaya red-rimmed and zonally incised styles; (2) cal AD 240 (190–340, 95% probability): a tentative estimate of the final deposition of Kalasasaya zonally incised vessels; (3) cal AD 420 (380–470, 95% probability): the final deposition of Kalasasaya red-rimmed vessels; and (4) cal AD 590 (500–660, 95% probability): the first deposition of Tiwanaku Redwares. These four modeled boundaries anchor an updated Late Formative chronology, which includes the Initial Late Formative phase, a newly identified decorative hiatus between the Middle and Late Formative periods. The models place Qeya and transitional vessels between inflection points 3 and 4 based on regionally consistent stratigraphic sequences. This more precise chronology will enable researchers to explore the trajectories of other contemporary shifts during this crucial period in Lake Titicaca Basin's prehistory.