How did they reproduce?

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.