Uppermost Cretaceous megaloolithid eggs from the Haţeg Basin, Romania, associated with hadrosaur hatchlings: Search for explanation

Abstract

Four localities in the Upper Cretaceous (Maastrichtian) of the Haţeg Basin (in order of discovery, Tuştea, Toteşti-baraj, Nălaţ-Vad and Livezi) have yielded clutches of megaloolithid eggs. Egg morphology and size, eggshell thickness, external ornamentation and internal microstructure, pore density and geometry, as well as morphology of the clutches (where this could be observed) are all similar among the four localities, allowing the assignment of the eggs to Megaloolithus cf. siruguei. Most egg occurrences are related to paleosols of variable chemical composition and maturity, developed within different parts of the floodplain. The nesting horizon from Tuştea was partially reconstructed on a 160 m 2 surface, allowing a thorough analysis of the taphonomy of the eggs and clutches. It has also yielded hatchling remains at different ontogenetic developmental stages, and even, more rarely, embryo remains as well. The bones occur in the vicinity of or even within the clutches; in a single case, incompletely ossified embryonic skeletal remains, including a dentary tooth, were found inside an incomplete egg. Without exception, the neonate remains belong to the basal hadrosaurid Telmatosaurus transsylvanicus, one of the common dinosaur species in the faunal assemblage. No sauropod neonate remains were found in the nesting horizon from Tuştea; only a fragmentary pelvic girdle and caudal vertebrae of an adult titanosaurian were unearthed recently 20 cm above the nesting horizon. None of the other three megaloolithid localities in the Haţeg Basin provided neonatal remains of any dinosaur species. Despite the general consensus that the Megaloolithus oogenus belongs exclusively to titanosaurian sauropods, the co-occurrence of megaloolithid eggs and hadrosaurid neonatal remains at Tuştea seems to contradict this view. Previous cladistic analyses of dinosauroid ootaxa might offer an explanation of this controversial issue. These analyses have revealed that Megaloolithidae appears to be the sister group of Spheroolithidae, usually regarded as a hadrosaurid egg family. Perhaps a significant amount of homoplasy is present in the evolution of dinosaurian eggs, whose structure depends on incubation environment as well as biology and physiology of the reproductive system itself, and that the oospecies of Megaloolithus might have been laid by different higher-level taxa, including both titanosaurian sauropods and (basal) hadrosaurids. Understanding the significance of this paraphyletic distribution requires further study.