As the earliest identified fossil group produced by non-avian dinosaurs, the classification of elongatoolithid eggs has attracted considerable attention in the field of paleobiology.
A team led by Wang Xiaolin from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, in collaboration with the National Museum of Nature, conducted in-depth research on the microscopic structure of elongatoolithid eggshells found in China. They have redefined the current classification of elongatoolithid eggs in China, identifying two families and five genera, namely the elongatoolithid family with the genera elongatoolithid, gigantoolithid, and furculoolithid, and the giant elongatoolithid family with the genera giant elongatoolithid and giant oofundus.
The histological sections of eggshell tissues in this study are shown in the microphotographs (left, normal light; right, orthogonal polarized light). The research team/credit conducted a study that recharacterized the microstructure of eggshells of the genus Qidanzu in the family Qidanzu, previously classified as elongatoolithid eggs in China, based on traditional hard tissue histological sections combined with scanning electron microscopy and electron backscatter diffraction techniques. This research has provided a wealth of new insights, offering a new reference basis for the sub-system classification of related taxa. The related paper was recently published in the international professional academic journal "Paleobiology."
The corresponding author of the paper, Associate Researcher Wang Qiang from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, pointed out that elongatoolithid eggs, including the elongatoolithid family and the giant elongatoolithid family, are common groups in Asia and North America and are one of the earliest fossil groups to adopt sub-system classification. Among them, the genera Elongatoolithus, Oofundus, and Macroelongatoolithus are known to be directly related to oviraptorid dinosaurs.
The backscattered electron diffraction image of the eggshell in this study. Research team/Provided image With the discovery of more and more dinosaur egg materials, the challenge arises as to whether the remaining elongated egg members, apart from the three mentioned categories, also belong to the Oviraptorosauria. Previous studies have shown that the eggshells of Oviraptorosaurs are very similar to those of the elongated egg members attributed to Oviraptorosaurs, based solely on histological sections and scanning electron microscopy, thus requiring new technological means for differentiation.
Researcher Wang Xiaolin stated that the application of electron backscatter diffraction technology with scanning electron microscopy to obtain crystallographic information of polycrystalline samples not only provides information on grain size, shape, and distribution but also yields information on the orientation, phase distribution, grain boundary types, and other related aspects of each grain. This technology has been widely used in the metallurgical and geological industries.
Scanning electron microscope images of Constanti's black mountain egg fossils provided for this study. Research team/Image source. In recent years, electron backscatter diffraction technology has played an important role in the study of eggshell microstructure, gradually becoming a conventional research method that can objectively characterize the crystal arrangement features of eggshells. This study represents the first domestic use of this technology to analyze elongated eggs, providing further clarification and refinement to existing classification schemes.
Dr. Zhu Xufeng, the first author of the paper and an assistant researcher at the National Museum of Nature, who graduated from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, explained that the crystal boundary images obtained through electron backscatter diffraction technology have led to several important discoveries in this study.
Scanning electron microscope images of Peng's ripple egg fossils in this study. The research team/credit 1 has identified for the first time an outer band near the outer surface of the continuous layers of Changshihe eggs, a structure more common in modern bird eggshells but less so in non-avian dinosaur eggshell types. Combining the scattered nodular ornamentation features on the eggshell's surface, the research team no longer supports its classification as a member of the elongatoolithid family based on current evidence, suggesting further research on eggshells with similar microstructural features, such as the continuous egg genus in North America, to determine their relationships.
2 The discovery of needle-like cones, columnar continuous layers, and reticular ornamentation in the subelongatoolithid genus is essentially identical to known oviraptorid eggshells, differing significantly from typical elongatoolithid members like Elongatoolithus and Macroolithus, thus should not be considered part of the elongatoolithid family.
3 Similar to the subelongatoolithid genus, the Nanxiong egg genus also features needle-like cones and columnar continuous layers, but due to the lack of suitable specimens, the morphological characteristics of its ornamentation cannot be determined. If future studies confirm the presence of reticular ornamentation, it may be treated similarly to the subelongatoolithid genus.
Scanning electron microscope images of Peng's ripple egg fossils from this study. The research team further discusses the criteria for determining scale-like ultrastructures in grain boundary maps and proposes two types of scale-like ultrastructures, Type I and Type II, based on different grain boundary features. Upon reexamining the electron backscatter diffraction data of all published eggshells, it was found that several egg genera identified as oviraptorids are highly developed Type II, while egg genera previously excluded from elongatoolithids and possibly belonging to caenagnathids or therizinosauroids exhibit only Type I development or Type II development exclusively on the outer side of the band.
Additionally, the research team also discovered the first histological evidence of egg resorption in elongatoolithid eggs in Peng's ripple egg samples. Significant abnormal layers were observed in the grain boundary maps of the eggshell, appearing outside the normal continuous layers, a structure generally attributed to maternal stress, resulting from egg resorption in the oviduct leading to interrupted eggshell deposition, a phenomenon previously documented in titanosaur eggshells.
Based on the above research findings, the research team concludes that elongatoolithid eggs in non-avian dinosaur egg fossils in China include elongatoolithid eggs of the elongatoolithid egg family, giant egg genus, ripple egg genus, as well as giant elongatoolithid egg genus and giant spindle egg genus of the giant elongatoolithid egg family, while the Nangong egg genus is temporarily classified under the elongatoolithid egg family. (End)
