During the later part of the Cretaceous global sea levels were extremely high, and much of Europe was flooded, leaving he remaining land masses as a string of islands. One of the best studied of these is Haţeg Island, in modern Romania, where the dinosaur population showed signs of island endemism, including Dwarf Titanosaurs - in other places the group of dinosaurs that reached the largest sizes of any terrestrial vertebrates that ever lived.
On 8 March 2012, in a paper in the journal PLoS ONE, a team of scientists lead by Gerald Grellet-Tinner of the The Field Museum in Chicago, The Journey Museum in Rapid City, South Dakota and Consejo Nacional de Investigaciones Científicas y Técnicas in Argentina, discuss the discovery of a number of clutches of Titanosaur eggs (Grellet-Tinner et al. avoid the use of the term 'nest' since there is no sign of any construction around the eggs) from the Haţeg Basin; the modern analogue of the Cretaceous Haţeg Basin.
Clutches of eggs from the Haţeg Basin site, now on display in the University of Cluj and the Royal Belgian Institute of Natural Sciences. From Gerald Grellet-Tinner et al. (2012).
The eggs date from the Maastrichtian, the final stage of the Cretaceous, which lasted from 70.6 to 65.5 million years ago. There are two Dwarf Titanosaur species described from this stage on Haţeg Island, Paludititan nalatzensis and Magyarosaurus dacus, although the later may be more than one species. A large number of dinosaur eggs have previously been recovered from the area.
Sketch of Paludititan nalatzensis, from palaeoartist Robinson Kunz.
Some larger, thicker-shelled, eggs from Haţeg Island have previously been described as Titanosaur eggs, though Grellet-Tinner et al. do not agree with this. The new eggs were assigned to Titanosaur origins on the basis of shell microstructure, which closely resembled that of previously described Titanosaur eggs from Mongolia and South America. The clutch sizes were smaller than those of Titanosaurs from elsewhere, which Grellet-Tinner et al. interpret as a symptom of the small size of the adults. In modern reptiles small species typically produce smaller clutch sizes than larger, related species.
The microstructure of Titanosaur eggs. (A & C) Eggs from the Haţeg Basin in Romania. (B & D) Eggs from Auca Mahuevo in Argentina.
The eggs were buried on four discrete levels in fine grained mudstone, a large number of individual clutches on each level, suggesting that the eggs were laid communally. There is no way to know which species of Titanosaur laid the eggs, indeed it is possible that not all the eggs were laid by adults of the same species.
Intrusions of the mineral palygorskyte were found in pores of some of the eggs. This is often, but not exclusively, associated with hydrothermal springs. The mudstone matrix was also rich in strontium and barium and other rare earth elements, which is also commonly a sign of hydrothermal activity. Taken together Grellet-Tinner et al. suggest that this was evidence that the Titanosaurs may have used heat from hydrothermal activity to help incubate the eggs. This behavior is known from some species of Megapode Birds in Polynesia, and has previously been suggested at dinosaur egg sites in Argentina.
The location areas where Titanosaur eggs have been found in Romania (black stars). Green areas with red dots show signs of volcanic activity, denser dots showing more volcanic influence).
See also A Mamenchisaurid Sauropod Dinosaur from Thailand, A Titanosaur from the Antarctic, An American Titanosaur and Dinosaurs on Sciency Thoughts YouTube.