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.”

Giant Mesozoic badger turned mammalian dogma on its head

Juvenile badger with dinosaur dinner

Check your biology book. If it says anything about mammals during the age of dinosaurs, it probably depicts the mammals as small, shrew-like animals scuttering around at night, barely scratching out a living as they scurry away from the thudding feet of a Tyrannosaurus rex.

Mammals ate dinosaurs–tasted like chicken

Banish the thought and rewrite the book. Yes, many of the mammals that lived in the Mesozoic—from about 248 to 65 million years ago—were shrew- or rat-like critters that probably stayed out of the way of most dinosaurs. But recent fossil finds demonstrate that some of the mammals in the age of the dinosaur not only got in the path of dinosaurs, they ate them.

In China, there is a famous fossil bed best known for the fossils of feathered dinosaurs it has yielded. But paleontologists have also turned up some other intriguing remnants, among them the mineralized bones of species from the Repenomamus genus. These animals were long, squat-bodied creatures with strong jaws and very sharp, pointy teeth. Researchers at the site had already reported finding R. robustus, a carnivorous mammal weighing in at about 15 pounds.

Giganticus, indeed

But two other finds reported in Nature flip common Mesozoic mammal dogma upside down. The first discovery was that of a fossil species now dubbed Repenomamus giganticus, a cousin of R. robustus, but with some distinctive features: this specimen probably weighed about 30 pounds and grew to be up to a meter long. Think about a mid-sized dog, say a large basset hound, with a badger-like face and rodent-like sharp teeth, and you’ve got your R. giganticus. Not something you’d want to go hand-to-tooth with when it’s in a bad mood.

Died with dinner inside (& more dog breed comparisons)

That, at least, is what researchers concluded after their second find: a fossil of R. robustus, the smaller species, with a juvenile dinosaur skeleton where the R. robustus stomach would have been. Not only did these hardy Repenomamus species look scary, for juvenile, leaf-eating dinosaurs, they were deadly. Experts estimate, based on mammalian habits of today, that mammals can kill and consume prey that is up to half of their body weight. If R. robustus could snack on a 5-inch dinosaur baby, then presumably R. giganticus could have put back a dinosaur the size of a dachshund.

The scientists who identified and named R. giganticus had a couple of hurdles to overcome. First, they had to determine that this was a genuine average version of R. giganticus, not simply R. robustus with a pituitary problem. The error that would result would be akin to finding the skeleton of the world’s tallest man and assuming that it represented our entire species.

Badger or human, your teeth show your age

But they looked at the teeth that accompanied the skull and jaw fossils, and the molars held the clues to the animal’s age at death. The last molar of the lower jaw appeared to have just erupted when the animal died, and it had little wear. Based on this clue, the researchers concluded that fossilized remains were from a juvenile representative of the new R. giganticus species.

Making the case that a Mesozoic mammal had actually consumed a dinosaur also required some consideration and discarding of various possibilities. The little dinosaur skeleton, from a Psittacosaurus, was a small patch of bones within the ribcage of some R. robustus fossil remains. The bones were located right where the stomach is on today’s mammals, and appeared to have been broken, torn apart, and displaced from one another. The fossil bones of the accompanying R. robustus skeleton were not in this condition. The Psittacosaurus specimen also had teeth, most of which were worn, implying that this animal was not scavenged from an egg as an embryo. Based on these clues, the researchers concluded that this R. robustus had caught and eaten the hapless Psittacosaurus—dismembering it and swallowing it in chunks—shortly before meeting its own death.

Ancient warfare captured in amber

The soldier vs. the cockroach

About 100 million years ago, a soldier beetle of an ancient type found itself under investigation by the antennae of a much larger animal, possibly a giant cockroach. Understandably alarmed, the small soldier beetle, only about a quarter of an inch long, immediately fired off a shot of offensive chemical from one of its rear glands. Just as it released its toxic dose, a wave of sap engulfed it, forever freezing the moment in amber and taking with it the antennae of the larger predator.

We would never know a thing about this tiny event from millions of years ago were it not for the preservative properties of amber. It starts out as sap dripping down a tree but has captured for us some of the most amazing, ancient finds in the history of fossils. Recent reports have included the discovery of the oldest bee species and even the presence of the malaria pathogen in the blood of an amber-preserved mosquito. Finds like this make amber a window into an ancient world showing us things we otherwise would never see.

Small-scale wars in a big world

The scene between the cockroach and the soldier bug would have been lost to history without the amber. But now we know that chemical warfare, today widely practiced by a variety of insect species, existed at the time of the dinosaurs. As Tyrannosaurus rex made its way across the landscape leaving its tri-toed footprints in the mud, soldier beetles were busy waging their war against killers on a much smaller scale.

Insects today employ this tactic so frequently that it featured as the central trait of the most frightful insect villain in movie history, the sulfuric-acid squirting super-killer in the Alien series. Some modern insects do actually shoot acid as a form of defense; in fact, the modern version of the soldier beetle fires off a mean shot of carboxylic acid at anything trying to eat it. Other nasty mixtures insects use today include chemicals that make the predator vomit or at least spit out the intended snack.

The amber that captured the David vs. Goliath battle between the hapless soldier beetle and its cockroach attacker was found in an amber mine in Myanmar, formerly Burma. This mine has been a treasure trove of biological finds trapped in amber, discoveries that tell us things about ancient ecosystems that we otherwise never would know.

What we do know is that this ancient species of soldier beetle liked to eat aphids, other little insects, and plant pollen, which may have been why it was on the tree in the first place. It had seven pairs of chemical-firing glands along its abdomen and was able to pick and choose which ones to use, depending on the angle of the predator. In the case of the amber-encased soldier beetle, it was employing only one gland and had actually achieved a successful shot, engulfing the predator’s antennae in the presumably noxious chemical before succumbing to the flowing sap.

Don’t crack my amber!

No one knows exactly what the chemical was because the person who owns the amber and the scientist who discovered the beetle refuse to allow eager entomologists to examine the encased remains. Many entomologists would be thrilled to extract DNA from this specimen, to be able to compare it to other sequenced ancient and modern samples. Others would like to identify what this earliest known practitioner of chemical warfare was using in those seven accurately firing glands. But to get at these samples, the entire specimen would have to be ground up and completely destroyed, something the amber’s owner, a collector of such pieces, refuses to allow.

And, for the geeks…a new dating for the soldier beetle?

As consolation, entomologists will have to accept another prize. This particular fossil pushes the dating of this species of soldier beetle back by 60 million years, newly placing the little bug squarely in the time of the dinosaurs.

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