Evolution of Vertebrate Reproduction

The discovery of reproductive structures and embryos in basal jawed vertebrates, in addition to nursery sites, has resulted in a greater understanding of the diversity of reproductive strategies present in early gnathostomes. The presence of both dermal and perichondral components to the pelvic girdle in placoderms, in addition to complex musculature, absent in extant gnathostomes, indicates that the pectoral and pelvic girdles are not as morphologically distinct as previously thought. This suggests that arguments against homology, based on dissimilar skeletal and muscular morphology need to be revised. Evidence that oviparity preceded viviparity is present in the fossil record of placoderms (antiarchs versus ptyctodonts and arthrodires), coelacanths and teleosts. The identification of intromittent structures in placoderms, separate from the pelvic skeleton, suggests that the intromittent organs of placoderms and sharks are not homologous, nor are the gonopodia, modifications of the male analfin in actinopterygians. These diverse reproductive structures reveal that internal fertilisation and live birth evolved independently and multiple times within the jawed vertebrates.