A team of paleontologists has discovered the world's oldest animal sperm to date, which were permanently preserved in mesozoans in Burmese amber 100 million years ago. It can be up to one-third the size of the entire body length of the mesozoite and is called a "giant sperm".
Mesosome mating behavior restoration diagram
The research team consisted of researchers from China, Germany and the United Kingdom. The study found that the use of giant sperm for sexual reproduction by mesozoans existed 100 million years ago, bringing forward at least 50 million years of the oldest known animal sperm record.
The results were published online on September 16 in the Proceedings of the Royal Society B.
Some personnel from the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences participated in the study. The researchers say mesozoans are a class of aquatic miniature crustaceans with double-petaled shells that have lived on Earth for nearly 500 million years and can be found today in many oceans. Fossils of the formation that preserve its double-lobed shell are abundant, but its soft parts are usually not easily preserved.
The researchers found that the Cretaceous Burmese amber was wrapped in 39 fossil specimens of mesozoans, including males, females and larvae, preserving their reproductive organs in unprecedented detail. The Cretaceous period was between about 145 million and 66 million years ago.
Three-dimensional reconstruction of different mesozoans
The amber weighs 0.676 g and is 0.59 mm long and 0.24 mm in the individuals of mesozoans wrapped in it.
The research team used high-resolution microtomography 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 organs, half penises, seminal sacs, eggs and giant sperm) were finely restored.
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
Some female mesenchymal seminal vesicles are in a state of expansion and are already filled with sperm. In other words, they have just successfully mated and then are permanently preserved as a research sample for scientists 100 million years later.
Why is it called a giant sperm? Because its individual length is actually at least 200 μm (0.2 mm), which is equivalent to one-third of the entire body length of the mesenchymal.
The study believes that this class of mesozoans has provided conclusive evidence that sexual reproduction was the norm as early as 100 million years ago. Its body structure suggests that during mating, the male passes through a hook-like gripper, grabs the female and then pumps the giant sperm into the female through the sperm. The giant sperm and eggs then complete the active fertilization process in the female's seminal vesicles.
The researchers note that modern mesozoans still mate in very similar ways, meaning that this mechanism has not changed for more than 100 million years.
Most males in the animal kingdom, including humans, produce tens of millions of tiny sperm cells, but mesomorphs are different—for them, quality is more important than quantity.
Scientists have been studying Burmese amber for decades and have found a variety of preserved "treasures," including frogs, snakes and a feathered fossil of a dinosaur's tail.
Nandu reporter Shi Minglei
Source: Website of Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences