Learning science includes learning to argue with "inscriptions": images used to symbolize information persuasively. This study examined sixth-graders learning to invest inscriptions with representational status, in a geographic information system (GIS)-based science investigation. Learning to reason with inscriptions was studied in emergent participation patterns in groups, operationalized as "roles". Cross-case analyses compared developmental trajectories for two roles in each group: "competitive challenger" and "quiet bystander". Role development mediated learning to reason with inscriptions, including (1) co-assembling "representational states" of data and (2) managing dialectical tensions of argumentation. "Role" was operationalized as a site of learning, intersecting individual and collective processes, rather than a mechanism of one impacting another. Distributed conceptions of practice support this approach for understanding how students learn to do science. (Contains 2 tables, 12 figures and 6 footnotes.)