As problem-based learning (PBL) has gained popularity across disciplines, its move from small medical-school inquiry groups into large-class undergraduate inquiry has led to an increasing need to understand the elements of successful PBL implementations in large classrooms. In this study, we investigated how PBL was appropriated among students to develop historical thinking skills in a 96-person introductory undergraduate history survey course. The video analysis demonstrated that students initially found the appropriation of PBL routines and norms to be challenging. However, instructor interaction with both the students and representational tools in a large classroom provided multiple co-occurring and dynamic supports. This synergistic scaffolding structured around representational tools was instrumental in a semester-long intervention in which we supported student learning of historical thinking skills by encouraging appropriation of the activities that govern PBL.