Placoderms had the “fun kind” of sex

Dunkleosteus, a Devonian placoderm. Pencil drawing, digital coloring, Nobu Tamura, Obtained from Wikimedia Commons.

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.”

Fish can count, too

One, two, three...

Timeline, 2008: We tend to think of a few things we do as being uniquely human. And then we keep finding other organisms that can do them, too. Walking on two legs? Meet the orangutan, walking upright in the trees. Tool use? Crows can make a hook to fish meat out of a tube. The ability to talk? Seems that Neanderthals might have had that, also. OK, well what about counting, having number sense? Baby chickens share this trait with us. To the growing list of other animals that do as well—which includes dolphins, rats, and some monkeys—you can now add the mosquitofish.

Mosquitofish vs Munduruku

Yes, apparently fish can also count, in some cases as well as infants ages 6 to 12 months. In fact, when compared to some natives of the Amazon, the Munduruku, which have limited number language, the fish may even be comparable. The Munduruku people see no value in having a construct for counting beyond five. The mosquitofish, on the other hand, can count about that high and estimate with even higher numbers.

Number sense: It’s not just for people any more

Number sense can be broken down into three paths of perception. We can visually estimate what we see, as people do when they report crowd counts for huge parades or demonstrations. We can also visually count individual units, as we might do just looking at the fingers on one hand. And humans also have the ability to verbally count, theoretically to infinity given sufficient time. While mosquitofish obviously do not count out loud, they do appear to have visual estimation and counting abilities.

Neither is sexual harassment

Their estimation abilities first emerged as a result of sexual harassment. Researchers studying the guppy-like fish noticed that when a male harassed a female, the female fish would take refuge with a group of fish nearby. If there was a choice of groups, or shoals, of different sizes, she would choose the larger of the two. Of course, her ability to tell “larger” might have had nothing to do with actual numbers but instead with the area that the fish occupied. To assess this possibility, researchers performed a number of complex experiments. Their results showed that the females were not relying in occupied area to figure out which group had more fish. They really were using visual number estimation to decide.

In fact, they seem to use ratios in their determinations, but the ratios need to meet a threshold of difference for the estimations to work. For example, a mosquitofish seems able to distinguish a group of 16 fish as being larger than a group of 8 fish, a ratio of 2:1. But the fish cannot tell a group of 12 from a group of 8, proving unable to distinguish a 3:2 ratio.

Estimating, counting: These fish are brilliant

With lesser numbers, up to about four, however, the fish discard visual estimation and rely instead on actual visual counting. In what really was a clever set of experiments, the research team let an individual female fish spend an hour exploring two areas of an aquarium. In one area, she could see a group of four fish but could only see each fish one at a time. In the other area was a group of three fish, again only visible to the female one at a time. After letting her explore, the researchers then determined where the female spent more time. The fish spent about twice as long swimming close to the larger group. In other words, the fish seems to have counted the number of individuals in each group and based on their counting, figured out which area of the aquarium had the larger group.

Pretend you’re a fish

To get in tune with how meaningful this ability is, visualize the experiment yourself as a human (you’re human, right?). Stand in front of two open doorways. In one doorway, four people appear, one at a time. In the other doorway, three people appear, one at a time. You can count them, distinguishing each different individual, and can tell which doorway leads to the larger group of people. That’s how smart the mosquitofish is.

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