Abstract:
Brachiopods have been extensively used in paleoclimatic and paleoecological reconstructions, but their utility would greatly increase if a method were developed to determine paleoseasonality. Currently, there is no accurate and efficient method of determining seasonal seawater temperature variations from fossil brachiopods because doing so requires knowledge of specimen ontogenetic ages, which are difficult to determine. In this study, the spiral deviation methodology for determining specimen ontogenetic ages and paleoseasonality is tested using four species of fossil brachiopods, including Laqueus rubellus, Terebratula terebratula, Pseudoatrypa sp. and Platystrophia ponderosa, which range in age from the Pleistocene to the Ordovician. Specimens were analyzed for spiral deviations using R computer code developed for brachiopod shells. These shells were then analyzed for preservation using electron backscatter diffraction and scanning electron microscopy. Finally, well-preserved fossil specimens were analyzed for oxygen isotopes and Mg/Ca ratios using a mass spectrometer and laser-ablation-inductively-coupled-mass spectrometer, respectively. Chemical analyses revealed that locations of spiral deviations on shells of L. rubellus displayed a strong direct relationship with Mg concentrations, and resulting paleotemperatures were seasonal. Conversely, specimens of T. terebratula and P. ponderosa did not show a consistently strong relationship between Mg concentrations and spiral deviations, although resulting paleotemperatures from T. terebratula agreed with those from previous studies. Overall, the results from this study indicate that the spiral deviation methodology combined with chemical proxies presents great potential for utility in past seasonal seawater temperature reconstructions in pristinely preserved, biconvex fossil brachiopods.
