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An international team of paleontologists has discovered giant sperm in a 100-million-year-old female ostracod preserved in an amber sample. Clearly, the little crustacean had mated shortly before being entombed in a drop of tree resin.
In another fascinating snapshot from the deep, an international team of paleontologists have reported the discovery of specimens of a tiny crustacean dating back to the Cretaceous (about 100 million years ago), preserved in amber samples from Myanmar. The most spectacular find is a single female, which upon closer inspection, turns out to contain giant sperm in her reproductive tract. In fact, this is the oldest fossil in which sperm have been conclusively identified. Furthermore, the specimen represents a previously unknown species of crustacean, which has been called Myanmarcypris hui. M. hui was an ostracod, as clearly indicated by the pairs of calcareous valves that form the shell, whose shape resembles that of a mussel shell. Ostracods have existed for 500 million years and thousands of modern species have been described. They are found in the oceans and in freshwater lakes and rivers. The fossilized shells of these crustaceans are by no means rare, but specimens preserved in Burmese amber reveal details of their internal organs, including those involved in reproduction. “The findings gave us an extremely rare opportunity to learn more about the evolution of these organs,” says geobiologist from Ludwig-Maximilians-Universitaet (LMU) in Munich Renate Matzke-Karasz, who played an important role in the morphological analysis of the fossils.
During the Cretaceous period, ostracods must have lived in the coastal and inland waters of what is now Myanmar, which were bordered by forests dominated by trees that produced enormous amounts of resin. The newly described specimens are among the many organisms that became trapped in the oozing droplets of the sticky substance. In recent years, amber found in Kachin province has produced a spectacular trove of fossils, including frogs and snakes, as well as part of a putative dinosaur (according to new evidence, that specimen may actually represent an unusual lizard). During the last 5 years, hundreds of previously unknown species have been described based on these inclusions. In fact, many of them have forced evolutionary biologists to reconsider conventional hypotheses about phylogenetic and ecological relationships.
The new ostracod specimens were analyzed with the help of computer-assisted 3D X-ray reconstructions. The images revealed amazing details of the anatomy of these animals, from their tiny limbs to their reproductive organs. – And in a female sample, Matzke-Karasz and his colleagues discovered mature sperm. The cells were discovered in the paired sperm receptacles where they were stored after copulation, ready to be released when the female’s eggs matured. “This female must have mated shortly before being encased in the resin,” says He Wang of the Chinese Academy of Sciences in Nanjing. The X-ray images also revealed the sperm pumps and the pair of penises that male ostracods insert into the female’s twin gonopores.
Findings in Burmese amber provide unprecedented insights into an unexpectedly ancient and advanced case of evolutionary specialization. “The complexity of the reproductive system in these specimens raises the question of whether the investment in giant sperm could represent an evolutionarily stable strategy,” says Matzke-Karasz. Males of most animal species (including humans) produce very large amounts of very small sperm. Sperm Comparatively few animals, including some fruit flies and, of course, ostracods, have taken a different approach: they produce a relatively small number of large sperm, the movable tails of which are several times longer than the animal itself.
“To show that the use of giant sperm is not an outlandish fad on the part of evolution, but rather a viable strategy that can confer a lasting advantage that allows species to survive for long periods of time, we must establish when this mode of reproduction appeared for the first time, “says Matzke-Karasz. Examples of fossilized sperm are extremely rare. The oldest known ostracod sperm (before the new discovery) is 17 million years old, and the previous record age, 50 Myr, was in the hands of some kind of worm. The new evidence extends that age by a factor of at least two. The fact that animals had already developed giant sperm 100 million years ago implies that this reproductive strategy may be successful in the (very) long term, Matzke-Karasz notes. “This is a pretty impressive record for a trait that requires considerable investment from both males and females of the species. From an evolutionary point of view, sexual reproduction with the help of giant sperm should therefore be a strategy. completely profitable. “
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