Fish and aquatic invertebrates can simply release their eggs and sperm into the current or deposit them side by side on the seafloor or in a nest, said Dunlop. Sharks, as one exception to the rule, do have paired structures for injecting sperm, called hemipenes.
However, "when any animal comes onto land it is faced with the common environmental problems of having to breathe, prevent water loss, and reproduce," said Dunlop.
Sperm would dry out rapidly on land, he said. To get round that problem, different animals have traveled down different evolutionary paths, but have come up with a similar solution. A penis for injecting sperm directly into the female has evolved on many separate occasions, he said.
In contrast, land plants—which also began life in the sea—have overcome the problem by enclosing their male sex cells in hardy desiccation-proof pollen granules or spores.
Oldest Organ in the Land
This is most likely the oldest fossil terrestrial species to be found with a penis, said Paul Selden, president of the International Society of Arachnology, and paleontologist at the University of Manchester in England. However, other aquatic animals known from fossils such as sharks, and extinct sea scorpion-like euryptids, probably had sperm transfer organs, added Selden. "But I don't know whether these have been found in the fossil record before."
These type of harvestmen "have relatively large genitalia, compared to their body size," said Selden—the fossil male has a penis two-thirds the length of his body. "I suppose it is to get past those long legs," said Selden.
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