November 9, 2010 Leave a comment
Timeline, 2008: From about 420 to 350 million years ago, the rulers of Earth’s seas were an unattractive-looking armored fish known today as the placoderms. This group, consisting of many species, were the bulldogs of the fish world, heavy-bodied with big ugly mouths full of protruding, potentially dangerous bony plates. Some of them were quite small, but a few species grew as large as 20 feet in length. They were the dominant vertebrate worldwide for about 70 million years.
Conventional scientific wisdom would say that these ancient fish reproduced the way modern representatives of ancient lineages do: external fertilization, the sperm fertilizing the egg with a little help from water. The wisdom was so conventional, in fact, that experts placed the rise of internal fertilization—delivery of the sperm into the female via an act of copulation—a good 200 million years after the placoderms swam the seas.
A catastrophe on the reef
In what is now Western Australia, something terrible happened about 380 million years ago in the shallow seas covering a coral reef: the oxygen that fed the reef suddenly plummeted, leaving the coral starved and unable to support the food web built around it. The outcome was a rapid, catastrophic loss of all of the species on the reef, including the placoderms. Thanks to stable plate tectonics and some good sediment coverage, these hapless animals remained preserved for the subsequent millions of years until a team of fossil hunters uncovered them. They now populate one of the most famous fossil finds in the world, the Gogo fossil sites, which are packed with perfect specimens of long-lost species.
The role of Sir David Attenborough, the world’s coolest naturalist
Among those perfect specimens—so perfect, in fact, that three-dimensional samples are available—is a species that now has the name Materpiscis attenboroughi. The name means “Attenborough’s mother fish” and requires a bit of explanation. Back in the late 1970s, Sir David Attenborough produced a wonderful nature and science series called Life on Earth. In the series, he highlighted the Gogo sites, and his interest led researchers to name the fish after him. But the first part of the name, the genus name Materpiscis, means “Mother fish.” Why? Because when this 10-inch fish died during that catastrophic reef loss, she died just before becoming a mother.
We know this because a couple of researchers working on her fossilized remains decided at the last minute to expose the fossil to one more round of acid treatment. They had pretty much decided to write her up as she was, which would have been plenty because of the preserved 3D perfection of her remains. But they agreed to that last treatment, which gently etches away layers of the fossil to reveal what lies beneath. They are glad they did, because what that last treatment exposed, inside of the adult fish, is a tiny, fossilized fish embryo, about a quarter of the size of its mother.
Eureka! Again, and again, and again
Anyone looking at that embryo, inside of that fish, might have had any number of “Eureka” thoughts in that moment. Eureka! It’s a fish embryo, 380 million years old! There aren’t that many of those lying around. But even more important, Eureka! It’s a fish embryo inside of the mother. That means that the egg was fertilized inside of the mother, where the embryo grew, nourished in her body, just as mammals do it. The embryo was even attached by a tiny, fossilized umbilical cord. A final Eureka! just might be that we can confirm the sex of this fish just based on the fact that she was pregnant when she died.
This just in: Sex is fun
The presence of an internally developing embryo in this placoderm sets the assumed evolutionary timing of internal fertilization back about 200 million years. No one would have guessed that these ancient, armored bulldog-like fish would represent the earliest-known internal fertilization. And the fact that fertilization was internal means that these animals must have copulated, the standard mechanism for getting sperm into the female to meet the egg. That recognition led one of the embryo’s discoverers to remark that this animal represents the earliest example a species engaging in “sex that was fun.”