Mesomorphs, an ancient living fossil that has been around since the early Ordovicians and has lived on Earth for nearly 500 million years and is still quite prosperous today. Recently, it has provided many interesting and worthy of study for the sexual reproduction of animals in nature, and its origin and evolution history.
Recently, a project led by the Chinese Academy of Sciences and jointly with German and British researchers was conducted to study a fossil mesozoan with a soft body preserved in Cretaceous Burmese amber. Through high-resolution micro-CT, the characteristics of the shell and soft body of these 39 little guys with a history of 100 million years but less than 0.6 mm were restored in a full year.
And the most interesting thing is that 100 million years ago, the moment they were coated in resin to form amber, some female mesembranous seminal vesicles were in a state of expansion and were already full of sperm. In other words, they have just successfully mated and are then permanently preserved as a research sample for scientists 100 million years later.
From the cross-sectional view of the amber fossil scan, the male mating organ "beat" of the mesozoate is still clearly visible, as well as the sperm pump, giant sperm, eggs, and the most research-worthy female container filled with giant sperm - sperm sacs. Why is it called a giant sperm? Because its length is actually at least 200 μm (0.2 mm), which is 1/3 of the entire body length of this ancient creature, almost 3 to 4 times that of humans.
Studies believe that this type of mesozoan has provided conclusive evidence that sexual reproduction was the norm as early as 100 million years ago! Their body structure suggests that during mating, the males pass through hook-shaped grippers, grab and fix the females, and then send the giant sperm through the sperm pump into the female's body. The giant sperm and eggs then complete the active fertilization process in the female's seminal vesicles.
This relatively complex mating and reproduction behavior has been fully preserved in this hundred million years, and even evolution seems to be at a standstill. Researchers believe that this reason is likely to be because of complex reproductive behaviors, which improve the mating success rate of mesembles and promote the preservation of this ancient breeding pattern.
The sperm sample this time is also the oldest animal sperm sample found by humans so far. Prior to this, as early as 2014, more than 16 million years old freshwater mesembles, about 1.2 mm long sperm fossils, were also found in a cave in Australia. But the discovery of the Myanmar sample directly pushed the time forward by a full 83 million years.
Earth creatures, so far, also have a very interesting point, many very small creatures, but can always produce some small sperm. On the contrary, primates like humans, on whom men depend on the little ones who have been passed down from generation to generation, are far less than them. And how did this huge feature evolve? This is still a mystery!
However, biologists also speculate that a breeding pattern with a larger sperm army like that of humans may be more conducive to finding better offspring through competition, but this is just speculation.
But at least, this study of amber fossils has made it clearer that the stability of sexual reproduction may be older and more stable than we think.