A few "eggs" actually facilitated the "return to one's roots" across billions of years?

Recently, "National Science Review" published a study on dinosaur reproductive biology led by Chinese scientists. The article focused on a unique buried fossil group discovered in Pingba District, Anshun City, Guizhou Province, including adult dinosaurs, dinosaur egg nests and embryos. It reported a new sauropod dinosaur - Qianlong shouhu, and innovatively proposed the origin of dinosaur leather eggs, which is different from the traditional origin of hard-shell or soft-shell eggs .

Hard-shelled eggs? Soft-shelled eggs? Leathery eggs? Are there so many secrets about eggs? Not only that, but the seemingly insignificant eggs have also helped many people to "return to their roots" across billions of years! This story starts with a boiled egg that was peeled...

1. Let’s start with a peeled boiled egg

When we peel a hard-boiled egg, there is a milky white membrane between the edible egg white and the calcium eggshell - the eggshell membrane (bird eggs usually have two layers). This eggshell membrane is usually composed of fibrous protein and is the starting point for the growth of the calcium eggshell. Calcite or aragonite crystals then grow on the outer layer of the eggshell membrane to form the outermost hard calcium shell of the egg to protect the embryo.

Basic structure of eggshell (Image source: Wikipedia)

Soft-shell eggs are usually composed of an eggshell membrane and a very thin amorphous calcium layer (less than 60 μm), such as snake eggs or most lizard eggs . This type of eggshell has good toughness, and when subjected to external force, the eggshell will bend or slightly fold.

Hard-shell eggs are made of an eggshell membrane and thick calcium carbonate crystals , and the calcium carbonate crystals (generally called shell units) present a specific shape and arrangement . The fragments of the hard shell are relatively large and have sharp edges. Crocodiles, birds and most dinosaur eggs are typical hard-shell eggs .

Leathery eggs are somewhere in between. Although the calcium layer of the eggshell is still thin (70-200 μm) and the eggshell has a certain toughness , the edges of the eggshell fragments are sharper and have a specific form of shell unit (calcite or aragonite). Among living animals , turtles, alligator turtles and some lizard eggs belong to this type.

Soft-shell, leathery-shell, and hard-shell eggshell breakage patterns (Photo courtesy of Han Fenglu)

The eggshell of Guardian Qianlong preserves the most complete microstructure of basal sauropods: the eggshell microstructure can be divided into two layers:

The calcite of the inner surface milky cones is arranged radially around the growth core ;

The calcite on the outer surface is composed of closely arranged columnar shell units.

Microstructure and EBSD image of the eggshell slice of Hukou Qianlong (Photo courtesy: Han Fenglu)

The microstructure of the eggshell is similar to that of the hard-shell dinosaur eggs of the Cretaceous period. However, the calcium layer is still relatively thin , and the fragmentation pattern is closer to that of leathery eggs , so the eggs of Qianlong Shouhu should be leathery. The appearance of the calcium layer of the eggshell is of great significance to the reproduction of dinosaurs. Compared with soft-shell eggs, the thickened calcium layer and shell unit of leathery and hard-shell eggs provide more protection for the embryo, protecting the egg from external damage during incubation and limiting the evaporation of water in the egg. At the same time, the pore structure in the eggshell can ensure normal gas exchange during embryonic development.

2. Eggshells: Unlocking the "Code" of Dinosaur Reproduction

Compared with the eggshell membrane composed of organic matter, the hard shell of calcium carbonate is easier to be preserved in fossils , so the eggshell is usually an important research object in the study of dinosaur eggs .

Scientists have established a sub-classification system for dinosaur egg fossils based on their macroscopic features such as shape, size, and eggshell patterns, as well as their microscopic features such as the morphology, arrangement, and pore distribution of the calcite crystals that make up the eggshell .

Based on this, scientists are able to scientifically and systematically understand the evolution of dinosaur eggshell structure and the reproductive characteristics and methods of dinosaurs .

Eggshell slice structures of different types of dinosaur eggs (Photo courtesy of Wu Rui)

Of course, since most dinosaur egg fossils only retain their appearance or eggshell fragments , it is difficult for us to directly link them with egg-laying dinosaurs, unless there are egg fossils preserved in association with bones, or more specifically, embryonic egg fossils preserved in egg fossils that have not yet hatched are found.

3. The unexpected egg that helped ancient creatures to “return to their roots”

However, the former is sometimes just a coincidence. For example, a nest of eggs was preserved next to the Mosaiceratops fossil found in the Nanyang Basin in Henan Province, China. Subsequent detailed research on the egg fossils and the discovery of embryonic egg fossils in the same area showed that this nest of eggs should have been laid by a large turtle (Nanxiong turtle) .

So far, only embryonic eggs of a few dinosaur groups have been discovered, including Torvosaurus, Lourinhanosaurus, Oviraptorosaurs, Troodontosaurs, Titanosaurs, Hadrosaurs, Therizinosaurs, and Protoceratops. The journey of other dinosaur eggs to "return to their roots" still has a long way to go.

Therefore, the unique burial conditions that protect the "trinity" of Qianlong adults, egg nests, and embryos provide an extremely important basis for studying the reproductive behavior of dinosaurs .

Oviraptorosaur embryo egg fossil (Image source: Wikimedia Commons)

Restoration of a sickle-like embryo egg (Image source: Wikimedia Commons)

Guarding the Qianlong embryo egg fossil (Photo source: Wikimedia Commons)

4. Rare soft-shell egg fossils

In addition to the hard-shelled dinosaur egg fossils (Protoceratops had soft shells), the soft-shelled egg fossils of some extinct animals have also been preserved due to the unique burial conditions.

Pterosaur eggs have been found in Argentina, as well as in western Liaoning and Hami, Xinjiang, China. The eggs of Hamipterus preserved in 3D in Hami have obvious plastic deformation depressions and are mainly composed of fluorapatite . This indicates that pterosaur eggs are likely to be soft-shelled, and the protein shell membrane was phosphated during the diagenesis process, forming a very thin calcium layer .

In addition, egg fossils that may belong to Mosasaur, Antarcticoolithus bradyi, were discovered in the Antarctic region, as well as embryonic eggs of Lycosaurs discovered in western Liaoning, China, all of which are also soft-shelled eggs.

Ecological restoration of Antarcticoolithus bradyi, a soft-shell egg found in Antarctica that is believed to have been laid by a mosasaur (Source: Wikimedia Commons)

Dinosaur egg fossils contain a wealth of information about dinosaur evolution, physiology, reproduction and even extinction. In the future, more well-preserved specimens like the Guardian Qianlong and new technologies will undoubtedly further our understanding of these extinct creatures.

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Author: Wu Rui, PhD student at the School of Earth Sciences, China University of Geosciences (Wuhan)

Reviewer: Han Fenglu, Associate Professor, School of Earth Sciences, China University of Geosciences (Wuhan)

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.