Reading Rocks - The Secret Of Joggins (Part 2 of 2)
Last time, we learned about the contentious origins of coal and how the cliffs at Joggins helped solve that 300-million-year-old mystery, with the help of its perfectly preserved fossil trees. But those trees have another secret - one that will make Joggins world famous, influence the writings of Charles Darwin, and give us an unprecedented look at a pivotal moment in the history of life on earth.
EPISODE NOTES
TRANSCRIPT
Last time, we learned how the fossil cliffs at Joggins helped to settle a raging debate about the origins of coal. Charles Darwin thought it formed in submarine environments. Charles Lyell said terrestrial environments.
Lyell and William Dawson’s expeditions to Joggins gave them their answer, thanks to petrified tree stumps frozen in the rock amongst seams of coal. You could see these trees were living and growing in the same place the coal started to form: a terrestrial environment.
But those tree stumps are hiding something else. Something that would be an even bigger influence on Darwin, and make Joggins one of the most important fossil sites in the world.
When these trees are growing here, 300 million years ago, the area is a complex river system through jungle and forest covered plains. You can actually still see traces of those rivers today as infilled channels on the beach, a little more resistant to the waves than the rock around them.
And the cliffs provide a temporal record of that environment. Thanks to their diagonal slant, you can travel through time just by walking up and down the beach. North take you further back, South is more recent. A ten million year slice of the carboniferous, from 310 to 300 million years ago, is laid out here.
Within those cliffs, the complete ecosystem is preserved in rock. 195 species have been found here, from the primary producers to detrivores to carnivores.
And that’s all known by the time our pals Lyell and Dawson arrive for their now-famous 1851 expedition. It already makes Joggins a significant site - one of the most comprehensive cross-sections of terrestrial life in the carboniferous.
But they find something no one else had - hidden inside those fossilized trees.
Most of the trees you find at Joggins are lycopods or ‘scale trees’ - the ancient ancestors of modern day club mosses. Once they’re exposed on the cliff, they typically only last about four years before erosion and the tides eat them away. But they’re here in the first place thanks to a very specific series of events 300 million years ago.
In the prime of life, those trees need to be suddenly buried up to a point in sediments, say from a flash flood.
That kills the tree. And as the exposed part becomes less stable from decomposition, eventually it’ll get snapped off by wind - leaving just the buried stump.
The inner part of that stump rots away first. And this is the key moment in the story. When that hollowed out, buried stump… becomes a trap. Small animals can fall in - and not get out again.
Then, if another flood covers them in more sediment layers, they’ll be preserved inside that stump as both tree and critters are fossilized.
And this is what Lyell and Dawson discover here - sometimes as many as 17 skeletons in a single trunk.
Among them is the crown jewel: this lil fella. Hylonomus lyelli. An adorable tetrapod who is also a seismic evolutionary shift.
The late carboniferous frozen in the Joggins cliffs is an era of profound change. Up until now, terrestrial vertebrates are all amphibious, needing water to keep their skin moist and as a medium to lay eggs.
Hylonomus lyelli is a great leap forward. The earliest true reptile that we’ve found. With a tougher skin letting her survive much better out of the water, and laying hard-shelled, amniotic eggs.
She represents the true shift of vertebrate life from the water to the land. Critters like her are the ancestors of all dinosaurs, all modern reptiles… all mammals. Us.
And it’s a miracle we know about her at all.
More than 90% of fossils depend on the remains of animals being quickly buried in sediments. Those conditions are much more likely near or in water.
So the most common fossils are of aquatic and semi-aquatic animals, like amphibians. It’s much rarer relatively, for terrestrial animals - especially this far back in time.
The fossils we do have of hylonomus lyelli depend totally on that really unlikely series of events: lycopod trees being partly buried, knocked over, hollowed out and functioning as traps that are then filled with more sediments. It’s only because of that process that we have a window on this miraculous, transitionary animal. And that’s why the Joggins cliffs stand as a UNESCO world heritage site - globally recognized for their fossils.
Finding these little tetrapods also sealed the deal for Darwin and the terrestrial origin of coal. And they significantly helped him build his theory of evolution, which he would publish as ‘On The Origin of Species’ just 8 years after Dawson and Darwin’s friend Lyell made their expedition to Joggins.
A final footnote on Lyell and Darwin. While by all accounts close friends, Lyell was no fan of the theory of evolution, being a deeply religious man himself. But he still supported and encouraged Darwin’s work and the publishing of that work.
In fact by 1863 Lyell had actually come around, embracing Darwin’s heretical theories as scientifically sound. And that’s a miracle I’d put on par with, say, a perfectly preserved tetrapod.