Ancient relative of shrimp turns out to be manly little chap

If size really does matter then the "man among men" award in the fossil record surely goes to a 100 million-year-old, millimetre…

If size really does matter then the "man among men" award in the fossil record surely goes to a 100 million-year-old, millimetre-long organism which leaves other males in the dust.

It boasted the highest body to penis ratio yet discovered and had not one but two appendages.

Prof David Siveter, professor of palaeontology at the University of Leicester, described the unusual creature during a session at the British Association meeting in Leicester.

In keeping with the themes discussed, he titled his paper: "Making the earth move: fossil sex in invertebrates".

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"My particular brief is to look at sex in animals," he said, calling a eukaryote inseminator an eukaryote inseminator.

Eukaryotes are those in the animal kingdom who procreate by internal insemination. A clear sign of this practice is "sexual dimorphism", he said, which is obvious differences between males and females.

His dimorphism dated back to between 515 million and 520 million years, he said, with male-female sex existing from that period.

Aeons would pass, however, before we got a look at what ancient love might have looked like, with the discovery of a 100 million-year-old penis in Cretaceous sediments.

The appendage was attached to a millimetre-long animal known as an ostracod, an ancient relative of crabs, shrimp and water fleas. It was trapped in sediments and fossilised "in flagrante delicto", Prof Siveter said, giving us a clear view of the stuff this miniature bivalve was made of.

Its penis makes up a third of its entire body volume, he said, and the ostracod was fitted with not one but two. This gives the ostracod the record for the longest sexual organ compared to body length of any creature known. The sperms delivered by the ostracod were even more impressive.

"Ostracods are very sexy animals; they have the second longest sperm in the fossil record," he said. Each sperm measures 10mm and some effort was needed to deliver this to the female.

Although the ostracod was beaten into second place for sperm length - the longest being a 13mm offering by another invertebrate from the period - it does retain pride of place for having the longest in relation to its body length.

While the ostracod fossils recovered in Brazil did show the male primed and ready for action, there was no waiting female to confirm how this union might have occurred.

Prof Siveter said, however, that a female would have been there to prompt the male "unless it was a 100 million-year-old flasher".

The research does not represent some new and unique form of voyeurism. It is a serious scientific study about the evolution of a key component of the one activity which keeps life on the planet.

"It is important for palaeontologists to understand when sexual reproduction emerged," Prof Siveter said.