Determining paleotemperatures in terrestrial environments are much more challenging than those in the ocean because of stratigraphic inconsistencies, strong spatial and temporal variations in temperature, and a paucity of well-tested methods. Here we utilize the ganoine scales of gars from the family Lepisosteidae to calibrate a new terrestrial paleothermometer. Gars are widespread both in the modern and in the past, as they are a freshwater fish lineage that extends back into the Cretaceous (100 Ma) and have remained relatively unchanged during that time span. Gars constantly record water temperatures, whose yearly average is closely related to mean annual temperature, in their body tissues, including scales. These scales grow continuously throughout life, are >95% hydroxyapatite and thus are highly resistant to diagenetic alteration. Oxygen isotopes in both biogenic phosphates and carbonates have been used to reconstruct environments on land with varying degrees of success. Phosphate-oxygen isotopes are more resistant to post-mortem alteration as the phosphorus-oxygen bond is stronger than the carbon-oxygen bond. We investigate the application of phosphate oxygen isotopes to gar scales by collecting scales from modern individuals from a north-south transect across the United States, exploiting the latitudinal temperature gradient in mean annual temperatures, measuring δ18Ophosphate of those scales, and comparing these values to the average δ18Owater and temperature of each locality. We compare our δ18Ophosphate calibration to previously published curves. Our work demonstrates that the δ18Ophosphate values of gar scales are robust recorders of temperature and δ18Owater.

Carbonate clumped isotope thermometry has been calibrated for a wide variety of carbonates, including calcite, aragonite, dolomite, siderite, and many of their biogenic forms. The clumped isotope composition of the carbonate group substituting for phosphate or hydroxyl in bioapatite (Ca(PO4,CO3)(OH,F)) has also been temperature calibrated using vertebrate tooth enamel from a range of endothermic body temperatures. We apply this method to other bioapatite-bearing taxa and the calibrated temperature range is extended to lower paleoclimatologically relevant temperatures. Furthermore, because relatively large bioapatite samples are required for carbonate clumped isotope measurements (Δ47), replicate sampling of thin tooth enamel may not be feasible in many situations. Here, we use gar fish (Lepisosteus sp.) scales to extend the calibration. These fish are unique in that they are entirely covered in ganoine scales, which are >95% hydroxyapatite. Their enamel structure also makes them resistant to diagenesis. Additionally, gar fossils are common in lacustrine, fluvial, and near-shore facies, and have a wide distribution in time (Cretaceous to modern) and location (North America, South America, Europe, India, and Africa). We have developed a reliable lab protocol for measuring Δ47 in gar bioapatite. We estimate the standard error (SE) for a single measurement as 0.027‰, which is based on replicate analyses and Student T-distribution to account for sample size. We report results for modern gar scales from seven North American localities with mean annual water temperatures (MAWT) ranging from 9 to 26 °C. These data give a temperature calibration curve for gar scales of Δ47 = (0.1095 ± 0.0159) x 106/T2 – (0.5941 ± 0.0548) (R2 = 0.74) and a curve for pooled bioapatite of Δ47 = (0.1003 ± 0.0144) x 106/T2 – (0.4873 ± 0.0495) (R2 = 0.76).