I love outdated museum displays. They marvelously represent the “history” part of natural history exhibits – dusty dioramas of old ideas that are a baseline for how much our understanding has changed.
One of my favorite displays is tucked away in a dim corner of the Smithsonian National Museum of Natural History. If you just amble through the main gallery – beneath the osteological frames of the ever-popular dinosaurs – you’re bound to miss it. The humble little scene, displayed in a glass case along a wall running behind the Mesozoic celebrities, envisions one of the most important moments in evolutionary history. On a dried, cracked mudflat, an intrepid Devonian fish makes a foray out onto land. The scene represents the first steps – so far as a fish can be said to step – any vertebrate took on into the terrestrial realm, and an artistic rendition of the salamander-like Ichthyostega in the background signifies where that uncomfortable piscine shuffling would eventually lead.
Starring the fleshy-finned fish Eusthenopteron, the Smithsonian scene is a distillation of the romantic idea that the first tetrapods – the four-legged vertebrates that colonized the land, our forebears among them – were forced to adapt to a terrestrial world as arid climates dried up the ponds archaic fish had relied upon. This idea, proposed at the beginning of the 20th century, became the dramatic canon for the origin of tetrapods. There were only two options for any creature trapped in the shrinking mudholes – evolve or die. Dire necessity was the mother of anatomical invention.
But the heroic story of brave fish biting their fins at fate and adapting to a harsh, changing world has faded into a footnote in the history of science. Even though paleontologists were correct that the stacks of bones inside Eusthenopteron fins represented the archaic beginnings of what would become our own arms and five-fingered hands, the ecological backdrop was all wrong. The earliest tetrapods lived in lush, swampy environments that were not in risk of evaporating.
The anatomical evidence played a part in scuttling the traditional story, too. Eusthenopteron would have been useless on land, and recently-discovered “fishapods” such as the roughly 380 million year old fish Panderichthys and the celebrated, 375 million year old Tiktaalik used their modified, limb-like fins to get around in the water. The origin of arms, legs, fingers, and toes wasn’t a transformation that happened on land. Limbs were an aquatic innovation that just so happened to be advantageous when tetrapods began to venture out of the water.
Even the abilities of Ichthyostega and kin have come into question. With four limbs and differentiated digits, Ichthyostega would seem to be a creature that lived right at the margin between water and land. But the creature’s anatomy doesn’t match our expectations of terrestriality.
When paleontologists Per Ahlberg, Jennifer Clack, and Henning Blom reexamined and reconstructed the skeleton of Ichthyostega in 2005, they found that the overlapping ribs of the tetrapod would have significantly restrained the kind of side-to-side walk expected for such a salamander-like animal. Instead, Ichthyostega might have had a weird shuffle – in which the legs moved but the creature’s torso was kept rigid – or used an “inchworm” kind of crunch. Simply put, the tetrapod was rubbish on land, and the authors concluded that “Ichthyostega appears to be an early and ultimately unsuccessful attempt at adapting the tetrapod body plan for terrestrial locomotion.”
As Ahlberg, Clack, and Blom pointed out, though, their conclusion was preliminary. Additional study and modeling of the tetrapod’s joints would hopefully better constrain our ideas about how Ichthyostega moved. That research has now been published. In today’s issue of Nature, anatomists Stephanie Pierce, Jennifer Clack, and John Hutchinson presented the results of their investigation into how Ichthyostega got around.
Using a high-resolution, three-dimensional computer model made from skeletal scans, Pierce, Clack, and Hutchinson put their reassembled Ichthyostega through its paces. And, just as the 2005 Nature paper suggested, Ichthyostega wasn’t capable of the classic side-to-side walk used by modern newts and salamanders. In fact, the strange anatomy of Ichthyostega constrained the animal’s movements in such a way that the tetrapod must have used an unusual mode of transportation for a creature with limbs.
The tetrapod’s forelimbs were very limited in their range of motion, the researchers found, and the hindlimbs of Ichthyostega would have been virtually useless for support on Devonian mudbanks. “[T]he most likely mode of forelimb movement on land/substrate,” Pierce and co-authors concluded, “involved synchronous mudskipper-like ‘crutching’ motions.” Ichthyostega didn’t so much walk as plop around supported by its arms.
The skeleton of Ichthyostega was a weird mosaic of features relating to both life in the water and occasional forays on land. The forelimbs of Ichthyostega and similar tetrapods were the first to be co-opted for moving on land, with the hips and hindlimbs following later. And, even then, the forelimbs of Ichthyostega were not novel appendages that evolved for the purpose of walking. Crutching around in the mud was an extra advantage to limbs that evolved to allow “both station-holding and lifting of the head of of the water to breathe and potentially feed.” Strange as it seems, the limbs of Ichthyostega had more to do with life in the water than on land.
And there’s another little wrinkle to this story. Two years ago, paleontologist Grzegorz Niedźwiedzki and co-authors announced that they have found exceptionally early tetrapod tracks – footprints and trackways dating almost 20 million years before Tiktaalik. Tetrapods capable of walking on land might have evolved far earlier than anyone expected.
There was already plenty of reason to approach these trace fossils with a skeptical eye. For one thing, trace fossils created by invertebrates have frequently been mistaken for early tetrapod footprints. But the new study by Pierce, Clack, and Hutchinson provide another reason to take another look at those controversial traces. If other early tetrapods were like Ichthyostega, they would have been physically incapable of making the kinds of alternating trackways Niedźwiedzki and colleagues described. Trackways created by Ichthyostega and similar tetrapods would probably look like sets of parallel hand prints with a body impression in the middle. Pierce and colleagues caution that as-yet-unknown tetrapods with different capabilities might have created the especially old tracks, but the gap in time, resemblance of the tracks to invertebrate traces, and the inability of Ichthyostega-like tetrapods to make the tracks raises the question of what, exactly, the trace fossils from Poland represent.
Evolution defies simplistic storytelling. As much as I loved the story of the Eusthenopteron that could, that narrative has been completely tossed out. Even after specialized limbs evolved, early tetrapods had not yet settled in to life on land. What was once a nice, neat story has transmuted into a wonderful mystery. When did those fishy Devonian tetrapods start to spend more time ashore, and how did the change happen? Ichthyostega, mudskipping along in the ancient Devonian, is a fantastic clue that we are only just getting acquainted with the creatures that passed down the body plan that lets me sit here, typing away with my extremely-modified fins.
Ahlberg, P., Clack, J., & Blom, H. (2005). The axial skeleton of the Devonian tetrapod Ichthyostega Nature, 437 (7055), 137-140 DOI: 10.1038/nature03893
Niedźwiedzki, G., Szrek, P., Narkiewicz, K., Narkiewicz, M., & Ahlberg, P. (2010). Tetrapod trackways from the early Middle Devonian period of Poland Nature, 463 (7277), 43-48 DOI: 10.1038/nature08623
Pierce, S., Clack, J., & Hutchinson, J. (2012). Three-dimensional limb joint mobility in the early tetrapod Ichthyostega Nature DOI: 10.1038/nature11124