By Katrina Jones, The Center for Functional Anatomy and Evolution, Johns Hopkins University
Around 400 million years ago, animals underwent a transition that changed the history of life on Earth: vertebrates ventured onto land. The evolution of four-legged tetrapods from fish is recorded by remarkable fossils like Ichthyostega. Scientists originally believed this early tetrapod walked on land like a salamander. However, new research from the Royal Veterinary College shows that Ichthyostega could not ‘walk’ like any living tetrapod. Instead, it dragged itself by its forelimbs like an amphibious fish.
Since the 1920’s scientists have been traveling to Kaiser Franz Joseph Fjord, Greenland, to unearth fossils that provide clues about the first vertebrate invasion of land. These rocks were formed in the Devonian period, 360 million years ago, when Greenland’s icy tundra was a tropical, monsoonal river basin. Ichthyostega flourished here in freshwater lakes and rivers. Many fossil remains of the first tetrapods have been discovered at this site, yet their biology remains mysterious. Which of these animals could actually live and walk on land? What was the sequence of evolutionary events that led to a vertebrate invasion of land?
A team from the University of Cambridge and the Royal Veterinary College, spearheaded by Dr. Stephanie Pierce, has set out to understand the locomotor evolution of early tetrapods. “Understanding early tetrapods and reconstructing their biology is incredibly important to understanding the first stages of the invasion of land, and how that set up biodiversity for the rest of Earth’s history,” says Dr. Pierce. Tetrapods first evolved in water, and then later used their limbs for terrestrial locomotion. Ichthyostega is exceptionally important because it represents an intermediate stage; it had tetrapod-like limbs, yet it spent time in the water and possibly on land. Therefore, if Dr. Pierce’s team could reveal how Ichthyostega moved on land, it could help them to understand the origin of terrestrial locomotion.
To test the capabilities of Ichthyostega’s limbs, Dr. Pierce created a 3-D computer model of its skeleton from 12 different fossils. The starting point for the model was a remarkable fossil, affectionately known as ‘Mr. Magic’. “Mr. Magic almost looks like a mummified animal. You’ve got all the ribs and the vertebral column entombed inside. You’ve also got a beautifully preserved shoulder girdle and forelimb.” To create their 3-D model the team used an X-ray CT scanner, similar to those used in the imaging of medical patients, to map the fossil bones. Dr. Pierce explains, “It was a real challenge; to not only scan the fossils, but to pull the data out of those X-rays. We’re talking about 360 million year old fossils. The fossils have a similar density to the rock that’s surrounding them, so when you pass X-rays through, it’s hard to tell the difference between the fossil and the rock. That was really, really challenging.” When scans of all the fossils were pieced together, the result was breathtaking: a complete 3-D skeleton of Ichthyostega.
Dr. Pierce used the 3-D model to measure the amount of mobility in the shoulder, elbow, hip and knee joints of Ichthyostega. She then compared it to mobility in five modern animals that can move both in water and on land: a salamander, a crocodile, a platypus, an otter, and a seal. The results were surprising. Ichthyostega had less mobile joints than any of the living animals. In particular, it could not rotate either its hip or its shoulder as they can, which is very important when walking on four legs. This lack of rotation means that Ichthyostega would have been unable to move its limbs one after the other, as in normal tetrapod walking. Instead, it would have moved both forelimbs at the same time. Another big difference is that the hindlimb of Ichthyostega was oriented at 45 degrees to the ground. This orientation, combined with the lack of rotation at the hip and length of the leg, means that its hind foot could not have made contact with the ground. Ichthyostega would have been unable to lift its body up onto four legs. “Comparing Ichthyostega to the modern animals that we looked at, we thought that we could probably rule out a typical quadrupedal gait” remarks Dr. Pierce. “The forelimbs are crutching the animal along, but the hindlimbs would have just been there trailing for support. When the animal was swimming, we could envision the hindlimbs, which were very paddle-like, being used in co-ordination with the tail to create propulsive forces.”
These findings caused Dr. Pierce and her colleagues to reconsider their choice of living animals, to better represent Ichthyostega. “So this got us thinking about what kinds of animal might use synchronous movement of the forelimb and not really use their hindlimbs.” She continues “The one we decided to go for was the mudskipper, because it showed a very similar type of movement to what was potentially used in the forelimb of Ichthyostega.” Mudskippers are amphibious fish that use their pectoral fins to navigate intertidal beaches searching for food, and use simultaneous, crutch-like motions of both forelimbs to drag themselves across the mud. Dr. Pierce’s reconstruction of Ichthyostega’s joint motions matched those observed in mudskippers by other scientists. The odd gait of this peculiar fish may be the nearest thing today to the excursions of Ichthyostega onto land 360 million years ago.
However, many questions about early tetrapod locomotion remain unanswered. There are fossilized footprints from Poland that also date from the Devonian period and tell a different story of early tetrapod locomotion. The creature that made them alternated its left and right limbs during walking, unlike Ichthyostega. Therefore, some early tetrapods may not have moved like mudskippers. Next, Dr. Pierce’s team intends to use 3-D joint reconstruction to discover how other Devonian tetrapods walked. By using this state-of-the-art technology to reassemble ancient fossils, we may be able to finally understand how our ancestors took their first tentative steps onto land.
Katrina Jones began her scientific career in the UK, where she studied geology at Cambridge University (Ba and MSci). She made the transatlantic hop in 2009, to pursue her PhD in Functional Anatomy at Johns Hopkins, Baltimore. Katrina is fascinated by the processes that drive skeletal evolution. Her thesis asks how the spine has adapted to changing body size in living and extinct mammals.