New research recently published in Nature suggests the earliest known version of internal copulation occurred in placoderms, challenging previously held theories of early vertebrate evolution. If current research is correct, then “external fertilization and spawning employed by the majority of recent bony fishes and many lissamphibians must have evolved from clasper-mediated internal fertilization,” says Flinders University paleontologist John Long and colleagues.

Fossils involve Microbrachius dicki, an early vertebrate fish within a group called placoderms. Within this group, the first instances of jaws, teeth and paired limbs have been observed; they now may also be known for developing internal fertilization and sexual dimorphism.

Researchers investigated the fossil record of antiarchs, a primitive group of placoderms, and have found dermal claspers in the males, and paired dermal plates in the females.  This is the first example of sexual dimorphism known on record. They suggest these traits are homologous across all placoderms, citing evidence they argue supports the notion that these structures are not associated with pelvic fins.

Specific fossils suggest that the male Microbrachius dicki used its claspers to reach the specialized genital plates in the female, thereby fertilizing her eggs. In this fish, the claspers were made of solid bone, thus rigid and stationary. This leads to Longs hypothesis that these fish “did it sideways, square dance style.”

In 2008, Long and colleagues described another armored, 380-million-year-old fish that carried embryos inside its body. According to National Geographic, this motivated the search to find evidence of internal fertilization within placoderms, where Long came across a collection belonging to paleontologist Elga Mark Kurik last year in Tallinn, Estonia.

These fossils along with further discoveries of related fish belonging to a greater group of placoderms revealed specialized male claspers, which function like a penis, and female genital plates that the fish used to copulate.

If their hypothesis is true, then this is the first instance in vertebrates where external copulation evolved from internal copulation, and not the other way around. In invertebrates, this may have happened many times.