The first dinosaur eggs were not what we expected

A recent study of the newly discovered, early Qianlong Shuhu Jurassic Sauropodomorphs indicate that eggs of the earliest dinosaurs had leathery shells. This finding, derived from comprehensive analyzes of dinosaur fossils, indicates major evolutionary changes in the shape of dinosaur eggs. The study also contributes to our understanding of the ancestral reproductive traits of various reptile species.

Reproductive biology of dinosaurs

The discovery of numerous exceptionally preserved reproductively related dinosaur specimens over the past three decades has improved our knowledge of dinosaur reproductive biology. However, due to limited fossil evidence and the lack of large-scale quantitative analysis of dinosaur evolution, much about dinosaur reproduction, especially pre-Pleistocene, has remained unclear.Cretaceous period Evolutionary history.

Breakthrough in egg morphology research

However, the recent fossil discovery by researchers from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences (CAS), as well as associated analyses, indicate that the first dinosaur egg was made of leather, and that the major transformation in Egg morphology occurred early in the evolution of theropod dinosaurs and not near the origin of birds.

Reconstruction of the adult and juvenile Qianlong skeleton. Credit: IVPP

Discovery of Qianlong Shuhu

The study was recently published in National Science Reviewannounces the discovery of specimens of a new sauropodomorph dinosaur from the early diverging Jurassic period Classify In Guizhou, China —Qianlong Shuhu– It consists of three adult skeletons and five clutches of eggs. This discovery may represent the oldest fossil record of an association between adult dinosaurs and nestlings, and the species name reflects this association: Qianlong Means “Guizhou Dragon” while shouhu Means “guarding” – a reference to the preservation of adult skeletal fossils in association with fossil eggs containing embryos.

Qianlong It was a medium-sized basal dinosaur, weighing one ton and measuring about six meters (20 feet) in length. Embryos show some differences from adults, for example, a relatively longer skull, a more vertical front edge of the snout, and fewer teeth.

Comparison of fossil eggs (c) with extant soft-shelled, leathery, and hard-shelled eggs. Credit: IVPP

Insights into Qianlong behavior and anatomy

Standard analyzes of limb proportions between adult and embryonic specimens point to that of the adult Qianlong It was able to walk on its hind limbs, but babies were likely quadrupeds. The general taphonomic and sedimentological characteristics indicate this Qianlong It may have practiced colonial nesting as a reproductive behavior, similar to other basal sauropods including Masospondylus And Moussorus.

The researchers also examined the microscopic structure of eggshells Qianlong Using multiple techniques, including histological thin section, electron backscatter diffraction, energy dispersive spectroscopy, and scanning electron microscopy. The results showed that Qianlong It had eggshell microstructures similar to other Cretaceous dinosaur egg fossils, which likely consisted of two layers—stratum mammillaria and stratum continuous—and had fully developed eggshell units. Limestone layer Qianlong The eggs were thicker than most soft-shelled eggs but thinner than hard-shelled eggs. Comparison of eggshell fractionation between different eggshell types also indicates that the surface of the eggshell Qianlong Small fragments, resembling leathery eggshells, appeared in contrast to the folded surface of soft-shelled eggs or the large segmented surface of hard-shelled eggs. These observations indicate that Qianlong She lays leathery eggs.

Summary of ancestral eggshell type for major nodes with maximum posterior probabilities in all ASR analyses. Credit: IVPP

Evolutionary trends in dinosaur eggs

In order to test macroevolutionary patterns for selected reproductive traits across the dinosaur-to-bird transition, the researchers compiled data from 210 fossil and extant species, representing all major reptile clades, and tested evolutionary trends using multiple time-referenced lineages.

They found that relative egg size decreased from the base of the diapsida to the base of the saurischia, but showed an increasing trend from the early theropods to the crownbird node. The most significant increase in egg size occurred early in the evolution of theropods. Regarding eggshell thickness, they discovered that thickness tended to decrease from the base of archosaurs to the base of saurischia, followed by a significant increase in eggshell thickness early in the evolution of theropods. An increasing trend in eggshell thickness also occurred in the evolution of sauropodomorphs.

Egg shape was generally preserved during the evolution of diploids into living birds. For example, although theropod egg elongation reached its peak in oviraptorosaurs—with the greatest egg elongation among diploids—it would later revert to its ancestral state. As a result, all branches of crown birds inherited only slightly elongated eggs.

Overall, the reconstruction of the ancestral state of different types of eggshells supports the conclusion that the first dinosaur egg was probably leathery, relatively small, and oval in shape. Furthermore, leathery eggshells may have been the ancestral condition of Avemetatarsalia, Arcosauria, and Testudines.

Reference: “Extraordinary Early Jurassic Fossils with Leathery Eggs Shed Light on Dinosaur Reproductive Biology” by Fenglu Han, Yilong Yu, Shukang Zhang, Rong Zeng, Xinjin Wang, Huiyang Cai, Tianzhuang Wu, Yingfeng Wen, Sifu Cai, Chun Li, Rui Wu , Chi Zhao and Xing Shu, October 9, 2023, National Science Review.
doi: 10.1093/nsr/nwad258