Second author

In 2008, scientists in Australia and the United Kingdom discovered embryos in fossilized placoderms — extinct, jawed fish covered in plates of bony armour. This confirmed placoderms as the earliest vertebrates known to be capable of live birth, and set in motion a re-examination of placoderm specimens in museums. A second look at one of these specimens has yielded a new first — a fossilized pelvic clasper, the male's copulatory structure (see page 888). Kate Trinajstic, a palaeontologist at Curtin University in Perth, Australia, tells Nature how the team's work sheds new light on prehistoric animal behaviour as well as biology.

How did the embryo's discovery affect your research?

Once we knew that live births were possible, we were eager to look for more evidence of embryos in placoderms. We also began to search for the reproductive organs necessary for internal fertilization.

How did you find the fossil clasper?

We first found it in 2001 and labelled it as the pelvic girdle. At that time, we didn't realize that live births were possible. Fortuitously, Per Ahlberg, an evolutionary biology at Uppsala University in Sweden, was working with me on a different project earlier this year and looked at this specimen with a new set of eyes. As soon as he got it under a high-power microscope, we both realized we had found the missing clasper.

What else has the new work revealed about placoderm reproductive behaviour?

When we re-examined our specimens, we realized that males of the Arthrodires (the largest group of placoderms) are rare in the collection from the Gogo Formation in Western Australia. Out of thirty fossils of these fishes, only two are males. This tells us that males and females lived separately and probably only came together at certain times of year to reproduce. So, as well as insight into reproductive strategies, we're also getting an idea of reproductive behaviour from fossils — which is a rare opportunity.

How much more of this story is there?

It's hard to say. We've got nearly 50 species of fish from the Gogo formation. Technological advances are key to how we interpret fossils. For example, we can use computed tomography to get cross-sectional images and confocal microscopy to create three-dimensional images and see a whole new level of detail in these specimens. I am often awestruck that we are able to peer through this window onto the past to make new discoveries.