The motherfish and the stunned mullet

John Long admits that his jaw dropped and he was “stunned like a mullet” when he first realised that he was looking at a 375 million-year-old embryo nestled within the fossilised remains of its mother.

While there was evidence that copulation and internal fertilisation were characteristics of the motherfish, a ptychtodontid, it was assumed that they, like other placoderms, were still oviparous, much like some of today’s sharks.

But an embryo it was, pushing back the known record of viviparity, or live birth, by 200 million years. And more was to come. The fossil, named Materpiscis Attenboroughi, was found on a Museum Victoria expedition to the Gogo Formation in the Kimberley, where 20 years earlier Long had found another ptyctodont, Austroptyctodus gardineri. That fossil had an unusual series of what he thought were scales, which he described in one of his first published papers.

“We found the Materpiscis and wrote our paper and sent it off to Nature,” Long says. “Nature was very interested and it went straight out to review, which doesn’t happen very often, and while it was in review I woke up one night almost in a cold sweat, thinking, I’ve seen those structures before somewhere.

“I scratched my brain and remembered that back in 1986, on my very first trip to Gogo, I found one of these ptyctodonts, and it was complete and I prepared it in resin. It had what I thought were a series of scales along the body and that’s what I labelled them in the paper.

“I woke up, went to the computer, which had a photo of this specimen that I blew up, and I could see immediately that these weren’t scales – they were little clusters of bones and each one was an embryo. My collaborator Kate Trinajstic, who is one of my ex-PhD students, had a look at the specimen in Perth and rang me back that day and confirmed that it was a mother with three embryos inside her.”

It’s not as if Long hasn’t had major finds before, but this one was special. One the same trip in 2005, he found a complete fossil of Gogonasus, which he had initially described from a snout back in 1985. Gogonasus is an important link with tetrapods, having a well-developed humerus, ulna and radius in its front fin, and large holes in the top of its head called spiracles, for taking in air.

On the newly discovered embryo, Long and his team – including Trinajstic, now at the University of WA, along with Gavin Young and Tim Senden, specialists in micro-tomography and scanning electron microscopy from ANU – first thought was that it was something the fish had eaten.

“But these things have highly specialised durophagous (shell-crushing) diets so they wouldn’t have been predatory on other fish,” Long says. “And then we looked at the position of it and there are several bits of evidence.

“One is that embryo is located close to the axial skeleton, where the ovaries would have been, not down in the gut region. And it is an exact miniature of the adult in every respect; even the tooth-plates, which are highly characteristic of each species, show that it is the same species.

“And even more importantly we have the first ever – preserved in any fossil known – a maternal feeding structure, a mineralised umbilical cord. There is also a hole in the rock that has been filled in with coarse crystals, so we are only suggesting that this may have been the position of the yolk sac.

Materpiscis has similar reproductive biology to the viviparous sharks, as has the Austroptyctodus, Long says. Other specimens of Austroptyctodus have been found with claspers preserved in males and females with smooth basal plates, showing they are sexually dimorphic.

“There was speculation on how they may have reproduced, however, because unlike sharks, which have a soft, cartilaginous clasper which the male inserts into the female, these ptyctodonts had a bony short of sheath on the clasper covered in bony spikes, which was quite odd,” he says.

In 2006, Long published a very well received popular science book called Swimming in Stone: the Amazing Gogo Fossils of the Kimberley, and speculated in it that ptyctodonts may have reproduced just by the claspers holding the female and then inserted. “Then lo and behold, as soon as I write that and publish it we find actual evidence that they were almost certainly internally fertilising them.”

(Long also wrote a book called Gogo Fish! The story of the Western Australian state fossil emblem, which won the Australian Children’s Book Council Honour Award. It tells the story of how a group of children from the Sutherland Primary School in Dianella successfully lobbied the WA Government to embrace Mcnamaraspis kaprios, also found by Long, as the state’s fossil emblem.)

While Long has many important finds to his credit, Materpiscis is one of the most important he or anyone else has ever made. David Attenborough, after whom the fossil is named, was pretty chuffed too.

“I am very, very flattered and I am very undeserving,” Attenborough, who first visited Gogo in the late 70s and has his own fossil fish from the area, says. "It is a favourite object to pass around during an evening spent with friends. And when they fail to get anywhere near identifying it, I say that it is 'the dermal scute of a placoderm', which is quite enough to baffle them."