Devonian Earth – The Age of Fishes and Forests

Devonian Earth

419.2 to 358.9 Million Years Ago

Zdeněk Burian

This painting depicts a typical Devonian landscape with archaeopteris (top-right), prototaxites (top centre) and various ferns and horsetails creating the first forests.

By the end of the Silurian period, 419.2-million years ago, the Earth’s continents were home to the first terrestrial ecosystems, but it was a very different land to what we know today. The Devonian period saw one of the most important adaptive radiations in the history of evolution, characterised by an explosion of both land-based plant and animal life and fishes. In the seventh part of my “A Journey through the History of Earth” series, we’ll be exploring the first forests and more.

Part 1: Hadean Earth – The Violent Creation of Our World

Part 2: Archean Earth – Signs of Life

Part 3: Proterozoic Earth – The First Animals

Part 4: Cambrian Earth – An Explosion of Evolution

Part 5: Ordovician Earth – Colonising a Barren Land

Part 6: Silurian Earth – The First Breath of Air

Part 7: Devonian Earth – The Age of Fishes and Forests

Part 8: Carboniferous Earth – The Age Bugs

Part 9: Permian Earth – The Age of Amphibians

Part 10: Triassic Earth – The Rise of the Dinosaurs

Part 11: Jurassic Earth – The Land of Giants

Part 12: Cretaceous Earth – The Reign of Tyrants

Part 13: Paleogene Earth – The Rise of Mammals

Part 14: Neogene Earth – Human Ancestors

Part 15: Quaternary Earth – The Age of Man

For hundreds of millions of years before the first colonisation of the land by the first primitive plant life in the Ordovician period, the seas were teaming with life. The Silurian saw the first animal take a breath of air, living permanently on the land and forming the first purely land-based ecosystems. For millions of years, these primordial terrestrial ecosystems remained primitive and undeveloped, but that was all about to change with the appearance of the first true forests during the Devonian.

The Devonian is the fourth period of the Phanerozoic aeon and the third of the Palaeozoic era. It’s named after the southwestern English county of Devon, a region of the UK that’s famous for its numerous fossil discoveries. Like the other periods of the Palaeozoic, the Devonian was first defined in the 1830s. Despite being the period that also saw the rapid spread of terrestrial life, the Devonian is often known as the ‘Age of Fish’ owing to an equally impressive marine radiation during the time.

Highlights of the Devonian

• Forests spread across the land
• The Golden Age of Fish
• Tetrapods colonise the land
• Development of reefs
• First insects learn to fly
• Mass extinction event

Ancient Bugs Vie for Terrestrial Dominance

One could describe the Devonian Earth as a tropical paradise. Oceans covered some 85% of the world’s surface, and global temperatures rose to an average of 20 °C, which is some 40% higher than they are today. It’s likely that even the poles were free of ice during this period. Oxygen levels continued to rise as well, peaking at about 15% during the Devonian as photosynthesising plants spread across river banks and coastlines and beyond, correlating with the evolution of larger plants and animals.

The foundations to the world’s first true terrestrial ecosystems were already well-established by the beginning of the Devonian. Though still lacking in proper roots or hard bark-like tissues, vascular plants were rapidly spreading and evolving, with some species reaching the size of small bushes. The fern-like lycopods, such as baragwanathia, had spread throughout the globe. They remain one of the most successful plant subgroups that ever existed, and they’re still going strong today. Nonetheless, the bizarre giant fungus prototaxites remained by far the largest organism on the land.

Long before mammals and reptiles existed, it was bugs that ruled the land, bugs that would, in the Carboniferous Period, reach some rather ungodly proportions. With the spread of terrestrial plant life providing sustenance for all manner of new terrestrial animals, more and more life started to crawl out of the sea. You may remember from Part 6 of this series that the first animal known to live on the land was a millipede. These tiny creatures evolved into arthropleurids, a family that includes the largest arthropod that ever lived, the gigantic arthropleura, which lived during the Carboniferous and grew up to 7.5 feet long (2.3 metres). No longer was pneumodesmus newmani, the tiny Silurian millipede, alone in the terrestrial world. By the Late Devonian, it had plenty of company in the form of scorpions, carnivorous centipedes and the first flightless insects.

… and Placoderms Dominate the Oceans

Nobu Tamura

Dunkleosteus was one of the largest animals in the world when it hunted in the Devonian seas.

The Devonian Period is often described as the Age of Fish, a fact perhaps best exemplified by the enormous dunkleosteus. Although the now long-extinct placoderms first evolved during the Silurian, they enjoyed unprecedented evolutionary success by the middle of the Devonian to such an extent that they dominated virtually every marine ecosystem. Dunkleosteus weighed over a tonne and grew up to 20 feet (6 metres) long, rivalled only in size by its relative, the aptly named titanichthys. These fearsome predators preyed on early ray-finned and lobe-finned fish, as evidenced by fossilised, partially digested remains found with placoderm fossils.

Numerous other orders of fish evolved during the Devonian, including the coelacanth, a lobe-finned fish that appeared some 409-million years ago. Considered a living fossil, the species survives to this day. Among its contemporaries were the first lungfish, such as the 3-foot-long (90cm) dipnorhynchid. Primitive sharks were also well-established, diversifying greatly towards the end of the Devonian to include species like the peculiar stethacanthus, a shark characterised by its anvil-shaped dorsal fin. Nonetheless, these early sharks were a far cry from the ferocious predators they later became known for, and likely made for easy pickings for roaming dunkleosteus.

Invertebrates continued to enjoy a great deal of success throughout the Devonian too, as sponges and corals built vast reefs, forming the foundations of ever-increasingly advanced marine ecosystems. Among the most significant is the iconic ammonite, a marine cephalopod that resembled nautiloids but is actually more closely related to octopi and squid. Amazingly, ammonites survived from some 334-million years, only perishing with the Cretaceous mass extinction event alongside the dinosaurs.

Nobu Tamura

The lobe-finned fish tiktaalik, is widely regarded as the missing link between fish and amphibians.

The Late Devonian saw another extraordinarily important evolutionary event take place. While small arthropods had already been enjoying life on land for some 60-million years, it was now time for our own ancestors to step out of the water. 375-million years ago, a lobe-finned fish called a tiktaalik lived in the shallows. Discovered in 2004 in Greenland, this creature grew up to six feet (180cm) long and developed leg-like appendages for fins, earning it the affectionate nickname of ‘fishapod’. This remarkable creature is now widely seen as the missing link between fish and terrestrial amphibians from which reptiles, mammals and, ultimately, us evolved. Although tiktaalik was probably predominantly amphibious, it would have used its limbs to push its way along the seabed and even lift itself up out of the water. It was also carnivorous, so it looked like the days of the poor Silurian millipede were numbered.

Forests Crawl across the Land

For billions of years, Earth’s lands had been barren and lonely. Even for the last 100-million years, terrestrial life had been mostly clinging to the coastal regions, apprehensive to evolve and colonise the lonely inland areas. However, that all changed in the Late Devonian when the continents underwent an unprecedented transformation, forming the very first forests. No longer were plants restricted to bush-sized lycopods, ferns and horsetails; now was the time of real woodland, such as the 100-foot-high (30 metres) archaeopteris. Although archaeopteris (not to be confused with the archaeopteryx!) wasn’t technically a tree, it very much resembled one, despite having fern-like structures in place of leaves. Finally, Earth’s continents were home to ecosystems that would be at least vaguely familiar to us all these hundreds of millions of years later. Towards the end of the Devonian, the proliferation of sophisticated terrestrial plant life was so great that atmospheric oxygen levels started to skyrocket, a change that had profound effects and largely shaped the Carboniferous Period that we’ll be exploring in Part 8.

Ocean Life Chokes to Death in a Catastrophic Extinction Event

Heinrich Harder

Coral reefs the world over were completely decimated by the Late Devonian mass extinction.

The Late Devonian saw one of the five greatest extinction events of the Phanerozoic aeon when the oceans appear to have choked to death with lack of oxygen. Exactly what triggered the extinction remains uncertain, although various possibilities have been proposed, including widespread volcanism, an impact event or possibly the evolution of terrestrial ecosystems itself. As the continents became green for the first time ever, the ubiquity of photosynthesising organisms may have drastically lowered CO2 levels, causing temperatures to plummet in the process.

Regardless of what triggered the extinction, anoxia, or reduced oxygen levels in the oceans, was the major killer, likely accompanied by dropping sea levels towards the end of the Devonian. Strong evidence for this includes the near complete disappearance of coral reefs and the extinction of many species of trilobite and brachiopods. Even the hardy dunkleosteus could not survive the profound changes to the ocean ecosystems; the most fearsome predator of the Devonian seas disappeared 360-million years ago.

Conclusion

While the Devonian ended shortly after the decimation of many of the world’s maritime ecosystems, life continued to thrive on the land as fish learned to walk and the first insects took flight. The rapid greening of the land presented a world of new opportunities in which our distant ancestors could thrive. Next week, we’ll be exploring the Carboniferous Period, during which terrestrial ecosystems went utterly wild, giving birth to insects the size of birds, scorpions the size of dogs and the very first reptiles.

Part 8: Carboniferous Earth – The Age Bugs