How Fishes Had Sex For The First Time

The fossilzed claspers of armored fish are the most primitive vertebrate sexual organs described to date.

AsianScientist (Oct. 27, 2014) – A paper published in Nature reveals how the intimate act of sexual intercourse first evolved in our deep distant ancestors.

In one of the biggest discoveries in the evolutionary history of sexual reproduction, scientists from Australia, China, Estonia, Sweden and the UK scrutinized a vast number of fossil specimens held in museum collections across the world. They found that internal fertilization and copulation was invented by ancient armored fishes, called placoderms, about 385 million years ago in Scotland.

Placoderms, the most primitive jawed vertebrates, are the earliest vertebrate ancestors of humans. The discovery shows that male fossils of the Microbrachius dicki, which belong to the antiarch group of placoderms, developed bony L-shaped genital limbs called claspers to transfer sperm to females; and females developed small paired bones to lock the male organs in place for mating.

Measuring about eight cm long, Microbrachius lived in ancient lake habitats in Scotland, as well as parts of Estonia and China.

As the paper’s lead author, Professor John Long, who is the Strategic Professor in Palaeontology at Flinders University in South Australia, discovered the ancient fishes mating abilities when he stumbled across a single fossil bone in the collections of the University of Technology in Tallinn, Estonia, last year.

The fossils, he said, symbolise the most primitive known vertebrate sexual organ ever found, demonstrating the first use of internal fertilisation and copulation as a reproductive strategy known in the fossil record.

Microbrachius means little arms but scientists have been baffled for centuries by what these bony paired arms were actually there for but we’ve solved this great mystery because they were there for mating, so that the male could position his claspers into the female genital area,” he said. “It was previously thought that reproduction spawned externally in water and much later down the track in the history of vertebrate evolution.”

The new discovery now pushes the origin of copulation back even further down the evolutionary ladder, to the most basal of all jawed animals.

“Basically it’s the first branch off the evolutionary tree where these reproductive strategies started,” Prof. Long added.

Microbrachius dicki claspers. Credit: Zhu You'an.
Microbrachius dicki claspers. Credit: Zhu You’an.

Dr. Matt Friedman of the University of Oxford, who was not involved in the study said: “The paper by Long and colleagues is nothing short of remarkable. Claspers in these fishes demand one of two alternative—but equally provocative—scenarios: either an unprecedented loss of internal fertilization in vertebrates, or the coherence of the armored placoderms as a single branch in the tree of life.”

“Both conclusions fly in the face of received wisdom, and suggest that there is still much to discover about this critical episode in our own extended evolutionary history.”

In one of the more bizarre findings of his research, Prof. Long said the fishes probably copulated from a sideways position with their bony jointed arms locked together.

“This enabled the males to manoeuvre their genital organs into the right position for mating,” he said. “With their arms interlocked, these fish looked more like they are square dancing the do-se-do rather than mating.”

Prof. Long said the discovery also signifies the first time in evolutionary history that males and females showed distinct differences in their physical appearance.

“Until this point in evolution, the skeletons of jawed vertebrates couldn’t be distinguished because males and females had the same skeletal structures.

“This is the first time in vertebrate evolution that males and females developed separate reproductive structures, with males developing claspers, and females developing fixed plates to lock the claspers in for mating.”

The discovery highlights the importance of placoderms in the evolution of vertebrate animals, including humans.

“Placoderms were once thought to be a dead-end group with no live relatives but recent studies show that our own evolution is deeply rooted in placoderms, and that many of the features we have, such as jaws, teeth and paired limbs, first originated with this group of fishes,” Prof. Long said.

“Now, we reveal they gave us the intimate act of sexual intercourse as well.”

The article can be found at: Long et al. (2014) Copulation in Antiarch Placoderms and the Origin of Gnathostome Internal Fertilization.


Source: Chinese Academy of Sciences.
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