First author

The history of reproductive biology is far from complete, with little evidence available in the fossil record. Last year, placoderms, a group of extinct fishes, were identified as the earliest vertebrates capable of live birth. John Long at Museum Victoria in Melbourne, Australia, and his colleagues reported fossilized embryos in specimens of a small placoderm subgroup found at the Gogo Formation, an exceptionally well-preserved ancient reef community in Western Australia. On page 1124, Long and his co-workers now show that two previously archived specimens of the largest placoderm group also contain embryos. This provides evidence of internal fertilization and may represent the first step of the radical change in vertebrate reproductive biology from egg-laying to live birth. Long tells Nature about the sex lives of fossil fish.

After last year's finding did you scour museum archives for embryos?

Yes. We examined Gogo specimens housed at Museum Victoria and at the Natural History Museum in London. We focused on those with reported 'stomach contents', thought to be ingested food. We found no evidence of digestive acids and were stunned to find two specimens containing embryos. This extends the variety of early fossil fishes that we know had live births, and helps us to understand the origins of vertebrate copulation.

How does the finding shed light on vertebrate reproductive biology?

This group of placoderms, the Arthrodira, lack external genitalia. Without specimens captured during gestation there was no reason to suspect internal fertilization. Once we had evidence in the form of embryos, we re-examined the pelvic girdle in the few intact specimens available and found a pelvic fin and cartilage that articulated towards the tail much like claspers — male sex organs that today's sharks use to copulate.

Of all your discoveries from the Gogo Formation, is this a favourite?

This is a career highlight because it has brought about such a big change in thinking about reproductive strategies throughout evolution. We once thought one of the ancient fish subgroups might have had a quirky form of internal fertilization. Now we think it was widespread across this major group.

Will your work shed light on fish evolution?

We hope the findings will help us to piece together a larger evolutionary puzzle — whether placoderms' closest living relatives are the cartilaginous sharks or bony fishes. Some scientists think placoderms are not related to any current species.