WHAT DO LINGULID BRACHIOPODS TELL US ABOUT PALEOTEMPERATURE AND PALEOENVIRONMENTAL CONDITIONS IN THE EARLY TRIASSIC?

Phosphatic skeletal materials provide a stable oxygen isotope paleothermometer independent of archives derived from calcite and aragonite, and more resistant to diagenesis. The wide distribution and common occurrence of phosphatic lingulid brachiopod fossils in early Triassic strata make them desirable alternative climate proxies compared to diagenetically-altered aragonitic, and their paleobiology is very well understood. Specimens of Lingula borealis collected from shallow subtidal, storm-dominated ramp facies of the Induan-age Dinwoody Formation of the western US have stable oxygen isotope compositions similar to those from modern Glottidia palmeri from the northern Gulf of California. Unfortunately, both silver phosphate and laser ablation analysis of modern Glottidia demonstrate high variability in stable oxygen composition not only between sites, but within and among shells collected at the same time from the same location, and their utility has thus been questioned. These variations have been reduced methodologically by milling and combining multiple shells for each sample in order to attain the mass required for silver phosphate analysis, providing a bulk-average value. Recent work suggests that the intra-shell carbonate fraction (< 5% of the lingulid shell) of modern shells was precipitated near equilibrium with the phosphate fraction, and may not support in-vivo fractionation. Paleotemperatures determined using multiple techniques coincide with the range of temperatures measured for the northern Gulf (from 5 to 45 degrees C). Lingulid brachiopods may secrete apatite while contemporary bivalves exhibit heat stress and interrupt shell growth at temperatures above 32 degrees C, and may better record the full range of temperatures experienced in extreme conditions. Triassic lingulid paleotemperature estimates from the Dinwoody Formation support elevated temperatures reported from conodonts in other studies, and the northern Gulf of California provides a strong modern environmental analogue. However, lingulid biomineralization and isotopic fractionation remains poorly understood, and requires further investigation.