The remains of one such male, nicknamed “Big Boy,” include the earliest known sexual organ for any vertebrate (an animal with a backbone or spinal column). Big Boy and his mate -- both primitive jawed fish -- are described in the latest issue of the journal Nature.
Their sexual intercourse, the first to involve internal fertilization, explains why these fish had tiny arms.
“We have found large L-shaped bony claspers (penis-like organs) on the males, which were so large they could have only reached the female’s genital region if the fishes were lying side by side,” lead author John Long told Discovery News.
“The tiny arms, which were jointed in these fishes, would have probably interlocked to help the male position the clasper into position for mating, hence it looked a bit like they were doing a square dance.”
Long, a paleontologist at Flinders University, and his colleagues investigated fossils for the fish, known as “antiarch placoderms,” which are the earliest primitive jawed fishes. Remains suggest that the fish from the genus Microbrachius (meaning tiny arms) lived in ancient lakes of Scotland, as well as in parts of Estonia and China.
The former, Long explained, requires more effort but protects young “from predation, allowing the mother to invest more time nurturing this young until they are ready to be born or hatch from large eggs.”
External fertilization, on the other hand, means the parents can produce numerous eggs, hoping that at least some will survive. He indicated that many fish in non-turbulent waters tend to reproduce this way, while some fish in fast-running streams tend to copulate, keeping their sperm and eggs inside their bodies, protecting them.
Long and his team now think that humans and all other vertebrates are distantly related to placoderms, since all share key traits in common: paired hind limbs; paired bony skull plates with sutures (a type of fibrous joint); similarly structured inner ears, jaws, teeth, and vertebrae; abdominal muscles; and, last but not least, sexual intercourse.
Co-author Zerina Johanson, a researcher at the Natural History Museum in London, said, “Now we know that internal fertilization is the general condition for placoderms and is the most primitive type of reproduction for vertebrates.”
Matt Friedman, a paleobiologist from the University of Oxford who did not work on the research, described the discovery as “nothing short of remarkable.”
“Claspers in these fishes demand one of two alternative, but equally provocative, scenarios: either an unprecedented loss of internal fertilization in vertebrates, or the coherence of the armored placoderms as a single branch in the tree of life,” Friedman said.
Read more at Discovery News
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