Placoderms are thought to be the first fish to reproduce
by internal fertilization. None of the early jawless fishes exhibited visible
sexual differences (sexual dimorphism). The first record of sexual distinction
appeared in the middle Devonian with the ptycotodontid placoderm
Rhamphodopsis, which had external clasping organs on males and wide pelvic
basal plates on females, indicative of internal fertilization and
thus also of sexual intercourse. Claspers are modified structures at the
base of the pelvic fins which transmit sperm directly inside the female.
It seems that ptyctodontids were the only vertebrates whose males were
armed with external bone covering the intromittent organ. Reproduction
in fishes today generally requires that the male shed sperm over the females's
eggs in the external environment.