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Did A Strong Tail Give Early Land-Invaders A Leg Up?

This guest post was contributed by David Moscato, a freelance science writer, following the recent Society of Vertebrate Paleontology Meeting in Salt Lake City, Utah. This guest post reflects the views solely of its author, which are not necessarily shared by PLOS. Thank you, David, for contributing to the PLOS Paleo Community!

Featured image: Atlantic mudskippers (Periophthalmus barbarus) photographed on the shore near Tenda-Ba in The Gambia. Image by Bjørn Christian Tørrissen, CC-BY.

It’s one of the most iconic stories of evolution: intrepid Devonian fish using their bony fins to drag themselves onto land, taking the first floppy steps in the transition from aquatic fish to terrestrial tetrapods. A good deal of research has focused on how these animals used their fins/legs to get around, but a presentation at this year’s meeting of the Society of Vertebrate Paleontology turned the focus to another important part of fish anatomy: the tail.

It may seem odd to investigate the tail as an organ for walking on land, but consider that for most fish, the powerfully-muscled tail is the main force for locomotion. It’s not far-fetched to imagine fish using their tails to thrust themselves (perhaps awkwardly) along the land.

To find out just how plausible this idea is, Dr. Rick Blob (that’s pronounced “blobe,” by the way), Professor of Biological Sciences at Clemson University, and colleagues turned to two modern-day examples: the famous mudskipper, and a charming little robot named MuddyBot.

Mudskippers are fish that spend most of their time flopping around on muddy shores in warm parts of Asia and Africa. They engage in what’s called “crutching” locomotion: using both front fins simultaneously to push themselves forward. They are also known to use their tails to propel attention-grabbing jumps.

Mudskipper 20 degree
Image provided by Clinton Colmenares

The researchers observed mudskippers on various sandy surfaces, and as it turns out, they do use their tails on slopes to give themselves an extra push. They swing their tail to one side of the body and thrust forward. On steep slopes the tail significantly improved the fish’s distance traveled, compared to a fins-only thrust, and the steeper the slope, the more frequently the fish used their tails for help.

So it seems a fish’s tail can indeed be used to improve performance on land. But the researchers wanted to ask even more detailed questions: Do the tail and fins have to be used simultaneously? Is the tail necessary for moving up slopes? Exactly how much does the tail improve forward movement? To answer these questions, the researchers needed a more precisely-controllable subject than a living, breathing mudskipper, so they built a robot.

MuddyBot is simple: two fins and a tail, designed to mimic the appendages and movements of the mudskipper, and whoever’s controlling the robot can tell it exactly how to move. So they ran MuddyBot through the same sort of trials as the fish. They didn’t use wet sand, though, for fear of the robot’s delicate machinery. Instead, it climbed up slopes of tiny seeds and plastic spheres, which served the same purpose.

The charismatic MuddyBot. Image credit: Rob Felt, Georgia Tech. Image provided by John Toon.

Those of us sitting in the room at the SVP conference were treated to videos of MuddyBot’s trials. It was clear to see how much more distance the robot gained using its tail and fins in unison, which was shown to be the optimal strategy. On the steepest slope (20°), when MuddyBot tried to use only its “fins,” it could hardly move at all (and the room of paleontologists laughed at the poor struggling robot, for we are cruel people).

But when it used its tail, MuddyBot significantly improved its trek up the steep slope. The trials showed that over the course of several “steps,” the robot could move more than three times farther with the tail-and-fins combination than with fins alone.

Being able to use a robot to test evolutionary hypotheses was very exciting for Blob, who spoke to me after his presentation. “I think that there’s really been this tremendous growth of openness to new techniques we can use to investigate Earth history,” he said.

He also pointed out, as any good scientist must, what was wrong with the study. For one thing, mudskippers and the mudskipper-bot are quite small compared to many early tetrapods. For another, crutching is not the only technique used by modern land-loving fish. African bichirs, for example, undulate their bodies, using their fins on alternate steps. And the bizarre waterfall-climbing gobies of Hawaii use suckers to inch along rocky walls.

Blob told me he is particularly interested in investigating how fish might use their hind fins on land (something mudskippers do not do), and also testing a wider selection of surfaces for them to walk over.  After all, early tetrapods came in a variety of body-types and lived in a variety of habitats. Tail-use may be only a small piece of the water-to-land puzzle.

As I watched the videos of fish and robot trudging over the sand, I couldn’t help but notice the distinctive tracks they left in the sediment. Could we find early tetrapod tail-prints? Blob told me early tetrapod trackways are generally difficult to evaluate, and nothing so far looks like those mudskipper tracks. But with a better understanding of how these living fish move through the sediment, he said, “we’ve got something to look for now.”


This research was presented at the Society of Vertebrate Paleontology, 2016.

Blob RW, McInroe B, Astley HC, Gong C, Kawano SM, Schiebel PE, Rieser JM, Choset H, Goldman DI (2016) Modeling the vertebrate invasion of land: tail use imporves terrestrial locomotor performance on soft substrates. Journal of Vertebrate Paleontology, Programs and Abstracts, 2016. 99 (Abstract Book)

The research was also published earlier this year. The videos of the mudskipper and MuddyBot trials are included in the supplementary material, so be sure to take a look!

McInroe B, Astley HC, Gong C. Kawano SM, Schiebel PE, Rieser JM, Choset H, Blob RW, Goldman DI (2016) Tail use improves soft substrate performance in models of early vertebrate land locomotors. Science 353(6295).

Supplementary Material


David Moscato is a freelance science writer from Long Island, NY with a background in paleontology. He is a contributor for Earth Touch News, and you can follow him on his blog, The Meniscus, or on Twitter @DMos150.

Are you interesting in contributing to the PLOS Paleo Community, too? Contact the editors or email us and pitch us your idea or blog post!

Wendiceratops in life, as restored by artist Danielle Dufault. CC-BY, from Evans and Ryan 2015.

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