Reproduction of mating behavior of mesozoans (drawn by Yang Dinghua)
Three-dimensional reconstruction of different mesozoans
Comparison of sexual reproductive organs of mesozoans (fossils on the left, modern taxa on the right): a and b: grippers; c and d: cross-section of the egg; e and f: sperm pump; g and h: sperm of the mesozoan; i and j: half penis; k and l: mesozoan sperm detail structure
According to the Nanjing Institute of Geology uux.cn and Paleontology of the Chinese Academy of Sciences: Mesetrozoans are a class of aquatic miniature crustaceans with double-petaled shells that have lived on Earth for nearly 500 million years. Fossils of the formation that preserve its double-lobed shell are abundant, but their soft parts are often not easily preserved. Recently, a team of researchers from China, Germany and the United Kingdom used high-resolution microtomography to study the fossil specimens of mesozoans with soft bodies preserved in Cretaceous Burmese amber, showing that the use of giant sperm for sexual reproduction of mesozoans existed 100 million years ago, and the results also advanced the earliest known animal sperm records by at least 50 million years. The research was published online as a cover paper on 16 September 2020 in the Proceedings of the Royal Society B.
Mesozoans are aquatic miniature crustaceans with a two-lobed shell , usually about 1 mm in size. Since the Ordovician, mesozoans have been one of the richest arthropod fossils in the geological record. Mesozoan fossils not only have important biostratigraphic significance, are one of the indispensable fossil categories in oil and gas exploration, but also play an important role in paleoenvironment reconstruction and biological evolution research. However , the vast majority of mesozoan fossils retain only calcified shells , and the soft parts ( appendages , bodies , etc.) are often difficult to preserve. But these soft structures are often able to provide a lot of important paleobehaviorological information, such as reproductive behavior.
Recently, Dr. Wang He and Researcher Wang Bo of the "Research Team on the Origin and Early Evolution of Modern Terrestrial Ecosystems" at the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences, in cooperation with German and British researchers, reported for the first time the fossils of mesozoans with soft bodies preserved in Cretaceous Burmese amber.
Weighing 0.676 g, the amber is 17.5 mm long, 13.5 mm wide and 4 mm thick, and a total of 39 fossil specimens of mesozoans have been preserved. Mesosomes are 0.59 mm in maximum and 0.24 mm in minimum. These mesozoans belong to three distinct taxa and contain a more complete developmental sequence from juvenile to adult. By comparing the living environment with the living environment of the living medium, it is believed that these mesozoans lived in the lagoon environment near the sea at that time.
The research team used high-resolution microtomography (micro-CT) to analyze 11 of these specimens, and through nearly a year of high-precision three-dimensional image reconstruction work, the morphological characteristics of the shells and soft bodies of these mesembranous insects (including appendages, grippers, Zengkel's apparatus (sperm pumps), half penises, seminal vesicles, eggs and giant sperm) were finely restored. Based on the analysis of fossil functional morphology and the morphological and ecological observation of living mesozoans, it is believed that when one type of fossil mesozoans undergoes sexual reproduction, the male grasps the female through the hook-like gripper and extends the half penis into the female's body, and during mating, the male sends giant sperm into the female through the Zengkel organ (sperm pump). Because of its well-developed muscles and threaded spines, Zengkelite is easily found in living individuals. Giant sperm will be stored in the female seminal vesicles, after which the sperm will begin to be active to complete the fertilization process.
In this study, the giant sperm of the current mesozoan was found for the first time in the Cretaceous mesozoan fossils, which are at least one-third the length of the mesozoan body. The discovery is also the earliest known record of animal sperm, advancing its fossil record by at least 50 million years. The fossil study shows that the reproductive organs associated with the sexual reproduction of modern mesembranths (such as grippers, Zengkel's organs, etc.) have been formed at least in the middle of the Cretaceous period, and their morphological characteristics have not changed over 100 million years, and further indicate that this behavior of mesozoans using giant sperm for sexual reproduction existed 100 million years ago, providing an important example of the phenomenon of evolutionary stagnation of reproductive behavior. This study also suggests that this complex reproductive behavior, which contains giant sperm, improves the success rate of mesozoan mating and promotes the radiation evolution of non-marine mesozoic species in the Late Mesozoic.
This research work is funded by the Chinese Academy of Sciences, the Ministry of Science and Technology and the National Natural Science Foundation of China. Wu Suping of the Nanjing Institute of Paleontology assisted in the micro-CT experiment, and Yang Dinghua drew an ecological restoration map.
论文信息：Wang H, Matzke-Karasz R, Horne DJ, Zhao X, Cao M, Zhang H, Wang B. 2020 Exceptional preservation of reproductive organs and giant sperm in Cretaceous ostracods. Proceedings of the Royal Society B 20201661. http://dx.doi.org/10.1098/rspb.2020.1661.