Exceptional embryonic dinosaur eggs from the Early Jurassic of China indicate leathery egg origin of the earliest dinosaur

SCIENCE CHINA PRESS

Skull and embryo of Qianlong shouhu. — IMAGE:
PHOTOGRAPH (A) AND LINE DRAWING (B) OF THE SKULL IN RIGHT LATERAL VIEW. A WELL-PRESERVED EMBRYO (C) AND ITS 3D (D) RECONSTRUCTION SHOWING THE PREHATCHING POSTURE.
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CREDIT: ©SCIENCE CHINA PRESS

Dinosaur reproductive biology is a well-attended but highly debated area. In the past 30 years, the discovery of several exceptionally well-preserved fossils related to reproduction has greatly improved our knowledge in this field. However, due to the scarcity of fossil evidence and the lack of quantitative analysis within a broad phylogenetic scale, much of this area remains unknown, especially the pre-Cretaceous evolutionary history.

The current study reported a new Early Jurassic sauropodomorph dinosaur discovered in Guizhou, northwestern China. The specimens contain three skeletons from adult individuals and five egg clutches (Figs. 1, 2). The genus name is the combination of the Mandarin Chinese “Qian” (an alternative name for Guizhou Province) and “long” (dragon), and the species name “shouhu” means “guarding” in Chinese, referring to the associated preservation of adult skeletal fossil and embryo-containing egg fossils. Qianlong is a medium-sized basal sauropodomorph dinosaur, which is about six meters long and one ton in weight. The embryos display some differences from the adults, like a proportionally longer postdentary bone, a more vertical anterior margin of the snout, and fewer teeth. Allometric analyses of limb ratios among adult and embryonic specimens indicate that adult Qianlong was bipedal, but the babies were likely to be quadrupedal. The general taphonomical and sedimentary features indicate that Qianlong might possess reproductive behavior such as colonial nesting, similar to other basal sauropodomorphs including Massospondylus and Mussaurus.

Extant reptiles and birds lay eggs and are protected by eggshells. Eggshells can be divided into three types: soft-shelled eggs, hard-shell eggs, and leathery eggs. Soft-shelled eggs possess a soft shell membrane that lacks or has a very thin calcareous layer (generally less than 60 μm), and modern lizards (except some geckos) and snake eggs are soft-shelled eggs. Hard-shelled eggs bear a thick calcareous layer, such as bird eggs, crocodile eggs, and some turtle eggs. The eggshell thickness of leathery eggs is between the soft shell and the hard shell, but it shows flexible properties, such as the eggshell of Chelydra serpentina. The researchers employed multiple techniques to examine the eggshell microstructure of Qianlong (Fig. 2), including histological thin-section, electron backscatter diffraction, energy-dispersive spectroscopy, and scanning electron microscope. The results show that Qianlong possesses eggshell microstructures similar to other Cretaceous dinosaur egg fossils, consisting of the mammillary layer and continuous layer with well-developed eggshell units. The calcareous layer of Qianlong is about 160 μm, which is much thicker than that in most soft-shelled eggs (usually less than 60 μm), but thinner than that in hard-shelled eggs (usually more than 200 μm). A comparison of eggshell fragmentation among different eggshell types suggests that the eggshell surface of Qianlong bears extremely small fragments, similar to those found in leathery eggshells but distinct from the folded surface in soft-shelled eggs and surface with large fragments in hard-shelled eggs. These observations indicate that Qianlong laid leathery eggs.

To test the macroevolutionary patterns of selected reproductive traits across the dinosaur-bird transition, the researchers assembled a dataset that includes the data from 210 fossil and extant species among all major reptilian clades. The dataset was then used to test the evolutionary trends with multiple time-scaled phylogenies. “Our ancestral state reconstruction with different time-scaled phylogenies supported that the first dinosaur egg was leathery.” Yu said. This differs from the previous hypophysis that the earliest dinosaur eggs are soft or hard-shelled. Archosaurs, the most recent common ancestor of dinosaurs, pterosaurs, and crocodiles, also originated from leathery eggs, whereas pterosaur eggs originated from soft-shelled eggs (Fig. 3). Quantitative analysis of the relative egg size showed that crocodiles, dinosaurs and pterosaurs had an increasing trend in the process of evolution, with the most significant change occurring during the origin stage of theropods (Fig. 4). For egg morphology, the earliest dinosaurs were subcircular or oval, and ornithischian and sauropods were more rounded. Theropods tended to be elongated, especially oviraptorosaurs. In addition, through the analysis of the relative (egg size) thickness of the eggshell (only calcareous layer), it was found that most reptiles tended to thicken eggshells during the process of evolution except for pterosaurs and lepidosaurs. Finally, the morphological changes of the eggshell units in archosaurs were analyzed, and it was found that most groups of dinosaurs exhibited an elongated overall evolutionary trend, especially in theropod dinosaurs. The bird eggshell unit became longer and then shorter, unlike other reptiles such as crocodiles and turtles, which became shorter and broader.

“These results provide important information for understanding the productive biology of early dinosaurs and the evolution of dinosaur egg size, egg shape, and eggshell type.” Xu said.

The comprehensive study has spanned over eight years. Xing Xu and Fenglu Han (associate professor at China University of Geosciences) initiated their research on these materials in 2016. However, the embryonic eggs were not fully prepared until 2019, when Shukang Zhang joined the team to study the eggshell microstructure. Xing Xu, Yilun Yu and Fenglu Han began to do Quantitative analyses of dinosaur egg fossils from 2020. “The dataset was collected and checked carefully to ensure its accuracy. Study of these materials was hard and time-consuming, but fortunately, we persevered with the research and obtained some convincing results!.” Han said.