Dubbed "mother fish" by the scientists who discovered her in northwestern Australia, Materpiscis attenboroughi is not only an entirely new genus and species, but pushes back the first known case of live birth in the animal kingdom by some 200 million years.
The tail-first birthing process was probably similar to that of some species of sharks and rays living today, says the study, published Thursday in the British journal Nature.
"The discovery is certainly one of the most extraordinary fossil finds ever made, and changes our understanding of the evolution of vertebrates," commented lead researcher John Long, head of science at Museum Victoria.
Long and his colleagues were particularly astounded to find such a sophisticated reproductive system so far back on the evolutionary clock.
"It shows us that live birth was occurring at the same time as egg laying, and that these mechanisms evolved together rather than sequentially," explained co-author Kate Trinajstic, who together with Long found the fossil.
The existence of the embryo and umbilical cord within the specimen also provides the first-ever example of "internal fertilization" -- that is, sex with penetration, the study says.
About 10 inches long, "mother fish" belongs to an extinct group of vertebrates, known as placoderms, that thrived during Middle Palaeozoic Era some 420 to 350 million years ago.