Placoderms were the dominant group of early jawed fishes in Devonian seas, rivers, and lakes, but their greatest diversity is seen in the arthrodire group. These “joint-necked” fishes included the mighty Dunkleosteus (one of the world’s first “megapredators,” reaching an estimated 24 feet in length with lower jaws over 3 feet long) and some 250 or so other known species.
Despite many fine complete specimens held in museums around the world, nothing was known of their reproductive behavior. Not one single arthrodire fossil showed any sign to distinguish either the males or females.
So, hot on the trail after discovery of our “mother” ptyctodontid placoderm fishes with embryos (see Part 1), our radar turned to looking at collections of arthrodires, and one specimen in particular that was published with a description of “stomach contents” containing other placoderm bones. This was a superb three-dimensional arthrodire from the same fossil site at Gogo, Australia, as our previous discoveries of ptyctodontid mother fishes.
My colleague Zerina Johanson was the curator fossil fishes at the museum, so she examined and photographed the specimen and sent me the images. We were convinced that the small fish plates inside the fish’s body was another embryo, so I flew over to London to examine it and was able to confirm it as an embryo. We then also found a second specimen of the same species with another complete embryo.
By now we had evidence that the major group of placoderms – the arthrodires – were also live bearers, but there was still no visible sign of sexual dimorphism on the rest of the skeleton. So how the heck were they doing it?
Like many breakthroughs in science, the answer came via a serendipitous event, when a colleague from Sweden, Per Ahlberg, visited one of our team, Kate Trinajstic, to look at Gogo fossils in the Museum in Perth a few months later. He identified the male clasper of Incisoscutum in a specimen that I had found earlier. I had had a student study it, but we did not see it for what it really was. Sure enough, it had a very long, slender shaft with a pointed head and many small ridges and barbs on its business end – quite reminiscent of the clasper of a living shark. With further searching, we soon recognized the female pelvic fin on another specimen (lacking the long clasper).
Diagrammatic reconstruction of the clasper (male reproductive organ)
of a 380-million-year-old placoderm fish (Incisoscutum).
It seems that even as late as 2009, the reproductive behavior of an entire class of extinct animals was finally resolved through the exceptionally well-preserved fossils from Gogo. The bizarre truth was that the most generalized group of jawed vertebrates was found to have one of the most derived methods of reproductive behavior – quite an unexpected situation.
So how do these discoveries relate to the evolution of complex sexual behavior in land animals? Do these strange, bony, placoderm claspers have anything at all to do with the reptilian or mammalian penis?
In my next two posts, I will look at what we know of sexual behavior through other fossil discoveries, and then swing back to look at the significance of placoderm mating behavior for interpreting human sexuality.
John A. Long is Vice President of Research and Collections at the Natural History Museum of Los Angeles County. His most recent book is Dawn of the Deed: The Prehistoric Origins of Sex, published by the University of Chicago Press.