Pelvic claspers confirm chondrichthyan-like internal fertilization in arthrodires

Recent finds1,2 demonstrate that internal fertilization and viviparity(live birth) were more widespread in the Placodermi, an extinctgroup of armoured fishes, than was previously realized.Placoderms represent the sister group of the crown group jawedvertebrates (Gnathostomata)3,4, making their mode(s) of reproductionpotentially informative about primitive gnathostome conditions.An ossified pelvic fin basipterygium discovered in thearthrodire Incisoscutum ritchiei was hypothesized to be identicalin males and females, with males presumed to have an additionalcartilaginous element or series forming a clasper.Herewe report thediscovery of a completely ossified pelvic clasper in Incisoscutumritchiei (WAM 03.3.28) which shows that this interpretation wasincorrect: the basipterygium described previously1 is in fact uniqueto females. The male clasper is a slender rod attached to a squarebasal plate that articulates directly with the pelvis. It carries a smallcap of dermal bone covered in denticles andsmall hooks thatmay behomologous with the much larger dermal component of theptyctodont clasper.