This month’s edition of Scientific American features a cover article entitled “Dawn of the Deed” by John A. Long. Long describes how fossil discoveries he and his colleagues made several years ago have changed the way we understand the evolution of copulation. While there has long been evidence that the sharks have practiced internal fertilization and gestation, new placoderm fossils discovered in Australia suggests that the ancestors of sharks, bony fish, and all other vertebrates may have also abandoned external fertilization. The placoderms are a now-extinct group of fish previously best-known for their armored faces and the development of the first jawbone. The authors hypothesize that placoderm internal fertilization and gestation may have been an adaptation to the predator-rich waters of the Devonian. They point to the fact that other fish lineages that have re-evolved internal fertilization show greater diversification than those which have remained external fertilizers.
One interesting aspect of this discovery is that it suggests the reproductive strategy of internal fertilization and gestation is far more ancient than previously believed. The return to external fertilization seen in modern-day amphibians and bony fish would therefore be a much later development than previously thought. Another interesting possibility suggested by these new fossil finds is that the jaw – almost universally used in vertebrates for feeding – might have originally evolved to aid males in successful copulation.Adaptation, Paleonotology, Sex and Reproduction