A fossil of the fish Incisoscutum ritchiei from the Upper Devonian period (350 Ma) of the Gogo formation of western Australia contains a 5 cm-long embryo. This specimen is one of the earliest examples of a pregnant vertebrate and shows that internal fertilisation, or sex, started far sooner than previously thought.The process of internal fertilisation and giving birth to live young, or viviparity, differentiates some fish and mammals from other animals such as reptiles and amphibians. Johanson and her colleagues believe it was the main reproductive method for early fish groups such as the placoderms and could have evolved in other fish groups.
‘Sex was far more common in these primitive prehistoric animals,’ adds Dr Johanson. ‘However, copulation appears to be the main way they reproduced, demonstrating that ‘sex’ started a lot sooner than we thought.’
‘This specimen shows just how important the Museum collections are because knowledge evolves and we have new interpretations that we can apply to fossils in the collections.' link.
Abstract : Here we report the discovery of embryos in the Arthrodira inside specimens of Incisoscutum ritchiei from the Upper Devonian Gogo Formation of Western Australia (approximately 380 million years ago), providing the first evidence, to our knowledge, for reproduction using internal fertilization in this diverse group. We show that Incisoscutum and some phyllolepid arthrodires possessed pelvic girdles … indicating that the pelvic fin was used in copulation. As homology between similar pelvic girdle skeletal structures in ptyctodontids, arthrodires and chondrichthyans is difficult to reconcile in the light of current phylogenies of lower gnathostomes, we explain these similarities as being most likely due to convergence (homoplasy). These new finds confirm that reproduction by internal fertilization and viviparity was much more widespread in the earliest gnathostomes than had been previously appreciated.