Evidence of reproductive biology is extremely rare in the fossilrecord. Recently the first known embryos were discovered withinthe Placodermi1, an extinct class of armoured fish, indicating a viviparousmode of reproduction in a vertebrate group outside thecrown-group Gnathostomata (Chondrichthyes and Osteichthyes).These embryos were found in ptyctodontids, a small group of placodermsphylogenetically basal to the largest group, the Arthrodira2,3.Here we report the discovery of embryos in the Arthrodira insidespecimens of Incisoscutum ritchiei from the Upper Devonian GogoFormation of Western Australia4 (approximately 380 million yearsago), providing the first evidence, to our knowledge, for reproductionusing internal fertilization in this diverse group. We show thatIncisoscutum and some phyllolepid arthrodires possessed pelvicgirdles with long basipterygia that articulated distally with an additionalcartilaginous element or series, as in chondrichthyans, indicatingthat the pelvic fin was used in copulation. As homologybetween similar pelvic girdle skeletal structures in ptyctodontids,arthrodires and chondrichthyans is difficult to reconcile in the lightof current phylogenies of lower gnathostomes2,3,5, we explain thesesimilarities as being most likely due to convergence (homoplasy).These new finds confirm that reproduction by internal fertilizationand viviparity was much more widespread in the earliest gnathostomesthan had been previously appreciated.
Long, J. A., Trinajstic, K., & Johanson, Z. (2009). Devonian arthrodire embryos and the origin of internal fertilization in vertebrates. Nature, 457(7233), 1124-1127. https://doi.org/10.1038/nature07732