1] School of Biological Sciences, Flinders University, 2100, Adelaide, South Australia 5001, Australia [2] Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, California 9007, USA [3] Museum Victoria, PO Box 666, Melbourne, Victoria 3001, Australia.
Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia.
Nature. 2015 Jan 8;517(7533):196-9. doi: 10.1038/nature13825. Epub 2014 Oct 19.
Reproduction in jawed vertebrates (gnathostomes) involves either external or internal fertilization. It is commonly argued that internal fertilization can evolve from external, but not the reverse. Male copulatory claspers are present in certain placoderms, fossil jawed vertebrates retrieved as a paraphyletic segment of the gnathostome stem group in recent studies. This suggests that internal fertilization could be primitive for gnathostomes, but such a conclusion depends on demonstrating that copulation was not just a specialized feature of certain placoderm subgroups. The reproductive biology of antiarchs, consistently identified as the least crownward placoderms and thus of great interest in this context, has until now remained unknown. Here we show that certain antiarchs possessed dermal claspers in the males, while females bore paired dermal plates inferred to have facilitated copulation. These structures are not associated with pelvic fins. The clasper morphology resembles that of ptyctodonts, a more crownward placoderm group, suggesting that all placoderm claspers are homologous and that internal fertilization characterized all placoderms. This implies that external fertilization and spawning, which characterize most extant aquatic gnathostomes, must be derived from internal fertilization, even though this transformation has been thought implausible. Alternatively, the substantial morphological evidence for placoderm paraphyly must be rejected.
有颚脊椎动物（颌口类）的繁殖方式包括体外受精或体内受精。通常认为体内受精可以从体外受精进化而来，但反之则不然。在某些盾皮鱼类（化石有颚脊椎动物，最近的研究中被归为颌口类的基干群的并系群）中存在雄性交配爪。这表明体内受精可能是颌口类的原始特征，但这样的结论取决于证明交配不仅仅是某些盾皮鱼类亚群的特有特征。在这种情况下，一直以来人们对最不具冠群特征的盔甲鱼目（antiarchs）的生殖生物学知之甚少。在这里，我们发现某些盔甲鱼目雄性具有真皮交配爪，而雌性则具有成对的真皮板，推测这些结构有助于交配。这些结构与骨盆鳍无关。交配爪的形态与更具冠群特征的盔甲鱼目相似，表明所有盾皮鱼类的交配爪都是同源的，体内受精是所有盾皮鱼类的特征。这意味着体外受精和产卵，这是大多数现生水生颌口类的特征，必须是从体内受精进化而来的，尽管这种转变曾被认为不太可能。或者，必须拒绝大量支持盾皮鱼类并系群的形态学证据。
