Seasonal Equability in Late Cretaceous Central-Eastern Iberia? Inferences from Isotopic Data on Vertebrates

After the mid-Cretaceous thermal maximum, the latest Cretaceous witnessed a long-term cooling trend (Santonian-Maastrichtian). It has been proposed that seasonal equability (low mean annual range of temperatures) accompanied the mid-Cretaceous greenhouse period, but was it also a climatic feature of the colder latest Cretaceous? Terrestrial proxies have proven useful in understanding past seasonality and in this vein, we performed oxygen isotope analyses of the phosphate (δ¹⁸O_PO4) on the rich and exceptionally well preserved late Campanian-early Maastrichtian vertebrate assemblage of 'Lo Hueco' fossil site (Cuenca, Spain). We analysed theropod and crocodilian tooth enamel, turtle shell, and gar ganoine with the aim of evaluating paleoclimatic conditions existing in the western area of the Tethys realm. The 'Lo Hueco' locality was situated at a paleo-latitude of 31°N and sedimentological and paleontological studies point to a coastal environment with distributary channels and sporadic sabkhas. Samples were collected from two different levels: G1 (proximal muddy floodplain) and G2 (distal muddy floodplain), with G1 being older. δ¹⁸O_H2O values were calculated from theropod, crocodilian and turtle δ¹⁸O_PO4 values using established equations and in all cases they are in good agreement with precipitation water from subtropical latest Cretaceous and modern settings. Theropods recorded consistently slightly lower δ¹⁸O_H2O values (G1: -4.1×1.4‰, G2: -3.5×0.5‰) than crocodilians (G1: -3.6×0.6‰, G2: -2.7×0.6‰) and turtles (G1: -3.8×0.6‰, G2: -2.9×0.5‰). This may be due to terrestrial endothermic taxa, such as theropods, recording ingested water year round, meanwhile semiaquatic ectothermic taxa, such as crocodilians and turtles, would record δ¹⁸O_H2O values representing local meteoric waters over the warm season, when conditions are favorable for apatite synthesis. With these δ¹⁸O_H2O values, we used gar ganoine δ¹⁸O_PO4 values as an independent proxy to calculate temperature values. As expected, temperature values estimated from theropods are lower (G1: 17.5×4.4°C, G2: 21.0×3.8°C), representing mean annual temperature (MAT), whereas temperature values yielded by crocodilians (G1: 19.6×4.4°C, G2: 24.4×3.8°C) and turtles (G1: 18.8×4.4°C, G2: 23.5×3.8°C) are slightly higher, reflecting the temperature of the warmest months (TWMs). Our record shows an increase in temperature values between G1 and G2, but they remain within expected temperature estimates based on other independent proxies (palynomorphs, vertebrates) and paleoclimatic models for the Late Cretaceous and the 'Lo Hueco' paleo-latitude. Maximum differences between TWMs and MAT are 2.1°C and 3.4°C for G1 and G2, respectively. These differences are in the low end-member of those observed in modern subtropical settings (~2.8-8.1°C) pointing to a slightly lower seasonal thermal varibility in central-eastern Iberia during the late Campanian-early Maastrichtian.