A trip back in time to the Devonian Period would initially reveal a peaceful world. Walking over the land, you might wonder if animals exist at all is this strangely quiet place. Then, along a nearby shoreline, a few animals make themselves known to you: invertebrates – like spiders, scorpions, and myriapods (a primitive type of insect) – are seen crawling on the ground. You are grateful, however, there are no mosquitoes; it will be quite some time before flying insects evolve!

A sudden splash attracts your attention. The underwater world appears to be teeming with life. Fish are abundant and diversified. Some species look familiar, having evolved into modern day representatives. Others seem bizarre, quite different in appearance from any of today’s fish, and you realize these are the fish species that eventually died out.

You also see many other animals in these Devonian waters. Available niches are occupied by various groups of invertebrates: corals, crustaceans, trilobites and other arthropods, molluscs with or without shells, medusas, sea worms, ascidians... Freshwater and marine environments alike are home to a rich and profuse variety Read More
A trip back in time to the Devonian Period would initially reveal a peaceful world. Walking over the land, you might wonder if animals exist at all is this strangely quiet place. Then, along a nearby shoreline, a few animals make themselves known to you: invertebrates – like spiders, scorpions, and myriapods (a primitive type of insect) – are seen crawling on the ground. You are grateful, however, there are no mosquitoes; it will be quite some time before flying insects evolve!

A sudden splash attracts your attention. The underwater world appears to be teeming with life. Fish are abundant and diversified. Some species look familiar, having evolved into modern day representatives. Others seem bizarre, quite different in appearance from any of today’s fish, and you realize these are the fish species that eventually died out.

You also see many other animals in these Devonian waters. Available niches are occupied by various groups of invertebrates: corals, crustaceans, trilobites and other arthropods, molluscs with or without shells, medusas, sea worms, ascidians... Freshwater and marine environments alike are home to a rich and profuse variety of animal life, albeit quite different from the world we are familiar with today.

© Miguasha National Park 2007

<i>Homo sapiens sapiens</i>

The appearance of humans is very recent in the history of the animal world. The latest discoveries indicate that the first lineages to eventually give rise to humans branched off from the big apes around 7 million years ago.

Miguasha National Park
2002
© Miguasha National Park


The seas and oceans of the Devonian Period provided highly favourable habitats for life. Sea levels rose considerably toward the middle of this period, and the edges of the continents were covered by warm, shallow seas – a situation that promoted the development of reef communities in equatorial regions. The main components of these great barrier reefs were rugose and tabulate corals, and stromatoporoids – a long-vanished sponge with a calcified skeleton. The extinction event at the end of the Devonian wiped out the classic coral-stromatoporoid association that had dominated the great reefs of Silurian and Devonian times.

The seafloor was still dominated by brachiopods during Devonian time, but only a few descendants survive in modern seas. Bivalve molluscs evolved into diverse and abundant shellfish in the shallow coastal waters, where they still thrive today. The great nautiloid molluscs began to decline, but gave rise to the ammonoids, who became omnipresent in Mesozoic seas. Echinoderms, mostly crinoids (“sea lilies”), created veritable underwater “fields”. And trilobites, although common, had suffered a series of extinctions and Read More
The seas and oceans of the Devonian Period provided highly favourable habitats for life. Sea levels rose considerably toward the middle of this period, and the edges of the continents were covered by warm, shallow seas – a situation that promoted the development of reef communities in equatorial regions. The main components of these great barrier reefs were rugose and tabulate corals, and stromatoporoids – a long-vanished sponge with a calcified skeleton. The extinction event at the end of the Devonian wiped out the classic coral-stromatoporoid association that had dominated the great reefs of Silurian and Devonian times.

The seafloor was still dominated by brachiopods during Devonian time, but only a few descendants survive in modern seas. Bivalve molluscs evolved into diverse and abundant shellfish in the shallow coastal waters, where they still thrive today. The great nautiloid molluscs began to decline, but gave rise to the ammonoids, who became omnipresent in Mesozoic seas. Echinoderms, mostly crinoids (“sea lilies”), created veritable underwater “fields”. And trilobites, although common, had suffered a series of extinctions and were less diversified than they once had been. Sadly, they would be practically decimated at the end of the Devonian.

Although already present during the Devonian, sharks were not yet the kings of the sea. Instead, the role of top predator belonged to the placoderms, particularly the arthrodire group. One arthrodire species found in Ohio, Dunkleosteus, could grow up to 9 metres long! Mingling with these giants were representatives of the classes of scaly fish: the actinopterygians, acanthodians and sarcopterygians.

Things changed dramatically, however, at the end of the Devonian Period when the retreat of the great epicontinental seas provoked a serious decline in the number of creatures living the world’s oceans.

© Miguasha National Park 2007

<i>Brachiopodes</i>

Spiriferid brachiopods preserved in the Lower Devonian layers of Percé Rock. Brachiopods were very abundant in Devonian seas.

Miguasha National Park
2003
© Miguasha National Park


When discussions turn to Devonian vertebrates, the first animals to come to mind are fish. Although it is true that tetrapods appeared at the very end of this time period, they were barely distinguishable from fish in anatomy and behaviour. It is thus reasonable to say that fish reigned during the Devonian, as dinosaurs reigned during the Mesozoic.

The Devonian Period is famous for being the “Age of Fishes”, even though plants and invertebrates also experienced important evolutionary changes. Throughout the 57 million years of Devonian time, fish evolution was marked by extreme diversification and the appearance of many new types of fishes.

Devonian fish species are divided among two large groups, agnathans, an assorted group of jawless fishes, and gnathostomes, the jawed fishes.

The agnathans would only narrowly survive the great biological crisis at the end of the Devonian. Of the six known agnathan groups, four would perish. And of the two that remained, only a handful of species still live today, including lampreys and hagfish.

Gnathostomes experienced diverse fates. The last of the acanthodians disappeared after the Devo Read More
When discussions turn to Devonian vertebrates, the first animals to come to mind are fish. Although it is true that tetrapods appeared at the very end of this time period, they were barely distinguishable from fish in anatomy and behaviour. It is thus reasonable to say that fish reigned during the Devonian, as dinosaurs reigned during the Mesozoic.

The Devonian Period is famous for being the “Age of Fishes”, even though plants and invertebrates also experienced important evolutionary changes. Throughout the 57 million years of Devonian time, fish evolution was marked by extreme diversification and the appearance of many new types of fishes.

Devonian fish species are divided among two large groups, agnathans, an assorted group of jawless fishes, and gnathostomes, the jawed fishes.

The agnathans would only narrowly survive the great biological crisis at the end of the Devonian. Of the six known agnathan groups, four would perish. And of the two that remained, only a handful of species still live today, including lampreys and hagfish.

Gnathostomes experienced diverse fates. The last of the acanthodians disappeared after the Devonian, as did the last placoderms, despite being the predominant group until their demise. Conversely, three other groups were very successful. Chondrichthyans, comprising sharks and rays, prospered and are still present to this day. Many sarcopterygian subgroups appeared in the Devonian, one of which was the ancestor of the first tetrapods. The evolution from sarcopterygian to tetrapod was a remarkable evolutionary achievement: 27,000 tetrapod species still exist, a number that does not include all those that became extinct along the way. In stark contrast, only a dozen or so fish species, including coelacanths and lungfish, are among the surviving sarcopterygians.

Actinopterygians, ray-finned fishes, were undeniably an evolutionary success. With nearly 29,000 species currently living in fresh and marine waters around the world, they constitute the planet’s largest vertebrate group.

© Miguasha National Park 2007

The Fish of Miguasha

The Parc national de Miguasha represents a window into the Devonian world, the famous Age of Fishes.

Illustration by François Miville-Deschênes
2000
© Miguasha National Park 2007


Tetrapods have walked on land since the end of the Devonian.A tetrapod is an air-breathing animal that has four legs, or has descended from a four-legged animal. Frogs, penguins, snakes, baboons, swallows, porpoises and even humans are all tetrapods. They live in nearly every ecological niche.

Tetrapods evolved from fish. It was long believed that tetrapods developed because fish succumbed to selective pressures that favoured life in the open air. We now know that the first tetrapods were entirely and exclusively aquatic: they could not linger on dry land despite having lungs and four feet with digits. For example, Acanthostega, found in Upper Devonian layers in Greenland, had ribs that were not sufficiently developed to support the weight of its body out of water. It would have collapsed on the first attempt, rendered unable to breathe. Ichthyostega compensated by developing a stronger ribcage that enabled it to overcome the burden of gravity.

A few million years later, when the first tetrapods worthy of the name began to walk on land, they were still dependent on their aquatic habitat, at least for reproduction. Their eggs, not so different from those of fi Read More
Tetrapods have walked on land since the end of the Devonian.A tetrapod is an air-breathing animal that has four legs, or has descended from a four-legged animal. Frogs, penguins, snakes, baboons, swallows, porpoises and even humans are all tetrapods. They live in nearly every ecological niche.

Tetrapods evolved from fish. It was long believed that tetrapods developed because fish succumbed to selective pressures that favoured life in the open air. We now know that the first tetrapods were entirely and exclusively aquatic: they could not linger on dry land despite having lungs and four feet with digits. For example, Acanthostega, found in Upper Devonian layers in Greenland, had ribs that were not sufficiently developed to support the weight of its body out of water. It would have collapsed on the first attempt, rendered unable to breathe. Ichthyostega compensated by developing a stronger ribcage that enabled it to overcome the burden of gravity.

A few million years later, when the first tetrapods worthy of the name began to walk on land, they were still dependent on their aquatic habitat, at least for reproduction. Their eggs, not so different from those of fish, had to remain submerged to survive. This is still the case for amphibians like frogs and salamanders. Their small size allows them to escape the drawbacks of gravity out of water, and they spend most of their time in the water. It was only with the appearance of impermeable membranes like the amnion (a membranous sac that surrounds and protects the embryo), that eggs could be safely laid on dry land by reptiles: a new group of tetrapods that appeared around 315 million years ago (315 Ma). Existing today as snakes, turtles, alligators and lizards, reptiles enjoyed considerable diversity when they included the celebrated dinosaur group between 230 and 65 Ma.

It was 220 Ma ago, in the shadow of the first dinosaurs, when a small group of reptiles, the synapsids, evolved into a new tetrapod form: mammals. It was only much later, once the dinosaurs became extinct and several ecological niches were left vacant, that mammals could express their true evolutionary potential.

Are dinosaurs extinct? Not really... 150 million years ago, some became covered in feathers and developed winged flight, giving rise to birds.

Each of these four great groups of tetrapods – amphibians, reptiles, mammals and birds – knew various evolutionary successes and failures over the course of time. They scattered across all regions of the Earth (some even returning to the oceans), and now interact with each other – and with all other living creatures – in the great ecosystem that we call the biosphere.

The evolutionary success of the tetrapods should not make us forget their ancient aquatic origins at the end of Devonian time. Paddling in mud near the soft, bushy banks of equatorial deltas and estuaries, a group of fish, the elpistostegalians, were poised to give rise to the first vertebrates with toed feet. Among these tetrapod forerunners were Panderichthys, now found fossilized in Latvia, Tiktaalik, in the Canadian Arctic, and Elpistostege, at Miguasha. The gradual conquest of land by vertebrates represented a major event in the history of the animal kingdom and raises an important question: what was the selective advantage of the first tetrapods? In other words, why did they leave the water? The land at that time, invaded by inedible plants and a few rare arthropods, offered little in the way of resources for vertebrates the size of a small crocodile, greedy for protein and with a talent for catching fish. It is plausible that, faced with competition from other predator fish of the same size like Eusthenopteron or Holoptychius, the tetrapod forerunners ventured on land to guard their prey. To seize a placoderm like Bothriolepis, a very common fish, and be able to devour it quietly in a place out of reach of envious competitors was undoubtedly a compelling advantage. It could finally eat in peace in the Devonian world where the calmest place – paradoxically – was outside the “silent world” of water!

© Miguasha National Park 2007

The evolution of sarcopterygian fishes towards the first tetrapods

For nearly a century, the osteolepiform Eusthenopteron has been considered an intermediate link in this transition. However, elpistostegalian fishes that include Panderichthys, Elpistostege and Tiktaalik resemble the first four-limbed vertebrates even more closely. The Elpistostege of Miguasha is considered to be an animal similar to Tiktaalik, an animal recently discovered in the Canadian Arctic.

Figure taken from Ahlberg and Clack 2006, Nature 440, 747-749.
2006
© Nature 440


<i>Eusthenopteron foordi</i>

It was during the Devonian Period that sarcopterygian fish gave rise to the first terrestrial vertebrates. Eusthenopteron foordi (shown here) was long thought to be the transitional animal between fish and tetrapods, sharing features with both, but recent discoveries have shown that the elpistostegalians are even more closely related to four-legged vertebrates.

Jean-Pierre Sylvestre
1997
© Miguasha National Park


Video

Interview with Philippe Janvier

Still picture of a prehistoric man followed by an interview with Mr. Philippe Janvier. At the end of the interview, we see a man walking on the moon.

Philippe Janvier
Take, for example, the invention of fire. This is particular to man. As far as I am aware, he is the only animal capable of making fire. One could not have invented fire in the water. Therefore, one had to be out of the water to invent fire.

If man had not invented fire, there would have been no rockets. Without rockets, man would have been unable to explore space. I believe that the conquest of space is inseparable from leaving the water and that leaving the water 350 million years ago was the first step towards the moon.

Astronaut on the moon
Oh My God! It’s unbelievable!

Miguasha National Park

© Miguasha National Park 2007


Learning Objectives

The learner will:
  • identify and classify different types of fossils;
  • explain the stages of fossilization and the best conditions to create and preserve fossils;
  • make assumptions about the evolution of living beings;
  • make assumptions as to the explanation of the disappearance of some species.

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