Eggshell fragments of ratite type (Ornitholithus, oofamily Elongatoolithidae)-attributed to giant birds-are described from Upper Paleocene deposits in the Pyrenean-Provence region. We study the porosity of a rich Ornitholithus sample from Claret-4 locality (Tremp Fm, Lleida, Spain), closely related to the global warming event which took place at the Paleocene-Eocene boundary (PETM), in order to infer relations between egg conductance, nesting habits and palaeoenviromental conditions. An egg mass of around 3 kg is inferred for the large Ornitholithus eggs, in agreement with the values previously estimated by Touraine [Touraine, F., 1960 Porosity observed in Ornitholithus eggshells is extremely reduced in comparison to bird eggs and even much more reduced in relation to theropod dinosaurs. Pore number is similar to that of bird eggs but most Ornitholithus pores are exceedingly small (micropores), thus pore radius and total pore area is much lower than in recent birds. Moreover, Ornitholithus micropores lack connectivity between them, thus decreasing functional pore area and permeability. The function of micropores could be related to weight spare, favorable hatching and/or thermal isolation. Ornitholithus eggshells are the most compact of all bird or theropod eggshells studied so far. Consequently, calculated water vapor conductance values in Ornitholithus eggs are in average 8 times lower than that for recent birds. The relation between egg conductance and nesting environment shown in recent birds suggests, in the case of Ornitholithus, an open and extremely dry nest environment. Anomalous compact Ornitholithus eggshells could help to keep eggs' inner relatively-constant temperature and humidity, in the extremely warm and dry palaeoenvironment inferred for the Pyrenean region at the Paleocene Eocene Thermal Maximum.