Dating habits of aquatic dinosaurs revealed
Artist's impression of Placoderms.
Image: Peter Trusler

Researchers at Curtin University have built upon previous studies to show that internal fertilisation and live birth in vertebrates is more than twice as old as previously thought.

The existing record was approximately 180 million years ago, but a recent discovery by an international team led by Curtin’s Dr Kate Trinajstic has added another 200 million years to this figure.

Discovering that the Incisoscutum species of Placoderms possessed penis-like appendages known as ‘claspers’ will have broad implications in the study of evolution in complex vertebrates, including humans.

Placoderms were primitive armoured fish that share some similarities with today’s sharks, and this latest find strengthens the ties between them: that of ‘erectile elements’ on males used to directly impregnate females.

“It tells us that many of the characteristics we consider as particular to more recent vertebrates have been present in the earliest vertebrates. We tend to think of internal fertilisation, live birth and providing nutrition to our young as mammalian characteristics, but now we know they’re not – these very ancient fish were reproducing not that differently from us and modern sharks,” Dr Trinajstic says.

“I think it also tells us that a lot of characteristics that we’ve considered as advanced aren’t really, and that live birth and internal fertilisation are very ancient forms of reproduction.

“The other thing that seems quite simple but is very significant is that for the first time, we can sex Placoderms – we can tell which are the males and which are the females.”

This allowed the team to identify most of their specimens as female, suggesting that males and females lived predominately separately, only coming together to reproduce.

By studying fossils found in a Devonian era reef in the north of Western Australia, the collaborative effort between Curtin, UWA, the Museum of Victoria, the Natural History Museum of London, and Sweden’s Uppsala University uncovered what may amount to the first evidence of ‘complex foreplay’ (In paleontology, ‘foreplay’ is purely pre-fertilisation activity).

“Well, simple foreplay is something like external fertilisation, so you just lay your eggs, and hopefully a male comes along and they get fertilised – there’s no interaction,” Dr Trinajstic says.

“With this, the very fact that there’s internal fertilisation means that the female has to show by some behaviour that she is receptive to the male, and the male has to indicate that he wants to reproduce and not eat her.
“So there has to be some kind of signalling, and the fact that these two animals are going to have to get together means that that signalling is probably quite complex.”

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