Amber, such as the lump that sits atop Dr John Hammond’s cane in Jurassic Park, makes fascinating fossils that offer us a glimpse into the past as they preserve living things frozen in suspended animation. They formed long ago as tree sap engulfed creatures on the trunks of trees, and notable examples have caught animals eating, fighting and, you guessed it, doing the nasty. Such specimens provide valuable information into the diets, behavior, and evolution of animals, which we can compare against extant species of the same family.
A new specimen described in the journal Proceedings of the Royal Society B has revealed what is thought to be the oldest fossilized animal sperm ever found. For 100 million years the amber preserved a collection of 39 ostracods, small bivalved crustaceans that are among the most abundant fossil arthropods since the Ordovician and have long informed paleoenvironmental reconstruction and evolutionary biology. The sample contains males, females, and juveniles and is believed to be from mid-Cretaceous Myanmar.
Most interesting of all about this particular specimen is that it has even preserved the soft tissues of the captive ostracods, all the way from their appendages to their private parts. More often than not all that remains in fossil ostracods is a calcified shell so to find such a detailed, well-preserved, and complete collection of specimens is both incredibly rare and very exciting.
Closer inspection using X-ray micro-computed tomography revealed that the amber had preserved the specimens’ genitalia in all its glory, detailing for the first time the morphology of the male clasper, sperm pumps (Zenker organs), hemipenes, eggs, and female seminal receptacles, which, incredibly, contained giant sperm. This marks the first time ostracod sperm has been found preserved within a Cretaceous fossil, and reveals the enormous swimmers were one-third the body length of adult ostracods. Even more impressive is that the discovery is also the earliest known animal sperm record, beating the last oldest specimen by 50 million years.
The incredibly well-preserved sex organs have shown that during sexual reproduction, male ostracods used a fifth limb, fitted with hook-like endopods, to grasp a female and insert its hemipenes into the female's paired vaginas. The extremely long but immotile sperm was then delivered to the hemipenes by the males’ Zenker organs and inserted into the female.
Amazingly, the sexual performance that we now know to have played out between mating ostracods, facilitated by a host of morphological adaptations, has remained unchanged for at least 100 million years. This sort of sustained adaptation is called evolutionary stasis, which is essentially Darwin for: if it ain’t broke, don’t fix it. The discovery constitutes a particularly impressive example of evolutionary stasis having stayed the same for such an incredible amount of time.
The emergence of this complex but efficient reproductive mechanism using giant sperm will have improved mating success and could even have been an important development that contributed to an explosive radiation of the superfamily Cypridoidea in the late Mesozoic, which to this day includes the majority of nonmarine ostracod species.