Latitudinal distributions of organisms may be limited by both mean temperature and seasonal temperature extremes, as well as a host of other environmental factors. Extant thecideide brachiopods are distributed throughout the tropics and into some warm-temperate regions (up to 35 deg N). Controls on this distribution are poorly understood. As a clade, they have broad temperature tolerances; Pajaudina is endemic to the Canary islands where seawater temperatures are as low as 17 degrees C, whereas Thecidellina and Lacazella live in waters in the Caribbean and Pacific that can reach a seasonal maximum of 30 degrees C. Both Thecidellina and Pajaudina, as well as Lacazella, typically inhabit waters that do not vary more than a few degrees C annually, a phenomenon that is reflected in the oxygen isotopic composition of their continuously accreted shell calcite. Fossil thecideides exhibit a broader latitudinal distribution (up to paleo - 45 deg N). Have thecideide brachiopods been limited latitudinally by seasonal extremes rather than mean temperature alone?

Thecidellina shells from Guam have a seasonal d¹⁸O range of 0.6 per mil, and Lacazella from the Mediterranean has a range of 0.8 per mil. However, fossil thecideides (with no evidence of diagenesis) exhibit greater seasonal isotopic ranges. Moorellina, from the Jurassic of England, has a range of 2.31 per mil and Thecidiopsis from the Cretaceous of Germany has a 1.16 per mil range. Assuming constant local salinity and invariant d¹⁸O_seawater, these values equate to seasonal temperature ranges of 12.5 and 6 degrees Celsius respectively. Within the complete fossil data set, absolute d¹⁸O values range from -0.46 to -4.42, similar to other published Mesozoic results. Extreme negative values could result from high summer temperatures combined with seasonal variation in salinity. Utilizing independent temperature proxies such as Mg/Ca ratios for fossil thecideide shells may constrain paleo d¹⁸O_seawater and help determine the relative roles of temperature and hydrography on shell d¹⁸O, and thus on thecideide distributions.