Evolution of turtle reproductive strategies. Courtesy of the research team
BEIJING, Aug. 19 (Xinhua) -- Chinese researchers accidentally obtained a strange-looking egg fossil during a field expedition in Henan three years ago — the size of the egg, the overall spherical shape, the eggshell is extremely thick, and the structure of the suspected bone was found in the broken eggshell. Who the hell laid the egg?
Eggshell diameter slice: a single polarized microscope image; b orthogonal polarized microscope image; c sketch diagram; d-scan electron microscope image; e cathodoluminescent image Courtesy of the research team
The research team of Associate Professor Han Fenglu of China University of Geosciences (Wuhan) and the latest collaborative research of Chinese and foreign counterparts confirmed that the egg fossil is an egg produced by the Nanyang Yu turtle, a member of the large nanxiong turtle family that lived in the late Cretaceous period (about 100 million to 66 million years ago).
Turtle embryo egg imaginary restoration diagram. Courtesy of the research team
This is the first detailed study of the fossils of Cretaceous turtles and turtles in Henan by the paleontological community, and it is also the first time that turtle eggs and specific adult genera species have been linked according to embryos. The result paper was recently published online by the international authoritative biological journal Journal of the Royal Society B-Biological Sciences.
Egg fossils are embryos of turtles
Originating in the late Triassic period, turtles and tortoises have lived for more than 200 million years and are found throughout most of the world, making them an evolutionarily successful reptile.
Since the Mesozoic Era, a large number of turtle and turtle skeletons and egg fossils have been preserved around the world, but turtle and turtle embryo fossils have rarely been found, so researchers usually lack sufficient evidence to determine the kinship of egg fossils, as well as the understanding of early turtle and turtle reproductive strategies, embryonic development and so on.
At present, only fossil turtle embryos have been officially reported in Germany, Mongolia, Brazil, the United States and Henan, China, but these studies have not conducted in-depth studies of embryonic bones.
In June 2018, after Han Fenglu and his colleague Jiang Haihai obtained egg fossils in the field in Henan, they realized that it might be a major discovery, and the research team then scanned the egg fossils with high-precision microscopic CT (electronic computed tomography), and the reconstructed 3D images clearly indicated that this was an embryonic fossil of a turtle, and it became the second turtle embryo fossil known in China.
The restoration and reconstruction of embryonic bones, the left figure is the 3D reconstruction map, and the red part of the right figure is the identified turtle embryo bone. Courtesy of the research team
Confirm the eggs laid by the Nanyang Yu turtle
Han Fenglu said that the egg fossil preservation bone is relatively loose, fragile, not easy to repair, in order to maximize the access to its embryonic bone information, while reducing the damage to the fossil, the research team decided to use micro-electronic computed tomography (micro-CT) to scan the egg fossil as a whole, and use 3D reconstruction software to model the internal embryonic bones in three dimensions.
Although the embryonic bone of the egg fossil indicates the morphology of the turtle, its eggshell is an outlier, especially the thickness of the eggshell reaches nearly 2 mm, which is thicker than the eggshell of all living turtles. "Which turtle lays this egg?" It is an urgent problem to be solved."
The research team joined hands with the University of Calgary, Canada, the Royal Tyrrell Paleontology Museum of Canada, the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences and other turtle and egg fossil research experts and scholars, they used the living turtle adult and the function relationship between the size and weight of the egg to infer that the adult egg fossil turtle body length reached 1.6 meters, and finally confirmed according to the body size, bone characteristics and origin and stratigraphy. The egg fossil is an egg laid by a group of extinct giant southern male turtle members of the Family Ofs Nanyang Yu.
Turtle embryonic egg fossil photo, white arrow in the picture indicates the exposed embryonic bone. Courtesy of the research team
Why do you have extremely thick eggshells?
Because the extremely thick eggshell of the Southern Male Turtle family has now completely disappeared, it is impossible to compare ancient and modern, and it is impossible to draw accurate conclusions. Therefore, researchers have also caused great controversy about this and proposed multiple hypotheses, including extremely arid environments, the greenhouse effect of the Cretaceous Period, and abnormal pH of the surrounding environment.
The research team conducted a detailed demonstration of the above hypotheses, and finally concluded that this thick eggshell of the Southern Male Turtle family is likely to adapt to an abnormal extreme environment, which is a specialization, and when the environment changes, it is also eventually eliminated.
It is worth mentioning that although the large-scale southern male turtle was completely extinct at the end of the Cretaceous Period, the rafters, which are closely related to the southern male turtle family and have a slightly thinner eggshell, have successfully continued to the Cenozoic.
In addition to the study of the fossil embryo of the Nanyang Yu turtle egg itself, Ke Yuzheng, the first author of the result paper and a master's student of China University of Geosciences (Wuhan), also studied, counted and discussed the reproductive evolution strategy of the whole turtle, and concluded that the breeding strategy of large-scale turtles in the evolutionary process generally showed the evolutionary law of "the size of the egg decreases, the eggshell becomes thinner, and the number of eggs increases".
"The embryo of this giant turtle is precious, it tells us that the turtles of the same era as the dinosaurs, there are many unknown worlds waiting for us to explore." Han Fenglu said. (End)
Source: China News Network