The transition from life in Earth’s watery depths to life on land occurred roughly 385–360 million years ago. All modern four-limbed land animals – “tetrapods” including mammals, birds, reptiles and amphibians – can trace their lineage back to pioneering land dwellers from this time.
But how aquatic animals moved about on land has been a matter of speculation. Until now, palaeontologists have largely relied on fossils and track marks to make their predictions.
The new study takes a different approach.
The team, led by physicist Daniel Goldman at the Georgia Institute of Technology, used mudskippers and a robot to model how early tetrapods moved.
“You certainly need the fossils,” says Goldman, “but for function, the fossils aren’t sufficient.” This is especially true for surfaces such as mud or sand, which yield and deform underfoot.
The African mudskipper acted as a real-life test case. The small amphibious fish takes frequent jaunts onto shoreline mudflats, planting its front pectoral fins on the mud and then flopping forward in what’s called a “crutching” motion – its fins acting as short crutches.
Every so often, the mudskipper uses its tail for extra oomph. By bending the tail at a right angle to its body, digging in and then straightening, the mudskipper limps forward further than it would with fins alone.
On level ground, mudskippers used their tail only rarely. But on a slope – such as a riverbank – the tail becomes more important. Mudskippers used their tail in a third of all steps on a 10° slope, and in over half of all steps when climbing a 20° slope. On the steeper slope, the tail doubled the distance the mudskipper moved.
To better gauge which aspects of the mudskipper’s walk are important, the team engineered the affectionately named “MuddyBot” robot.
The simple two-limbed, single-tailed contraption was made to scale beds of poppy seeds – analogous to sand, yet unable to jam its delicate motors – using different fin and tail movement parameters.
Similar to the mudskipper, MuddyBot’s tail was beneficial on the challenging sloped surfaces, but was of little help on level ground. Coordination between fins and tail was also essential.
In early tetrapods, says Goldman, “having an extra appendage, which would be coordinated properly, would have been useful to enable them to get up hills”.