This full paper considers how collaborative discourse can reveal ways upper-class engineering students mechanically reason about engineering concepts. Argumentation and negotiation during collaborative, multimodal discourse using speech and gestures helps establish common ground between learners and fosters reflection on their conceptual understandings. Gestures produced during speech can be either conceptually concordant (or redundant) with speech, suggesting a stable epistemology, or conceptually discordant (or complementary) with speech, suggesting an evolving epistemology. Epistemic Network Analysis (ENA) supports visualizing changes in students' connections between concepts, as revealed by their multimodal discourse, displaying their evolving epistemologies. To investigate changes in students' emerging epistemologies, we hypothesized (H1) students enrolled in a "Mechanics of Materials" course would initially describe their conceptualizations using course-specific, formalized speech, which would be correlated with their conceptually discordant gestures; once common ground was established, students' course-specific, formalized speech would become more strongly correlated with their conceptually concordant gestures, evidenced by a shift in students' epistemic networks. Results showed students initially displayed a weak correlation between course-specific speech and conceptually discordant gestures. They displayed a moderate correlation between course-specific speech and conceptually concordant gestures while establishing common ground. Once students established common ground, the correlation between course-specific speech and conceptually discordant gestures disappeared, while the correlation between course-specific speech and conceptually concordant gestures remained unchanged. These results show engineering students' emerging conceptualizations of a Mechanics of Materials concept as revealed by their shifting gestures during collaborative argumentation and negotiation, so they better align with the scientific epistemology. This is in contrast to information gleaned from traditional assessments that provide a static snapshot of students' knowledge (expressed almost exclusively in verbal and symbolic forms). Multimodal analyses and ENA offer alternatives to traditional assessment practices by revealing a richer, more detailed understanding of the cognitive processes students' thinking and learning that can inform and complement current engineering teaching and assessment practices and advance theories of learning that guide engineering education.