The media is getting another science story wrong. I keep seeing this discovery of an array of fossil placoderms as revealing the origins of sex, and that’s not right. Sex is much, much older, and arose in single-celled organisms. Come on, plants reproduce sexually. A fish is so far removed from the time of origin of sexual reproduction that it can’t tell us much about its origins.
Let’s get it right. These fossils tells us about the origin of fu…uh, errm, mating in vertebrates.
What we have are a set of placoderm fossils from the Devonian (380 million years ago) of Western Australia (The Aussies are going to be insufferable, now that they can claim to be living in the birthplace of shagging) that show two interesting features: some contain small bits of placoderm armor that show no signs of digestion, and so are not likely to be relics of ancient cannibal feasts, but are the remains of viviparous broods — they were preggers. The other suggestive observation is that the pelvic girdle has structures resembling the claspers of modern sharks, an intromittent organ or penis used for internal fertilization.
These animals are placoderms, an extinct group (which may be paraphyletic) of fish characterized by a massively armored skull. They are thought to belong on the gnathostome stem — that is, somewhere among this motley and diverse and successful collection of animals lies the ancestor to all the jawed vertebrates, such as sharks, bony fish, and tetrapods like us. That makes them an important focus of interest in understanding our own origins, and learning that they used internal fertilization, i.e., were shtupping each other, suggests that this is a very ancient practice.
Here are some photos of two of the fossils. You may not find them particularly exciting, because most of what you see below is a jumble of bony plates, the disarticulated pieces of the bony skull and scales. What was found was that, among the larger pieces of the adult fish, there were smaller pieces of the same species, in a state of early development. That only this species was found inside, and that they are in a consistent state (unlike a jumble of partially digested meals), suggests strongly that these are embryos.
The fins of some of the adults also show traces of a distinctive feature that all of us who have done shark dissections will find familiar: an extension of the pelvic fin to form a kind of paired penis called a clasper. That structure is a giveaway that these animals were copulating in those distant Devonian seas.
All this raises some interesting questions. We mammals, of course, use internal fertilization. The ancestral bony fish, the animals that gave rise to modern bony fish and modern tetrapods, are thought to have been spawners that practiced external fertilization, like salmon today. Modern sharks use internal fertilization. Placoderms, down there at the base of the family tree, are now known to have some members that used internal fertilization.
Now the question is where our reproductive practices came from. Did internal fertilization come first, then our bony fish ancestors threw it away (hard to imagine…), and we then re-evolved the ability independently? Or was it retained in the line that led to us, and bony fish secondarily lost it (given the ubiquit of spawning in fish, this seems unlikely)? Clearly, what we need now are more revealing fossils that expose ancient reproductive behavior.
Long JA, Kate Trinajstic K, Johanson Z (2009) Devonian arthrodire embryos and the origin of internal fertilization in vertebrates. Nature 457:1124-1127.
Ahlberg PE (2009) Birth of the jawed vertebrates. Nature 457:1094-1095.