Dentine exhibits micrometre scale growth marks which are due to the daily and weekly growth of bioapatite and associated tissues. It is unknown if banded dentine in fossils, which present colour differences from normal dentine, are formed during tooth development and are evidence of physiological processes, are a result of diagenesis post-mortem, or are a combination of physiology and diagenesis. Through research on mosasaur teeth collected from the Late Cretaceous Western Interior Seaway (WIS) of Manitoba using elemental, morphological, optical, textural, and isotopic analyses, the factors leading to banded dentine formation were investigated. Two different mechanisms of banded dentine development became apparent. The first mechanism was caused by diagenetic alteration of the surrounding tubules and dentine layers through the effects of fracturing with high concentrations of secondarily precipitated minerals. Although evidence of diagenetic alteration was present, secondary ion mass spectroscopy stable isotope analysis demonstrated that similar oxygen isotope values to those found in previous studies in the enamel of southern WIS mosasaurs also persisted in northern WIS mosasaurid dentine, suggesting a biological signal was not overprinted. The second mechanism found very little evidence of diagenetic alteration and more evidence of a physiological process generating banded dentine. Through TEM analysis, the structure of normal vs. occluded dentine was documented for the first time in a marine reptile, with potential implications for interpreting palaeopathologies in the fossil record. Each of these studies emphasizes the effects of diagenesis on dental tissues but also shows that physiological processes are preserved, giving insight into the lives of extinct marine reptiles.