<span>Changes in atmospheric composition, driven by fluctuating concentrations of greenhouse gases, are the primary driver of climate change, alongside variations in oceanic circulation. Reconstructions of temperature and climate over geological timescales rely on qualitative and quantitative proxies preserved in the geological record. Sedimentary deposits such as coal, evaporites, bauxites, and tillites, although they represent specific climatic regimes, have low temporal resolution. By contrast, quantitative geochemical proxies such as δ18O or δ13C, clumped-isotope thermometry, TEX86, and Sclerochronology offer higher temporal resolution. However, these proxies have limitations, reflecting, in some cases, local post-depositional variation or alteration, indicating a need to integrate proxy methods to establish reliable and robust paleoclimate reconstructions.</span></p> <p>In this paper, we present a new qualitative method for reconstructing temperature, based on the fractal dimension of ammonite suture lines, which can integrate with and support well-established paleoclimate proxies. Suture lines mark the contact between the septum, formed by the organism, and its shell. The linear pattern of fossil sutures exhibits a fractal dimension and resembles viscous fingering—a process that depends on fluid viscosity. Based on the fractal dimension of c. 450 taxa of Cretaceous ammonite sutures (143–66 million years ago), we present a fractal dimension curve that serves as a qualitative proxy for seawater viscosity and inversely correlates with seawater temperature. The curve reveals warm and cold snaps associated with geological events that may have triggered climate changes during the Cretaceous, such as Large Igneous Provinces, seafloor production, mid-ocean ridge length, and Oceanic Anoxic Events.