This study aims to explore the skeletal mineralogy of temperate and Polar bryozoans, investigating variability within and between species in relation to methodological, biological and environmental factors. Oceans are becoming warmer and more acidic and it is becoming increasingly important to increase our knowledge about the responses of marine calcifiers to environmental conditions. Bryozoans are important components of the benthic community globally, however prior to this study relatively little was known about their skeleton composition. This study has contributed over 1700 new mineralogical analyses to the field and provided new skeletal profiles for 115 bryozoan species. The study represents by far the most comprehensive regional profile of bryozoan mineralogy to date with 79% of Scottish species analysed. Targeted experiments have resulted in the documentation of impacts of curational procedures on skeletal mineralogy, resulting in recommendations which are pertinent across taxa in the wider fields of both curation and palaeoclimatography. Evidence is presented of mineral localization in specific skeletal features in bryozoan skeletons which adds to the growing body of data showing MgCO3 localization for mechanical and ecological advantage in marine invertebrates. Prior to this study it was proposed that both “active” biological and “passive” environmental controls influence bryozoan skeletal mineralogy. Through the examination of both Polar and Temperate species, this study has provided evidence of biological control of bryozoan mineralogy, while finding no evidence of passive environmental control. This finding precludes the use of bryozoan mineralogy for palaeoclimatic interpretation, for the species included in this study, and it is recommended that future species are chosen carefully and thoroughly calibrated prior to their use as palaeothermometers. Further investigation into the effects of ecological specification on the temperature/mineralogy response may, however, prove an area for fruitful research, enabling prediction of climate change effects on the bryozoan skeletons of ecologically specialised species and providing insights into future changes in community composition.