Paleobiological and isotopic studies of eggshells from a declining dinosaur species

Abstract

Late Cretaceous dinosaur eggshells from southern France and the Spanish Pyrenees, presumably belonging to the sauropodHypselosaurus priscusMatheron, are almost exclusively composed of primary calcite. Besides normal development of these eggshells, there appear two kinds of pathologic tendencies: bi- or multi-shells (infrequent), and shells with a reduced thickness (increasing in frequency until, in the uppermost horizon, they represent more than 90% of the sample). The extinction of the species is attributed primarily to the consequences of thinning of the eggshells.The physiological mechanisms producing pathologic dinosaur eggshells are evaluated in the light of homologous phenomena occurring in living birds and reptiles. On this basis, it is concluded that in the late Maastrichtian populations of“Hypselosaurus,”pathologic eggshells were caused by hormonal imbalances of the vasotocin and of the estrogen levels. On the same basis it is postulated that the teratological shell repetition led to embryo suffocation and that the pathological reduction in shell thickness caused shell breakage and dehydration of the embryo. The lethal results are evident from the frequent absence of “resorption craters” in the mammillary knobs of pathologic shells, a fact which indicates either lack of fertilization of the eggs or the perishing of the embryo prior to the calcification of its skeletal bones. A change in environmental conditions is the ultimate factor which caused hormonal imbalances and extinction. Such a change is indicated by a shift of the mean oxygen isotopic composition (δ18O) of eggshell carbonates from −0.6%oto −5.3%o, and by changes in Sr. Information of palaeo-climate is primarily derived from eggshells of living birds and reptiles. The correlation between temperature and oxygen isotopic composition of waters (and related carbonates) is less distinct than for marine carbonates. δ13C ranges from −16.5 to −4.5 of eggshells of extant species indicate food from “normal” C3metabolism and from C4metabolism of plants in a dry climate.“Hypselosaurus”populations probably consumed “normal” C3plants. Using isotopic calibration of eggshell carbonates for the interpretation of δ13C and δ18O values of dinosaur eggshells, a slight change from higher to lower temperatures or a change from a dry to a more humid climate during the time from Lower (and Middle) to Late Maastrichtian can be assumed. The latter explanation is favored because the exceptionally high Sr in the Early Maastrichtian eggshells could be a potential indicator of co-existing evaporites.