At the Swiss Light Source SLS, X-ray tomographic microscopy of tiny, 400-million-year-old fish shows how anatomy geared toward evading predators equipped it to become the hunter once jaws evolved.
An international team led by scientists from the Canadian Museum of Nature and the University of Chicago reconstructed the brain, heart, and fins of an extinct fish called Norselaspis glacialis from a tiny fossil the size of a fingernail. They found evidence of change toward a fast-swimming, sensorily attuned lifestyle well before jaws and teeth were invented to better capture food.
“These are the opening acts for a key episode in our own deep evolutionary history,” said Tetsuto Miyashita, research scientist at the Canadian Museum of Nature and lead author of the new study published in the journal Nature.
Jaws changed everything – but maybe not first
Fish have been around for half a billion years. The earliest species lived close to the seafloor, but when they evolved jaws and teeth, everything changed; by 400 million years ago, jawed fishes dominated the water column. Ultimately, limbed animals – including humans – also originated from this radiation of vertebrates.
It has long been a mystery, however, how this pivotal event occurred. The standard theory holds that jaws evolved first, and other body parts underwent changes to sustain a new predatory lifestyle. “But there is a large data gap beneath this transformation,” said Michael Coates, Professor and Chair of Organismal Biology and Anatomy at the University of Chicago and a senior author of the study. “We’ve been missing snapshots from the fossil record that would help us order the key events; to reconstruct the pattern and direction of change.”
The new study flips the “jaws-first” idea on its head. “We found features in a jawless fish, Norselaspis, that we thought were unique to jawed forms,” said Miyashita, who was formerly a postdoctoral fellow in Coates’ lab in Chicago. “This fossil from the Devonian Period more than 400 million years ago shows that acute senses and a powerful heart evolved well before jaws and teeth.”
But the team also needed a chance encounter and a special tool to gain these insights into the inner workings of Norselaspis.
Synchrotron X-rays reveal ghosts of organs never seen before
The fossil of Norselaspis the team studied is so exquisitely preserved in a fragment of rock that they were able to scan it and see impressions of its heart, blood vessels, brain, nerves, inner ears, and even the tiny muscles that moved the eyeball. The fossil was hidden in one of thousands of sandstone blocks collected during a French paleontological expedition to Spitsbergen, Norway’s Arctic Archipelago, in 1969.
Sorting through these rocks 40 years later, the study’s co-authors Philippe Janvier and Pierre Gueriau split one open, revealing a perfectly preserved cranium of Norselaspis barely half an inch long. The team took the fossil to Switzerland to scan it with high-energy X-ray beams at the TOMCAT beamline of the Swiss Light Source SLS.
“We used a technique known as X-ray tomographic microscopy,” said Federica Marone, TOMCAT beamline scientist at the SLS. “This allowed us to non-destructively study the 3D details of the fossil at very high resolution, and gain insights that have never been seen before,”
The result was jaw-dropping. Slice by slice, the X-ray images revealed with astonishing detail delicate bone membranes that enclosed the fish’s organs. These tissue-thin bones capture the ghosts of organs formerly held by the skeleton.
“Making use of the tiny refraction of the X-ray beam going through the sample, in addition to its commonly used absorption, we have been able to boost the contrast between similar tissues,” explains Marone. “This enabled us to image these tiny bones, only a hundredth of a millimetre wide, which show the imprints of now lost organs.”
Read more on SLS website
Image: Norselaspis glacialis was a jawless fish from the Devonian period 400 million years ago, which had anatomical features such as a larger heart and sensory organs that allowed later fish to adapt to a predatory lifestyle once jaws evolved.
Credit: Kristen Tietjen, University of Kansas
