Fossil captures 380-million-year-old live birth
PARIS (AFP) — She is the oldest mother of any species ever found, a 380-million-year-old fish immortalised in a fossil while still attached to her offspring by an umbilical cord.
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 fertilisation" -- that is, sex with penetration, the study says.
About 25 centimetres (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.
Often called the dinosaurs of the seas, they ruled the world's lakes and oceans for almost 70 million years.
The new find, remarkably preserved in three dimensions, contains a single, intra-uterine embryo connected by a calcified umbilical cord.
Partly hidden under a fossilised fin, the cord and embryo would have gone entirely unnoticed but for a last-minute hunch, said Trinajstic, a professor at the University of Western Australia in Crawley.
"John and I were just going to write up the fish, describe it anatomically," she recalled in a phone interview. "But we decided to give it one last acid bath to see if we could expose more of the shoulder from the rock."
It was a risky move, she said. Too much acid, and "the whole think crumbles."
The went for lunch and came back an hour later. "When we pulled it out of the acid, the embryo was just sitting there -- is was so perfectly preserved, so clear, it could not have been anything else."
Because they had already dated the fossil, the two researchers, who have been working together since the mid-1980s, knew immediately that they had a major find on their hands.
"I remember thinking, 'Oh wow, we've really scored big this time'," Trinajstic said.
They further examined the fossil with high-resolution scanning electron microscopy and computer tomography scanning, which also showed the path of a major blood vessel inside the umbilical cord.
The fish and its unborn offspring probably fell victim to rapid depletion of oxygen in the water, settling to the bottom of the sea where they were gently covered in layers of silt-like mud that hardened over time, according to the scientists.
Long and his colleagues named their find after pioneering naturalist Sir David Attenborough, who first discovered the Gogo site which during the Devonian Period was a 1400-kilometre coral reef off the Kimberley coast of northwest of Australia.
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Eine 375 Millionen Jahre alte Nabelschnur gibt Hinweise darauf, dass die Panzerfische, bei denen man bisher davon ausging, dass sie die Vorfahren der modernen Fische seien, eigentlich den Haien und Rochen näher stehen. Die derzeitige Theorie der Entwicklung der sexuellen Fortpflanzung müsste somit umgeschrieben werden.

Die Panzerfische, eine Klasse prähistorischer Fische, die auch als „Dinosaurier der Meere“ bekannt sind, stammen aus dem Erdzeitalter des Devon. Somit handelt es sich bei dem Fossil um den ältesten bekannten Fall eines Wirbeltiers, das lebende Junge geboren hat. Außerdem ist es die einzige versteinerte Nabelschnur, die je gefunden wurde.

Dr. John Long, der wissenschaftliche Leiter des Museums Victoria und Leiter des Teams, das die Fossilien untersucht, sagte, der Fund verändere „unser Verständnis der Evolution der Wirbeltiere“.

Er und Dr. Kate Trinajstic von der University of Western Australia haben eine wissenschaftliche Arbeit über das Fossil geschrieben, das während einer Expedition zur Gogo-Formation in der Nähe der Stadt Fitzroy Crossing in Western Australia gefunden wurde. Die Expedition wurde durch das Australien Research Council finanziell unterstützt. Als sie die letzten Untersuchungen an dem Fossil vornahmen, fanden sie den Embryo. Den Angaben des Forscherteams zufolge wird durch die Entdeckung die Entwicklung der Viviparie (Eigenschaft, dass Tiere lebende Tiere gebären) um 200 Millionen Jahre nach hinten verschoben. Weiterhin wird durch den Fund die erste Entwicklung interner Befruchtung gekennzeichnet – eine Besonderheit, die nun bei Haien und Rochen gefunden wurde.

Dr. Long gab der Art den lateinischen Namen „Materpiscis attenboroughi“. Materpiscis leitet sich von den lateinischen Wörtern „Mutter“ (mater) und „Fisch“ (piscis) ab, und „attenboroughi“ wurde zu Ehren von Sir David Attenborough gewählt. Er war der erste, der mit Hilfe einer Fernsehsendung aus den 1970er-Jahren die Aufmerksamkeit auf die Gogo-Region lenkte. Schon damals war er davon überzeugt, dass an diesem Ort Fossilien gefunden werden könnten.

Die Ergebnisse des Teams wurden im wissenschaftlichen Journal „Nature“ veröffentlicht. Das versteinerte Exemplar wird im Museum in Melbourne ausgestellt.