Some of the earliest vertebrates with four limbs enjoyeda decade-long childhood, but spent that whole time living in water – not on land.
A team of palaeontologists from Europe analysed upper arm bones of a pile of ancient part-fish, part-amphibian Acanthostega fossils and found the whole lot were still yet to reach adulthood when they died some 365 million years ago. Their remains, reported in Nature, also show the animals were purely aquatic as juveniles but could adapt quickly.
“It’s a really really cool discovery,” says Brian Choo, a palaeontologist at Flinders University in Adelaide, Australia.
“It was once thought maybe they evolved legs as babies so they could walk from pool to pool. But no, this work shows that’s not the case.”
When fish grew limbs and started dragging themselves out of the water in the late Devonian era, they represented one of the major leaps in our evolution.
But getting into the nitty-gritty of their lifestyle has been tricky. These first tetrapods (meaning ‘four-footed’) lived hundreds of millions of years ago and fossil remains are few and far between.
This changed in 1987, when Jennifer Clack from the University of Cambridge in the UK found a “mass-death deposit” in East Greenland of more than 200 skeletal remains, including 14 skulls belonging to the Devonian tetrapod Acanthostega.
It’s thought a school of the animals was washed into a pool of water during a flood – which then dried up.
To find out how old the individuals were when they died, Clack, along with Sophie Sanchez and Per Ahlberg from Uppsala University in Sweden and Paul Tafforeau from the European Synchrotron in France, analysed the four humeri – or upper arm bones – found in the deposit.
A non-destructive but highly sensitive scanning technique called propagation phase contrast synchrotron microtomography let the team visualise the bones’ fine structure in 3-D, as well as the amount of bone tissue in each sample.
Acanthostega’s skeleton began as a structure made of cartilage, but as the tetrapod grew, it laid down bone cells in a process called ossification.
Clack and the team concluded the four specimens were all juveniles when they met their untimely end, but different ages.
The tetrapods also spent at least six years, and more likely around 10, living in water before reaching adulthood.
Choo points out the same phenomenon is seen in the Queensland lungfish, a living fossil that’s remained unchanged for the best part of 100 million years and “spends more than 10 years a tadpole”.
Other ancient fishes and tetrapods had long larval stages too, with Neoceratodus spending up to 20 years a juvenile.
“Most of the fossils we are calling ‘tetrapods’ from this age could really just be advanced tadpoles,” says John Long, a palaeontologist also from Finders University.
“We might yet not know what an adult early tetrapod was like.”
Strangely, the Acanthostega humeri degree of ossification didn’t correlate with bone length. This means some Acanthostega individuals started laying down bone when they were younger than others – perhaps due to external pressures that forced them to use their limbs, perhaps in very shallow pools, earlier.
This is “most compelling”, writes Nadia Froebisch from the Museum of Natural History in Berlin, Germany in a News and Views article. Such developmental plasticity, where species respond to their environment throughout life, is thought to “have an important role in evolution”.
“Using life histories is another way to inform us about how complex the fish-tetrapod transition was,” Long says.
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