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"Probably the Oldest" Penis Found in Spider Fossil

John Pickrell in England
for National Geographic News
October 6, 2003
 
Scientists think they may have uncovered one of the earliest examples of
male genitalia recorded in stone. The 400-million-year-old fossil organ
belongs to a harvestmen or daddy longlegs, a non-web-spinning arachnid,
related to mites and ticks.

"This is probably the oldest example of what would anatomically be classed as a penis," said paleontologist Jason Dunlop of the Humboldt University in Berlin, Germany, who made the find. "Though the oldest fossil genitals may in fact belong to 450-million-year-old millipede fossils," he said.

Genitals are used as important clues to identify many spider species as they can vary widely in shape and form between species. This variation in shape is thought to ensure females can only mate with males of their own species.


Almost Identical

Fossils of both male and female harvestmen bearing genitals were first uncovered in 2001 by Dunlop and his team, in rocks from the village of Rhynie, near Aberdeen in Scotland. The female harvestmen has a genital opening in which the male deposits sperm in modern species and an ovipositor for laying eggs. Despite the great age of the fossils, they are almost identical to harvestmen that can be found in the garden today.

The discovery was reported by Dunlop at the Rhynie Hot Spring System: Geology, Biota & Mineralisation conference held at the University of Aberdeen in late September. The find will also be detailed in the science journal Nature later this year.

The fossils, which are the oldest harvestmen ever discovered, were found by carving fine slices of the very hard Rhynie chert rock. This rock is famous for including many invertebrate and plant fossils of a similar age. The oldest spiders yet found are 380 million years old.

400 million years ago, Rhynie would have looked something like Yellowstone park today: a hot springs environment with gushing geysers spraying water rich in dissolved silica, said Dunlop. The silica quickly crystallizes, rapidly preserving animals and plants in fine detail, he said.

Adapted to Land

Another interesting feature of the fossils is that they have large branching trachea—the harvestmen equivalent to lungs. This is the earliest known example of air-breathing apparatus of this type in arachnids, and suggests that the animals were land-living. Arachnids, along with all animal groups, began life in the sea.

A penis, along with a pair of lungs or trachea, is a common tool required for life on land. Many marine animals don't have a penis-like structure, because it isn't required.

"Reproduction typically [in aquatic animals] is accomplished by spreading a cloud of sperm close to the unfertilized eggs," said Adriano Kury, a fossil spider expert at the Federal University of Rio de Janeiro National Museum in Brazil. "Later, many groups developed [methods to] somehow bring the sperm to the eggs or into the female body," he said.

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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