Wednesday, 13 October 2021

FROM FISH TO TETRAPODS: ELPISTOSTEGE WATSONI

Elpistostege watsoni

In the late 1930s, our understanding of the transition of fish to tetrapods — and the eventual jump to modern vertebrates — took an unexpected leap forward. The evolutionary a'ha came from a single partial fossil skull found on the shores of a riverbank in Eastern Canada. 

Meet the Stegocephalian, Elpistostege watsoni, an extinct genus of finned tetrapodomorphs that lived during the Late Givetian to Early Frasnian of the Late Devonian — 382 million years ago. 

Elpistostege watsoni — perhaps the sister taxon of all other tetrapods — was first described in 1938 by British palaeontologist and elected Fellow of the Royal Society of London, Thomas Stanley Westoll. Westoll's research interests were wide-ranging. He was a vertebrate palaeontologist and geologist best known for his innovative work on Palaeozoic fishes and their relationships with tetrapods. 

As a specialist in early fish, Westoll was asked to interpret that single partial skull roof discovered at the Escuminac Formation in Quebec, Canada. His findings gave us the publication that would name Elpistostege watsoni and helped us to better understand the evolution of fishes to tetrapods — four-limbed vertebrates — one of the most important transformations in vertebrate evolution. 

Hypotheses of tetrapod origins rely heavily on the anatomy of but a few tetrapod-like fish fossils from the Middle and Late Devonian, 393–359 million years ago. These taxa — known as elpistostegalians — include Panderichthys, Elpistostege and Tiktaalik — none of which had yet to reveal the complete skeletal anatomy of the pectoral fin. 

None until 2010 that is, when a complete 1.57-metre-long articulated specimen was found and described by Richard Cloutier et al. in 2020. 

The specimen helped us to understand the origin of the vertebrate hand. Stripped from its encasing stone, it revealed a set of paired fins of Elpistostege containing bones homologous to the phalanges (finger bones) of modern tetrapods and is the most basal tetrapodomorph known to possess them. 

Once the phalanges were uncovered, prep work began on the fins. The fins were covered in wee scales and lepidotrichia (fin rays). The work was tiresome, taking more than 2,700 hours of preparation but the results were thrilling. 

Origin of the Vertebrate Hand
We could now clearly see that the skeleton of the pectoral fin has four proximodistal rows of radials — two of which include branched carpals — as well as two distal rows organized as digits and putative digits. 

Despite this skeletal pattern — which represents the most tetrapod-like arrangement of bones found in a pectoral fin to date blurring the line between fish and land vertebrates — the fin retained lepidotrichia (those wee fin rays) distal to the radials. 

This arrangement confirmed an age-old question — showing us for the first time that the origin of phalanges preceded the loss of fin rays, not the other way around.

E. watsoni is very closely related to Tiktaalik roseae found in 2004 in the Canadian Arctic — a tetrapodomorpha species also known as a Choanata. These were advanced forms transitional between fish and the early labyrinthodonts playfully referred to as fishapods — half-fish, half-tetrapod in appearance and limb morphology. 

Up to that point, the relationship of limbed vertebrates (tetrapods) to lobe-finned fish (sarcopterygians) was well known, but the origin of major tetrapod features remained obscure for lack of fossils that document the sequence of evolutionary changes — until Tiktaalik. While Tiktaalik is technically a fish, this fellow is as far from fish-like you can be and still be a card-carrying member of the group. 

Tiktaalik roseae
Complete with scales and gills, this proto-fish lacked the conical head we see in modern fish but had a rather flattened triangular head more like that of a crocodile. 

Tiktaalik had scales on its back and fins with fin webbing but like early land-living animals, it had a distinctive flat head and neck. He was a brawny brute. The shape of his skull and shoulder look part fish and part amphibian.

The watershed moment came as Tiktaalik was prepped. Inside Tiktaalik's fins, we find bones that correspond to the upper arm, forearm and even parts of the wrist — all inside a fin with webbing — remarkable! 

Its fins have thin ray bones for paddling like most fish, but with brawny interior bones that gave Tiktaalik the ability to prop itself up, using his limbs for support. I picture him propped up on one paddle saying, "how you doing?" 

Six years after Tiktaalik was discovered by Neil Shubin and team in the ice-covered tundra of the Canadian Arctic on southern Ellesmere Island, a team working the outcrops at Miguasha on the Gaspé Peninsula discovered the only fully specimen of E. watsoni found to date — greatly increasing our knowledge of this finned tantalizingly transitional tetrapodomorph. 

E. watsoni fossils are rare — this was the fourth specimen collected in over 130 years of hunting. Charmingly, the specimen was right on our doorstop — extracted but a few feet away from the main stairs descending onto the beach of Miguasha National Park. 

Miguasha is nestled in the Gaspésie or Gespe’gewa’gi region of Canada — home to the Mi’gmaq First Nations who self-refer as L’nu or Lnu. The word Mi’gmaq or Mi’kmaq means the family or my allies/friends. They are the people of the sea and the original inhabitants of Atlantic Canada having lived here for more than 10,000 years. 

The L'nu were the first First Nation people to establish contact and trade with European explorers in the 16th and 17th centuries — and perhaps the Norse much earlier. Sailing vessels filled with French, British, Scottish, Irish and others arrived one by one to lay claim to the area — settling and fighting over the land. As each group rolled out their machinations of discovery, tensions turned to an all-out war with the British and French going head to head. I'll spare you the sordid details but for everyone caught in the crossfire, it went poorly.

North America Map 1775 (Click to Enlarge)
Cut to 1760, the British tipped the balance with their win at the Battle of the Restigouche, the last naval battle between France and England for possession of the North American continent — Turtle Island. 

The bittersweet British victory sparked the American War of Independence. 

For the next twenty years, the L'nu would witness and become embroiled in yet another war for these lands, their lands — first as bystanders, then as American allies, then intimidated into submission by the British Royal Navy with a show of force by way of a thirty-four gun man-of-war, encouraging L'nu compliance — finally culminating in an end to the hostilities with the 1783 Treaty of Paris. 

The peace accord held no provisions for the L'nu, Métis and First Nations impacted. None of these newcomers were Mi'kmaq — neither friends nor allies.

It was to this area some sixty years later that the newly formed Geological Survey of Canada (GSC) began exploring and mapping the newly formed United Province of Canada. Geologists in the New Brunswick Geology Branch traipsed through the rugged countryside that would become a Canadian province in 1867. 

It was on one of these expeditions that the Miguasha fossil outcrops were discovered. They, too, would transform in time to become Miguasha National Park or Parc de Miguasha, but at first, they were simply the promising sedimentary exposures on the hillside across the water —  a treasure trove of  Late Devonian fauna waiting to be discovered.

In the summer of 1842, Abraham Gesner, New Brunswick’s first Provincial Geologist, crossed the northern part of the region exploring for coal. Well, mostly looking for coal. Gesner also had a keen eye for fossils and his trip to the Gaspé Peninsula came fast on the heels of a jaunt along the rocky beaches of Chignecto Bay at the head of the Bay of Fundy and home to the standing fossil trees of the Joggins Fossil Cliffs. 

Passionate about geology and chemistry, he is perhaps most famous for his invention of the process to distil the combustible hydrocarbon kerosene from coal oil — a subject on which his long walks exploring a budding Canada gave him a great deal of time to consider. We have Gesner to thank for the modern petroleum industry. He filed many patents for clever ways to distil the soft tar-like coal or bitumen still in use today.

He was skilled in a broad range of scientific disciplines — being a geologist, palaeontologist, physician, chemist, anatomist and naturalist — a brass tacks geek to his core. Gesner explored the coal exposures and fossil outcrops across the famed area that witnessed the region become part of England and not France — and no longer L'nu.

Following the Restigouche River in New Brunswick through the Dalhousie region, Gesner navigated through the estuary to reach the southern coast of the Gaspé Peninsula into what would become the southeastern coast of Quebec to get a better look at the cliffs across the water. He was the first geologist to lay eyes on the Escuminac Formation and its fossils.

In his 1843 report to the Geologic Survey, he wrote, “...I found the shore lined with a coarse conglomerate. Farther eastward the rocks are light blue sandstones and shales, containing the remains of vegetables. (...) In these sandstone and shales, I found the remains of fish and a small species of tortoise with fossil foot-marks.”

We now know that this little tortoise was the famous Bothriolepis, an antiarch placoderm fish. It was also the first formal mention of the Miguasha fauna in our scientific literature. Despite the circulation of his report, Gesner’s discovery was all but ignored — the cliffs and their fossil bounty abandoned for decades to come. Geologists like Ells, Foord and Weston, and the research of Whiteaves and Dawson, would eventually follow in Gesner's footsteps.

North America Map 1866 (Click to Enlarge)

Over the past 180 years, this Devonian site has yielded a wonderfully diverse aquatic assemblage from the Age of Fishes — five of the six fossil fish groups associated with the Devonian including exceptionally well-preserved fossil specimens of the lobe-finned fishes. 

This is exciting as it is the lobe-finned fishes — the sarcopterygians — that gave rise to the first four-legged, air-breathing terrestrial vertebrates – the tetrapods. 

Fossil specimens from Miguasha include twenty species of lower vertebrates (anaspids, osteostra-cans, placoderms, acanthodians, actinopterygians and sarcopterygians), a limited invertebrate assemblage, along with terrestrial plants, scorpions and millipedes.

Originally interpreted as a freshwater lacustrine environment, recent paleontological, taphonomic, sedimentological and geochemical evidence corroborates a brackish estuarine setting — and definitely not the deep waters of the sea. This is important because the species that gave rise to our land-living animals began life in shallow streams and lakes. It tells us a bit about how our dear Elpistostege watsoni liked to live — preferring to lollygag in cool river waters where seawater mixed with fresh. Not fully freshwater, but a wee bit of salinity to add flavour.  

  • Photos: Elpistostege watsoni (Westoll, 1938 ), Upper Devonian (Frasnian), Escuminac formation, Parc de Miguasha, Baie des Chaleurs, Gaspé, Québec, Canada. John Fam, VanPS
  • Origin of the Vertebrate Hand Illustration, https://www.nature.com/articles/s41586-020-2100-8
  • Tiktaalik Illustration: By Obsidian Soul - Own work, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=47401797

References & further reading:

  • From Water to Land: https://www.miguasha.ca/mig-en/the_first_discoveries.php
  • UNESCO Miguasha National Park: https://whc.unesco.org/en/list/686/
  • Office of L'nu Affairs: https://novascotia.ca/abor/aboriginal-people/
  • Cloutier, R., Clement, A.M., Lee, M.S.Y. et al. Elpistostege and the origin of the vertebrate hand. Nature 579, 549–554 (2020). https://doi.org/10.1038/s41586-020-2100-8
  • Daeschler, E. B., Shubin, N. H. & Jenkins, F. A. Jr. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature 440, 757–763 (2006).
  • Shubin, Neil. Your Inner Fish: A Journey into the 3.5 Billion History of the Human Body.