fossil – Prehistoric Otter

Enhydriodon: The Bear Otter

31st Jul 20201st Aug 2020andrewsimpson0194 Comments

Well it was coming eventually! After 24 blog articles on this humble little site, it’s time that an article on Prehistoric Otter was actually about a Prehistoric Otter! There were a few candidates for the otter of choice for this momentous occasion, but ultimately I decided that Enhydriodon shall be the one in the spotlight. So sit back, grab some snacks, and let’s learn about the life and times of the “Bear Otter”.

Enhydriodon lived in East Africa and India from 5 to 2 million years ago during the Pliocene period of the Cenozoic era. As previously stated Enhydriodon was an otter and like all otters it belonged to a larger mammalian group known as the Mustelids, whose members also include Wolverines (the animal not the superhero!), Ferrets and Badgers. The earliest otters evolved around 23 million years ago with the first “modern” otters arriving on the scene 7 million years ago; a full 2 million years before Enhydriodon. Otters evolved from land based ancestors who became semi aquatic partly due to the exciting food resources available (e.g. fish, crustaceans and shellfish) and partly to help escape larger, scarier predators. Modern Sea Otters take this concept to the extreme and have become fully aquatic marine animals but the majority of todays otters still maintain a tie to the land. Otters took to their watery home with gusto and over millions of years they evolved webbed feet and a snazzy waterproof fur coat. This coat is so snazzy that unfortunately humans would hunt otters specifically for it, or even just for sport, to use the fur in the fashion business. Once these adaptations were in place the otters diversified and spread across the globe producing a wide variety of species. Along the way many species came and went, and Enhydriodon was one of them.

The first Enhydriodon fossils were discovered way back in the early 19^th century, with the first known species, Enhydriodon sivalensis from the Sewalik hills in Northern India, being described and named by Dr Hugh Falconer of the British Museum. Since then further fossils have been unearthed of other Enhydriodon species. One notable species was described in 2011 and named Enhydriodon dikikae, which was found in East Africa; specifically Dikika in Ethiopia and Kanapoi in Kenya. What we know about Enhydriodon comes from a few fossils of a snout, lower jaw, back of the skull humerus and fragments of a femur. That’s not a lot to go on, but from what palaeontologists do have they can ascertain a few key features. For example, we know that its short snouted skull had a battery of broad incisors, powerful canines and crushing molar teeth. Luckily because we know of other similar extinct giant otters (such as the wolf sized Siamogale from South-West China, which lived just) we can compare these finds against these extinct otters and similar modern day otters in order to give us a rough reconstruction of its probable life appearance.

So what makes Enhydriodon a particularly special otter? Well, from the aforementioned fossils scientists have estimated that Enhydriodon sivalensis was roughly the size of a panther. However this was topped by Enhydriodon dikikae, which grew to over 2.1 metres long and weighed somewhere between 200-400 pounds, making it the largest otter (and largest mustelid) that has ever lived. By comparison Enhydriodon dikikae was larger than your average Leopard or Wolf and even approached Lion size! This was certainly not the small, cute animal that most people associate with otters today. Instead it was a large and powerful beast (though in all honesty it was probably still cute).

How this otter is thought to have lived is a matter of some debate. The original paper that described Enhydriodon dikikae (Geraards et. al. 2011) concluded that it was more land based than modern otters. However others have argued against this stating that it was more semi-aquatic like an Asian Short-Clawed Otter. However long it spent in water what is clear is that it would have ventured in at least on occasion as its variety of powerful teeth indicate a diet of water and land based prey. Potential items on Enhydriodon’s menu ranged from large fish, such as catfish, and shellfish to even small-medium sized land based mammalian herbivores like antelope. This wide ranging diet is plausible based on what we know of modern otter species. For example the modern day Giant Otter from South America is known to prey on small-medium sized Caimans and Anacondas alongside its usual diet of fish.

The East African landscape that Enhydriodon dikikae lived in consisted of open forest and Savannah grassland, crisscrossed by rivers that fed into the occasional huge lake (not to far removed from today). The animals that this Enhydriodon would see on a daily basis were a real uncanny mix of different species. On the one hand, there were animals familiar to anybody who has either been on an African Safari or watched nature documentaries, like Antelope, Hippos and Leopards to name a few. But on the flipside there were a few strange faces. These included extinct animals such as Deinotherium; a gargantuan 4.5-5 metre tall relative of elephants that weighed twice as much and possessed downward curving tusks; the “Scimitar-Toothed” cat Homotherium, Sivatherium; a 3 metre tall relative of giraffes; and giant species of Wolverines (Plesiogulo) and Baboons (Dinopithecus).

There was another, very special animal that I haven’t mentioned yet that also lived alongside the large and powerful Enhydriodon; our very own ancestors! Those same Dikika and Kanapoi deposits where Enhydriodon dikikae fossils were found also contain the fossils of the 3.5 million year old Australopithecus afarensis (specifically the skeleton of a youngster nicknamed “Salem”) and the 4.2 million year old Australopithecus anamensis respectively. Australopithecus is a landmark human ancestor because it was one of, if not the first, to walk upright, a key feature that distinguishes humans from other great apes. It’s unknown just how much Enhydriodon and Australopithecus would have interacted with one another, but based on Enhydriodon’s diet and the comparative sizes of the two (Australopithecus was only 1 to 1.5 metres tall at most) it’s probable that the Bear Otter wouldn’t have said no to hunting an Australopithecus that was lingering too close to the waters edge. That’s such a strange thought. Nowadays people generally adore otters but in the distant past your great great etc. grandparents lived in fear of being eaten by one!

Enhydriodon dikikae | Dinopedia | Fandom — *Enhydriodon* compared to one of our ancestors. From this image you can see why encountering an *Enhydriodon* might have been a bad thing for these early human relatives!
**Image Credit: Victor Leshyk, http://novataxa.blogspot.com/2012/04/2011-enhydriodon-dikikae-ethiopia.html**

So why is the bear otter no longer with us? Well this may again be linked to our human ancestors. Enhydriodon went extinct roughly 2 million years ago during a time of significant decline of multiple large African carnivore species. Coinciding with this was the evolution and diversification of a couple of different new species of human (or “Homo”) such as Homo Habilis; one of the first human ancestors to make stone tools. Some of these human species were starting to incorporate more meat into their diet becoming omnivorous and active hunter gatherers. As a result they competed with Enhydriodon and other carnivores for prey, and this increased competition may have played a role in the otter’s decline. On top of this the climate was also changing, becoming drier and promoting more open Savannah and less forest. This new habitat wouldn’t have been as suitable for Enhydriodon and would have definitely affected its population.

To me, the most fascinating aspect of Enhydriodon is not its size, it’s not whether it was semi-aquatic or not, or even its appearance (though otter fans would surely go bananas if it was alive today). It’s the connection that it has to the history of humankind. As time has passed the relationship between our ancestors and Enhydriodon changed from an animal that was feared by the Australopithecines into one that early human species actively competed with on an equal footing. The relationship between humans and otters developed further after Enhydriodon went extinct with humans hunting its otter cousins for fur and sport, and now to protecting them through conservation efforts. I guess it just goes to show that otters have always had a relationship with the human species in one form or another, and Enhydriodon was the start of it all.

References/Further Reading

• Geraards et. al. 2011 paper describing fossils of Enhydriodon dikakae from Ethiopia

Denis Geraads, Zeresenay Alemseged, René Bobe & Denné Reed (2011) Enhydriodon dikikae, sp. nov. (Carnivora: Mammalia), a gigantic otter from the Pliocene of Dikika, Lower Awash, Ethiopia, Journal of Vertebrate Paleontology, 31:2, 447-453, DOI: 10.1080/02724634.2011.550356

• A book titled “Palæontological Memoirs and Notes of the Late Hugh Falconer: Fauna antiqua”, written by Dr Hugh Falconer where he describes his paleontological discoveries, which included Enhydriodon sivalensis

Falconer, Hugh, 1868, Palæontological Memoirs and Notes of the Late Hugh Falconer: Fauna antiqua, Fauna Antiqua sivalensis.

• Mindat.org datasheet on Enhydriodon dikikae

Mindat, “Enhydriodon dikikae”, Mindat.org, https://www.mindat.org/taxon-8570226.html

• A 2012 Nature article, written by Jeff Tollefson, detailing Lars Werdelin’s research on how competition with human ancestors may have played a role in Enhydriodons and other large African carnivore extinctions

Tollefson, Jeff, “Early humans linked to large-carnivore extinctions”, News, Nature, 26^th April, 2012, https://www.nature.com/news/early-humans-linked-to-large-carnivore-extinctions-1.10508

• Bobe et. al. 2020 paper on the ecology of Australopithecus anamensis, which includes details of the animals it shared its environment with (e.g. Enhydriodon).

René Bobe, Fredrick Kyalo Manthi, Carol V. Ward, J. Michael Plavcan, Susana Carvalho, The ecology of Australopithecus anamensis in the early Pliocene of Kanapoi, Kenya, Journal of Human Evolution, Volume 140, 2020, 102717, ISSN 0047-2484, https://doi.org/10.1016/j.jhevol.2019.102717.

“All hail the Great Beast Megatherium!”

22nd May 202012th Jul 2020andrewsimpson019Leave a comment

File:Megatherium NT small.jpg - Wikimedia Commons — **A reconstruction of the Great Beast**
Image Credit: Nobu Tamura, (© N. Tamura), http://spinops.blogspot.com/2015/02/megatherium-americanum.html

“Deluded! Madman! Fake Scientist!”

The Cryptozoologist had been called these a lot throughout his professional career, (as well as other, more mean things). Time after time after time he had failed to discover any of the amazing creatures’ people claimed to have seen, with the majority being proven to never have existed in the first place. Now, nearing his retirement, he was wandering the amazon rainforest looking for yet another cryptid; the “Mapinguari”. “Just turn around” the voice in his head said (not for the first time). “They’re just stories made up to attract tourists or hoaxers trying to make a name for themselves or misidentified animals. None of them are real!”. The Cryptozoologist sighed, and for the first time in his life he wondered “Maybe I am a crazy old man”. Then he heard it, a crash of vegetation coming from the trees just to the left of him. He turned round, straining to locate exactly where the noise had come from. Then he saw it, and his jaw dropped. What he was seeing was an animal believed to have gone extinct 8,000 years ago. It was a great beast taller than an elephant and just as bulky, which possessed huge claws that it was currently using to pull down branches from a nearby tree towards its mouth. As he took out his camera and frantically took pictures two more large adults shuffled out of the forest, one of which had a baby clinging onto to its back. “They wouldn’t believe me” the Cryptozoologist thought. “But just wait till they see you!”

This “Great Beast” is known scientifically as Megatherium Americanum (meaning “Great Beast from the Americas”). Megatherium is an animal that palaeontologists have known about for a very long time. The first fossils were discovered in 1787, four decades before the first dinosaurs would be found, in Argentina by a man named Manuel Torres. After their discovery these bones were shipped to the Museo Nacional de Ciencias in Madrid, Spain, where they still reside today (another reason to visit Spain!). It was from these bones that French naturalist Georges Cuvier first described and named Megatherium, noting its close relation to modern day tree sloths. After these first fossils more were discovered, including bones found by Charles Darwin from 1832-1833 during the first Beagle expedition. Even nowadays new discoveries are revealing more insights. For example a paper published in 2017 (by Bocherens et. al.) looked at preserved collagen proteins in Megatherium fossils to give insights into its diet. Some people have gone a step further and claimed that Megatherium is still alive somewhere in South America. Stories from Brazil tell of the “Mapinguari” or “sloth monster”; a shaman who was transformed by the gods into a giant sloth-like creature. Cryptozoologists (like the one in the story) think the Mapinguari is actually a late surviving species of Megatherium, however scientists (and yours truly) don’t take these stories seriously due to absence of any concrete evidence.

**A mounted skeleton of Megatherium with a awe inspired human for scale!**
Image Credit: Beatrice Murch, https://www.flickr.com/photos/blmurch/3495336846

Megatherium belonged to a large order (or “superorder”) of mammals known as the xenarthans. Modern xenarthans include Tree Sloths, Anteaters & Armadillos, but during the Cenozoic era this group was much more diverse. From their origins in South America they ended up colonising North America, grew to a range of shapes and sizes and occupied a wide variety of habitats ranging from the treetops (e.g. modern tree sloths) to even the ocean (e.g. the swimming ground sloth Thalassocnus). Megatherium itself belonged to a sub-order of xenarthans commonly known as the “Giant Ground Sloths”. These sloths were very different from their slow moving and tree dwelling modern counterparts. They were bulky, ground living herbivores with large and sharp claws. While Megatherium itself was confined to South America other species of Giant Ground Sloths migrated across the Isthmus of Panama into Central and North America. This was during the great American interchange; a time where multiple species from South America migrated into North America (and vice-versa). As a result Giant Ground Sloths established populations in places such as Costa Rica, Texas and California.

Because multiple fossils of the “Great Beast” have been known to palaeontologists for some time we have a pretty good idea of what it would have been like. Megatherium roamed the South American pampas, mostly in Argentina, Bolivia and Uruguay, from the Pleistocene (roughly 400,000 years ago) to Early Holocene (roughly 8,000 years ago) periods of the Cenozoic era (a timespan commonly known as the “Ice Age”). This beast stood over 3.5 metres tall when fully upright and weighed up to 4 tonnes, making it the largest animal in South America during the Ice Age and the largest xenarthan ever. Its potbellied frame was supported by column-like hind legs that would have given it a long reach. Furthermore preserved Megatherium track-ways and its skeletal anatomy indicate that it could have walked on two legs as well as on all fours. Its front limbs were tipped with large, non-retractable claws which were used for pulling branches closer to them to eat and for digging up roots and tubers. In fact the claws were a reason that Megatherium was initially thought to have been a burrower, living like giant mole! Big claws would have undoubtedly been very effective defensive weapons with Megatherium using them, alongside its large size, to protect itself from predators, such as the large Sabre-Tooth Cat Smilodon populator. Other distinctive features include a relatively narrow snout, a prehensile upper lip (like a black rhino) and a thick shaggy coat. This coat is found on most Megatherium reconstructions and is based on the discovery of exceptionally preserved hair and hide specimens of related Giant Ground Sloths. However a study from 2002 (Fariña 2002) has speculated that Megatherium might’ve been nearly hairless! This is based on the observation that modern large mammals, such as elephants and rhinos, are mostly hairless to prevent them from overheating in hot climates (large animals produce a lot more excess heat). Megatherium may seem very different to what we would think of a typical large herbivore today. However the overall body plan of a large, bulky, bipedal herbivore with large claws has actually appeared a few times throughout earth’s history. One example is the Therizinosaur dinosaurs; a group which lived a full 65 million years earlier than Megatherium but is thought to have lived a similar lifestyle. This is an example of convergent evolution; where two completely unrelated organisms, often separated by millions of years of evolution, evolve similar body plans to live in similar ways. It’s a very fascinating phenomenon that has resulted in a lot of symmetry between modern and extinct animals (e.g. Dolphins and Ichthyosaurs).

**A *Megatherium* looking at the horizon as two glyptodonts waddle by!**
Image Credit: D. Bogdanov (DiBgd), https://commons.m.wikimedia.org/wiki/File:Pleistocene_SA.jpg#mw-jump-to-license

Such a majestic animal is another example how diverse the megafauna were during the last Ice Age. However the majority of these animals are not around anymore. Megatherium’s story is similar to other megafauna. Climate change at the end of the last Ice Age played a part, resulting in a loss of habitat and decline in population. This was combined with the arrival of modern humans into South America roughly 14,500 years ago. Some Megatherium bones bear distinct marks on them that indicate that they were cut by human tools. Furthermore other bones have been unearthed alongside human made stone tools and weapons. Tools, high intelligence and co-operation made humans a terrifying predator for a Megatherium to try and defend itself against and humans were so efficient that Megatherium numbers dwindled further. Eventually the dynasty of the Great Beast would come to a close 8,000 years ago. This unfortunate end makes one wish that the Cryptozoologists were right, and that Megatherium was somehow still living in South America to this day. If this were the case then I’m sure many more people would see what a “Great Beast” it really was.

References/Further Reading

• Bocherens et. al. 2017 paper reconstructing the diet of Megatherium from analysis of collagen in the fossils

Bocherens et. al. (2017), Isotopic insight on paleodiet of extinct Pleistocene megafaunal Xenarthrans from Argentina. Gondwana Research, 2017; 48: 7 DOI: 10.1016/j.gr.2017.04.003

• Billet et. al. 1997 paper examining the inner ear anatomy of Megatherium and what it tells us about its body mass and agility

Billet, G et al. “The inner ear of Megatherium and the evolution of the vestibular system in sloths.” Journal of anatomy vol. 223,6 (2013): 557-67. doi:10.1111/joa.12114

• Natural History Museum website article profiling Megatherium and detailing a project that was digitally scanning all the fossils Charles Darwin collected on the 1831-183 6 Beagle voyage

Brewer, Pip, “What was Megatherium?”, Natural History Museum, https://www.nhm.ac.uk/discover/what-was-megatherium.html

• Fariña 2002 paper suggesting that the largest Giant Ground Sloths, such as Megatherium, were mostly hairless

Fariña, Richard. (2002). Megatherium, the hairless: appearance of the great Quaternary sloths (Mammalia;Xenarthra). AMEGHINIANA. 39. 241-244.

• Politis et. al. 2019 paper, published in Sciences Advances, on the discovery of Megatherium remains that show evidence of Human Hunting

Politis, Gustavo & Messineo, Pablo & Stafford Jr, Thomas & Lindsey, Emily. (2019). Campo Laborde: A Late Pleistocene giant ground sloth kill and butchering site in the Pampas. Science Advances. 5. eaau4546. 10.1126/sciadv.aau4546.

Acanthostega: The shape of things to come

3rd May 202012th Jul 2020andrewsimpson019Leave a comment

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”.

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.