The international research team reporting the discovery includes Dr. Zoltán Csiki-Sava, professor at the Faculty of Geology and Geophysics from the University of Bucharest
A new research, recently published in the journal Papers in Palaeontology, yielded a conclusive, though surprising, answer concerning a strange type of fossil eggshells by using an innovative investigation technique.
The research was conducted by an international team led by Seung Choi from the Seoul National University (South Korea) and including Yuong-Nam Lee (Seoul National University, South Korea), together with Miguel Moreno-Azanza (Universidade Nova de Lisboa; Museu de Lourinhã, Portugal), Zoltán Csiki-Sava (University of Bucharest, Romania), and Edina Prondvai (Ghent University, Belgium, currently at the MTA-MTM-ELTE Research Group for Paleontology, Hungary).
The team succeeded to determine the true identity of a peculiar type of fossil eggshells called Pseudogeckoolithus, originating from the end of the Cretaceous Period (roughly 85 to 66 million years ago), eggshells that confused European paleontologists for several decades.
The Pseudogeckoolithus eggshells are widely distributed across southern Europe and also appear in northern Africa. In Romania, they are common in the dinosaur-bearing fossiliferous rocks of the Hațeg Basin, within the Hațeg Country UNESCO Global Geopark. The eggshells are thin (usually less than 0.3 mm). Whereas some aspects of their microstructure resemble those of the eggshells of some theropods (that is, predatory dinosaurs), other features are reminiscent of those of modern gecko (lizard) eggshells. Furthermore, they are characterized by a unique node-like ornamentation, being in this respect also remarkably similar to the eggshells of some modern geckoes. Thus, these fossils were commonly referred to as ‘geckoid’ eggshells by the experts.
“This was a case of a very confusing signal within the European Late Cretaceous fossil record. Due to its similarity to modern gecko eggshells, Pseudogeckoolithus was often regarded as a proof supporting the presence of ancient gecko-like lizards in the tropical archipelago covering the southern part of Europe at the end of the reign of dinosaurs” says Zoltán Csiki-Sava from the University of Bucharest, one of the co-authors of the study.
In order to clarify the identity of the Pseudogeckoolithus egg-layer, the team investigated such ‘geckoid’ eggshell fossils from Romania (originating from the Geopark, but also from the Transylvanian Depression, in the neighborhood of Sebeș, Alba County), Hungary, and Spain. They used a novel technique called Electron Backscatter Diffraction (EBSD) analysis to reveal their detailed crystallographic pattern, and compare it with that of modern geckos as well as of birds and extinct bird-like dinosaurs, the so-called maniraptorans.
“We are now fully aware that modern gecko eggshells and typical dinosaur (including bird) eggshells have completely different crystallographic arrangement. Thus, we decided to compare the crystallographic image of Pseudogeckoolithus to that of the eggshells of modern geckoes and birds, as well as those of small predatory dinosaurs, to solve the Pseudogeckoolithus enigma” explains Seung Choi, who had previously studied in detail both gecko and predatory dinosaur eggshells using the same technique.
Indeed, the researchers could confirm that Pseudogeckoolithus, with its eggshell crystals growing from the inner towards the outer shell surface, has a completely different crystallographic arrangement from that of modern gecko eggshell, where crystal growth appears to proceed from the outer surface inwards. Instead, Pseudogeckoolithus eggshells show a crystallographic arrangement typical to birds and their maniraptoran ancestors, to which the main villain from the movie Jurassic Park, Velociraptor, also belongs.
“Pseudogeckoolithus eggshells are so thin and fragile, that it is often difficult to recognize pristine fragments that preserve all the features typical to theropod eggshell, such us multiple layers with different types of crystals. Nevertheless, EBSD allowed us to identify these structures even in the less well preserved specimens, confirming that Pseudogeckoolithus is not a true gecko eggshell. Accordingly, there is no evidence that geckoes were common in the Upper Cretaceous of Europe, as often interpreted previously” comments Miguel Moreno-Azanza, from Universidade Nova de Lisboa and Museu de Lourinhã. “The misleading identity of Pseudogeckolithus was already addressed by Nieves López-Martínez and Monique Vianey-Liaud who named it back in 1997. They classified this eggshell as dinosaurian but named it ’gecko-like stone egg‘, which is what Pseudogeckoolithus stands for. Now, these new techniques allowed us to firmly corroborate their initial observation – these eggshells originate from eggs laid by dinosaurs” he adds.
In fact, the pan-European distribution of Pseudogeckoolithus eggshells actually implies the common presence of small, likely closely related bird-like predatory dinosaurs throughout Europe during the Late Cretaceous. “It is remarkable how widespread the Pseudogeckoolithus-type eggshells are. They turn up practically everywhere in the Late Cretaceous archipelago, regardless of geographic position, sedimentary environment or age of the rocks. Now, based on the abundance of their eggshells, we can envisage easily the large number of small feathered predatory dinosaurs that roamed these tropical islands, even when their body fossils are very rare or even completely absent.” explains Zoltán Csiki-Sava the significance of their results.
Shockingly, Pseudogeckoolithus has many features in common with the eggshells of megapodes, modern birds that do not brood their eggs; instead, they build large mounds of dirt and vegetation and bury their eggs inside. In the mound, organic matters are fermented, producing the heat required to hatch the eggs. Both Pseudogeckoolithus and megapode eggshells are thin, have funnel-like breathing canals, and disperse node-like ornamentation, a combination of traits that is otherwise very rare in modern bird eggshells. “Based on these features and the eggshell thickness proportions, the eggs themselves were probably no larger than the eggs of a crow (~ 3×4 cm) and were laid in a buried nest. However, the mother herself was likely larger than a crow, maybe about the size of a chicken, because the dinosaurian pelvis was narrower, and hence their eggs were proportionately smaller than those of modern birds.” says Edina Prondvai.
“This study is a very good example as to why and how advanced and sophisticated analytical methods become more and more important in paleontology. They can not only identify fossil organisms where other, traditional methods fail to deliver, but can also shed new light on their distribution, life and evolution” concludes Seung Choi, lead author of the study.
The paper reporting this research can be accessed at https://onlinelibrary.wiley.com/doi/abs/10.1002/spp2.1294.