Paleontologists found this bundle of "giant sperm" in the reproductive tract of an ostracod that's been preserved for 100 million years in amber from Myanmar. From the Ludwig-Maximilians-Universität München:
In fact, this is the oldest fossil in which sperm cells have been conclusively identified. Moreover, the specimen represents a previously unknown species of crustacean, which has been named Myanmarcypris hui[…]
The new ostracod specimens were analyzed with the aid of computer-assisted 3D X-ray reconstructions. The images revealed astonishing details of the anatomy of these animals, ranging from their tiny limbs to their reproductive organs. – And in one female specimen, Matzke-Karasz and her colleagues discovered ripe sperm. The cells were discovered in the paired sperm receptacles in which they were stored after copulation, ready for release 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 twin gonopores of the females.