Diet and climatic context of giant birds inferred from δ13Cc and δ18Oc values of Late Palaeocene and Early Eocene eggshells from southern France

Abstract

Abstract Abundant fragments from eggs laid by giant birds occur in the Palaeocene (Thanetian) and Eocene (Sparnacian) sedimentary deposits of southeastern France. In the Sparnacian, thick eggshell fragments, assigned to the oospecies Ornitholithus arcuatus, correspond to very large bird eggs that were most likely laid by Gastornis. The Thanetian thin eggshell fragments, assigned to Ornitholithus biroi, were presumably laid by a smaller, yet unidentified bird. In order to investigate ecology and environment of these egg-laying birds, stable carbon and oxygen isotope compositions of 125 fossil eggshell fragments were analyzed. After removing samples affected by diagenetic alteration of the calcitic shells, the measured range of δ13Cc values (− 11‰ to − 6‰ V-PDB) is interpreted as reflecting an herbivorous diet for these birds in a context of limited annual precipitation (≈ 500 mm y− 1). Stable oxygen isotope analysis of living ostrich eggshell calcite, along with that of the water extracted from their albumen and yolk, provided evidence to calculate isotopic fractionation factors between both calcite and body water (αcalcite–body water = 1.03041) and between body water and meteoric water (αbw–mw = 1.00399), using δ18O values of local meteoric waters identified as the source of the birds drinking water. Combined with the δ18O values of fossil eggshells, both isotopic fractionations provided calculated δ18O values of meteoric waters in the range − 9.5‰ to − 2.8‰ (V-SMOW) for the Thanetian, and in the range − 8.9‰ to − 1.7‰ (V-SMOW) for the Sparnacian. These large isotopic ranges likely reflect inter-annual temperature variations of the complete year, suggesting a year round egg-laying strategy. Corresponding Mean Air Temperatures (MAT) were comprised between 20 ± 4 °C and 22 ± 4 °C during the Thanetian, and between 23 ± 3 °C and 25 ± 3 °C during the Sparnacian. These giant birds likely lived under a warm and dry climate similar to that prevailing today in western Mediterranean islands.