最近关于褶齿鱼类（被称为“盾皮鱼”的已灭绝化石鱼的一个小类别）能够进行体内受精和胎生的证据的发现，为我们了解生殖生物学的一种远古形式提供了一个难得的机会。
现在，研究人员在另一种“盾皮鱼”——“Incisoscutum”保存完好的化石内也发现了胚胎。这个发现很重要，因为“Incisoscutum”是“节颈类”（“盾皮鱼”的一个多样化的大类别）的一个成员。这些发现表明，“Incisoscutum”的骨盆带发生了适应性变化，以支持用于体内受精的鲨鱼“鳍脚”等器官。这些新的发现证实，体内受精和胎生方式在最早的有颚类脊椎动物中要比人们以前所认为的普遍得多
1 Museum Victoria, PO Box 666, Melbourne 3001, Victoria, Australia
2 Research School of Earth Sciences, The Australian National University, Canberra 2600, Australian Capital Territory, Australia
3 School of Geosciences, Monash University, Clayton 3800, Victoria, Australia
4 School of Earth and Geographical Sciences, The University of Western Australia, Perth 6009, Western Australia, Australia
5 Natural History Museum, Cromwell Road, London SW7 5BD, UK
Evidence of reproductive biology is extremely rare in the fossil record. Recently the first known embryos were discovered within the Placodermi1, an extinct class of armoured fish, indicating a viviparous mode of reproduction in a vertebrate group outside the crown-group Gnathostomata (Chondrichthyes and Osteichthyes). These embryos were found in ptyctodontids, a small group of placoderms phylogenetically basal to the largest group, the Arthrodira2, 3. Here we report the discovery of embryos in the Arthrodira inside specimens of Incisoscutum ritchiei from the Upper Devonian Gogo Formation of Western Australia4 (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 with long basipterygia that articulated distally with an additional cartilaginous element or series, as in chondrichthyans, 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 gnathostomes2, 3, 5, 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.
