Recent finds1,2 demonstrate that internal fertilization and viviparity (live birth) were more widespread in the Placodermi, an extinct group of armoured fishes, than was previously realized. Placoderms represent the sister group of the crown group jawed vertebrates (Gnathostomata)3,4, making their mode(s) of reproduction potentially informative about primitive gnathostome conditions. An ossified pelvic fin basipterygium discovered in the arthrodire Incisoscutum ritchiei was hypothesized to be identical in males and females, with males presumed to have an additional cartilaginous element or series forming a clasper. Here we report the discovery of a completely ossified pelvic clasper in Incisoscutum ritchiei (WAM 03.3.28) which shows that this interpretation was incorrect: the basipterygium described previously1 is in fact unique to females. The male clasper is a slender rod attached to a square basal plate that articulates directly with the pelvis. It carries a small cap of dermal bone covered in denticles and small hooks that may be homologous with the much larger dermal component of the ptyctodont clasper.
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We thank M. Siverson for access to specimens at the Western Australian Museum. J.L. and K.T. are supported by Australian Research Council Discovery grants, and field work at Gogo in 2001 was supported by an Australian Geographic grant. P.A. is supported by the Swedish Research Council. K. Bifield prepared the specimen.
Author Contributions J.L. found WAM 03.3.28 and supervised the preparation, P.A. correctly identified the clasper, and all authors contributed to the description of the material.
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Ahlberg, P., Trinajstic, K., Johanson, Z. et al. Pelvic claspers confirm chondrichthyan-like internal fertilization in arthrodires. Nature 460, 888–889 (2009). https://doi.org/10.1038/nature08176
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