Acanthostega: The shape of things to come

File:Acanthostega MLCS.JPG - Wikipedia
Acanthostega wondering what you’re looking at!
Image Credit: Conty, https://en.wikipedia.org/wiki/File:Acanthostega_MLCS.JPG

Throughout earth’s history there have been many major leaps in evolution; the evolution of eyes, the first multicellular animals and (from our point of view) the first time our hominid ancestors walked upright. However one that sticks out in a few people’s minds is when vertebrates first hauled themselves out of the water and started walking on land. To illustrate just how big this step was, imagine an alternate reality where it never happened. This parallel world would contain no reptiles, birds or mammals, and human civilisation would’ve never emerged. The vertebrates in this world are comprised of a wide variety of fish species swimming in seas, lakes and rivers across the world alongside a range of molluscs, crustaceans and corals (to name a few). On land the world is still covered in thousands of plant species but the only animals are invertebrates. Beetles, arachnids, and ants of all possible sizes scuttle along the ground. Dragonflies, wasps and flies buzz and dance through the air and worms bury through the soil keeping the ecosystem together. All in all, it is a world radically different to what we know.

As a result documenting how and why this important moment in life on earth occurred is key to understanding the world around us. One animal that has helped palaeontologists to do this is a 60 cm long stem-tetrapod that swam the rivers of Greenland during the Devonian Period (360 million years ago). Its name was Acanthostega gunnari, meaning “Gunnars spiny roof”.

File:Acanthostega model.jpg
A model of a swimming Acanthostega
Image Credit: Dr. Günter Bechly, https://commons.wikimedia.org/wiki/File:Acanthostega_model.jpg

While fossils of Acanthostega were first discovered in 1933 (and described in 1952 by Gunnar Säve-Söderbergh, who the species is named after, and Erik Jarvik) the majority of what we know about it comes from a magnificent bone bed, part of the Celsius Bjerg Group rock sequence found in East Greenland, that was discovered in 1987 by a team led by Palaeontologist Jenny Clack. These beds contains the remains of multiple Acanthostega buried and preserved with their skeletons almost completely intact. A paper released in 2016 (Sanchez et. al. 2016) detailed another interesting observation about these fossils. Micro CT synchrotron scans of the interior of the arm bones showed that the bones were still reasonably cartilaginous and had yet to fully ossify (i.e. harden into fully formed bones). This ossification happens as animals mature, so it was deduced that all of the 1987 fossils were of juvenile Acanthostega (roughly around 6 years old) who seem to have been living together. The ossification process also seemed to have progressed further in some individuals than in others, suggesting that there was size variation between members of the group, either through genetic variation, sexual dimorphism or even both. Tragically for this ragtag group of youngsters, it seems that they all died together. It is thought that a flash flood might have washed all of them into a small pool of water. This then dried up after the flood receded leaving them stranded and exposed to the elements, away from the water that kept their skin from drying out.

Acanthostega is a great example of a transitional fossil. Its anatomy is comprised of both basal fish-like features (e.g. internal gills, fish-like teeth, fleshy tail fins and a lateral line system) and derived tetrapod-like features (e.g. simple lungs and limbs tipped with digits). Curiously all these features would have made Acanthostega perfectly suited for its river home. It used its fleshy tail to power itself through its river home, snapping at any fish that wondered too close, and to help locate its prey and navigate through its watery environment it used a lateral line system to sense movement and pressure gradient changes. These features (along with its internal gills) meant it stayed underwater for long stretches of time, though its simple lungs enabled it to take breathes of air if required. What surprised palaeontologists the most about Acanthostega was the structure and function of its limbs and the number of digits on each limb. The limbs were not large or robust enough to bear Acanthostega’s weight for long, meaning it would only rarely spend time on land (if at all). Instead the limbs acted as paddles, aiding with swimming and manoeuvring underwater. This is important because it showed that the early tetrapods didn’t evolve limbs when they started walking on land, but instead first evolved them to better aid them underwater. Then later in time they would adapt this pre-existing feature to use for walking on land. The story is the same for its digits. Each of Acanthostega’s limbs were tipped with 8 digits. This showed that the number of digits on stem-tetrapod limbs wasn’t restricted to a set number (originally thought to have been 5). These early digits would have had webbed and made the early limb a more effective paddle. Then later in evolutionary time digits (like limbs) evolved to help bear and spread out the vertebrate’s weight when it was on land.

File:Acanthostega gunnari.jpg - Wikimedia Commons
A skeletal of reconstruction of Acanthostega. Note its 8 digits, flat skull and paddle-tail.
Image Credit: Ryan Somma, https://commons.wikimedia.org/wiki/File:Acanthostega_gunnari.jpg

Looking at all of its features its certain that Acanthostega would have actually spent almost all of its time in water, patrolling the waterways and hunting for small fish and arthropods. Its fish like skull features and relatively weak bite force (adapted more for gripping prey) compared to later tetrapods were perfectly adapted for catching slippery aquatic prey, meaning it didn’t hunt terrestrial animals. Like modern day amphibians Acanthostega would have laid its eggs in water as the eggs lacked a hard watertight casing. Throughout its life Acanthostega would also have had to watch its back! Multiple species of large freshwater fish were alive during the Late Devonian and many of them would have seen Acanthostega as a tasty meal.

So while Acanthostega wouldn’t have been much of a “land lubber”, it was a shape of things to come. This small river dweller helped palaeontologists to figure out the early evolutionary history of the stem-tetrapods and showed that limbs and digits, those features that you use every day, were first developed for underwater use, and only later on evolved for use on land.

All that we know about Acanthostega, the evolution of limbs and digits and how vertebrates first ventured out of the water, couldn’t have been possible without the hard work and dedication of Jenny Clack. Before her work this evolutionary transition period wasn’t particularly well understood. However her meticulous research on every facet of Acanthostega (whose fossils she sometimes gave nicknames to, such as “Boris”, “Rosie” and “Grace”) and its relatives, revolutionised our understanding of this key period of vertebrate evolution. She was one of the world’s leading experts on stem-tetrapods and Acanthostega in particular. This is clear to see as almost every scientific paper released about Acanthostega over the last three decades has carried her name either as a researcher or as a source. Sadly Jenny Clack passed away in March of this year (at time of writing). She will be greatly missed by her friends, family and the wider scientific community. With her passing, the world has lost one of the great palaeontologists.

References/Further Reading

Sanchez et. al. 2016 paper detailing the growth and life history of Acanthostega

Sanchez, S., Tafforeau, P., Clack, J. et al. Life history of the stem tetrapod Acanthostega revealed by synchrotron microtomography. Nature 537, 408–411 (2016). https://doi.org/10.1038/nature19354

Clack 2002 paper on the skull roof of Acanthostega

Clack, J. (2002). The dermal skull roof of Acanthostega gunnari, an early tetrapod from the Late Devonian. Transactions of the Royal Society of Edinburgh: Earth Sciences, 93(1), 17-33. doi:10.1017/S0263593300000304

Neenan et. al. 2014 paper on the feeding biomechanics of Acanthostega

James M. Neenan, Marcello Ruta, Jennifer A. Clack and Emily J. Rayfield (2014) Feeding biomechanics in Acanthostega and across the fish–tetrapod transition, Proc. R. Soc. B.28120132689, https://doi.org/10.1098/rspb.2013.2689

Porro, Rayfield & Clack 2015 paper on a 3d reconstruction of an Acanthostega skull. This allowed the trio to infer how Acanthostega caught prey.

Porro, Laura B et al. (2015) “Descriptive anatomy and three-dimensional reconstruction of the skull of the early tetrapod Acanthostega gunnari Jarvik, 1952.” PloS one vol. 10,3 e0118882, doi:10.1371/journal.pone.0118882

Tree of Life web project section on Acanthostega, written by the Late Jenny Clack

Clack, Jennifer A. 2006. Acanthostega. Acanthostega gunnari. Version 13 June 2006. http://tolweb.org/Acanthostega_gunnari/15016/2006.06.13 in The Tree of Life Web Project, http://tolweb.org/

Another Tree of Life project section written by Jenny Clack on the definition of “Tetrapod” and how it is debated

Clack, Jennifer A. 1997. The Definition of the Taxon Tetrapoda, 1997, http://tolweb.org/accessory/Definition_of_the_Taxon_Tetrapoda?acc_id=471 in The Tree of Life Web Project, http://tolweb.org/

The University of Cambridge Department of Zoology news article on the passing of Professor Jenny Clack

Aucott, Rachael, “Professor Jenny Clack, FRS, 1947-2020”, University of Cambridge, 26th March, 2020, https://www.zoo.cam.ac.uk/news/professor-jenny-clack-frs-1947-2020

A Science Direct web page about lateral line systems

Science Direct, “Lateral Line System”, Science Direct, https://www.sciencedirect.com/topics/medicine-and-dentistry/lateral-line-system

Clack & Neininger 2000 paper on the Celsius Bjerg Group, a rock sequence that Acanthostega fossils have been found in

Clack, J. A. and S. L. Neininger (2000). “Fossils from the Celsius Bjerg Group, Late Devonian sequence, East Greenland; significance and sedimentological distribution.” Geological Society, London, Special Publications 180(1): 557-566.

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