Hundreds of millions of years before there were birds or bees, fish were pioneering the earliest known form of vertebrate sex, paleontologists report Tuesday in Nature.
The armor-coated swimmers, called antiarchs, lived around 385 million years ago. They were among the first vertebrates with jaws, and thanks to the discovery of some never before seen anatomy, paleontologists now know they were also the first to use internal fertilization to reproduce.
Flinders University paleontologist John Long and colleagues add new evidence that copulation goes far back in the vertebrate family tree. Previously, early fish like the antiarchs were thought to have relied on spawning: Females would drop their eggs into the water, and males would douse them with sperm.
But in 2008 Long and colleagues described another armored, 380-million-year-old fish they named Materpiscis—"mother fish"—that carried embryos inside its body. This required internal fertilization, and further discoveries of related fish belonging to a greater group called placoderms revealed specialized male claspers, which function like a penis, and female genital plates that the fish used to copulate.
The new findings, centered on a more archaic form of placoderm called an antiarch and named Microbrachius, move vertebrate sex yet another step down the vertebrate family tree.
Finding the Parts
The discovery came while Long was visiting paleontologist Elga Mark Kurik last year in Tallinn, Estonia. When she brought him a box of Microbrachius fossils, Long says, "I spotted one isolated plate with a strange tube of bone attached, and I couldn't figure out what it was. Then the penny dropped: It was a clasper!"
Today's sharks and rays have claspers, too, but the fossil structures were different. "They are solid bone and fixed to the trunk shield bones of the fish, so not movable at all," Long says. To get in position, then, Long suspects the fish used their weird bony arms to find the right interlocking position. "I'm saying they did it sideways, square dance style," Long says.
The initial find started a worldwide search for other antiarchs with claspers, which led him to private collections in England and the Netherlands. These fossils had not only the male claspers in place but also the genital plates of the females. These modified bones helped lock the male clasper in place during the brief passage of sperm from male to female.
The find comes as a bit of a surprise, says Imperial College London paleontologist Martin Brazeau. "Microbrachius has been known to science for nearly 130 years. How did we overlook this?" Brazeau says. "We've been assuming the evidence simply isn't there, but it's been right under our noses for a long time."
Evolution of Sex
It's not yet clear whether all early vertebrates used internal fertilization. The clasper-bearing placoderms might represent an ancient, common reproductive strategy or a group of their own that evolved internal fertilization separately from later vertebrates, Brazeau says.
"That we're dealing with such a fine line between two important competing hypotheses really highlights why this is such an important and exciting discovery," Brazeau says.
Not that the discovery is all about The Act itself. The appearance of bony claspers in males and specialized genital plates in females means that Microbrachius is the earliest known example of sexual dimorphism—or differences in appearance between the sexes—in the fossil record.
What's more, many of the evolutionary firsts important to humans' body plan—such as jaws, teeth, paired limbs, and internal fertilization—originated among these armored fish. Through these discoveries, Long says, "we've added another rich chapter to the story of vertebrate evolution by now including placoderms in the narrative."