Placoderms, mysterious prehistoric fish, were often mistaken for tortoises and other armoured animals in the 19th century. Placoderms were among the first animals with jaws but have no living representatives, a fact that led to a number of erroneous early interpretations based on their unusual forms. In general, placoderms are easily distinguished by their heavy armour made of large bony plates covering their head and thorax, and sometimes even their pectoral fins.

Placoderm armour was quite easily fossilized and is often all that remains of the animal, the tissue of their tail and fins being too delicate to preserve. The general absence of these non-armoured body parts helps explain the mistaken interpretations by paleontologists who studied the first placoderm specimens.

Even when correctly identified, these animals are still strange. Their dermal plates, for example, are not analogous to the skull bones of modern fish. In fact, they are not even similar between the different placoderm groups! A comparison with other fish, even with other placoderms, is therefore quite useless for understanding their anatomy. Their jaws and pectoral fins are also so unusual Read More
Placoderms, mysterious prehistoric fish, were often mistaken for tortoises and other armoured animals in the 19th century. Placoderms were among the first animals with jaws but have no living representatives, a fact that led to a number of erroneous early interpretations based on their unusual forms. In general, placoderms are easily distinguished by their heavy armour made of large bony plates covering their head and thorax, and sometimes even their pectoral fins.

Placoderm armour was quite easily fossilized and is often all that remains of the animal, the tissue of their tail and fins being too delicate to preserve. The general absence of these non-armoured body parts helps explain the mistaken interpretations by paleontologists who studied the first placoderm specimens.

Even when correctly identified, these animals are still strange. Their dermal plates, for example, are not analogous to the skull bones of modern fish. In fact, they are not even similar between the different placoderm groups! A comparison with other fish, even with other placoderms, is therefore quite useless for understanding their anatomy. Their jaws and pectoral fins are also so unusual that they bear little resemblance to those of other gnathostomes.

Placoderms first appeared at the very beginning of the Silurian Period, but did not become common until the beginning of the Devonian. At this point they underwent an extraordinary adaptive radiation that allowed them conquer every aquatic ecological niche, taking on forms that ranged from large predators to small bottom-dwelling fish that fed on filtered sediment. Placoderms were only partially affected by the great mass extinction of the Upper Devonian, but suddenly disappeared at the Devonian-Carboniferous boundary without leaving any descendants.

Although equipped with solid bony plates on the outside, they had a cartilaginous internal skeleton like that of sharks, which explains the difficulty in finding well preserved non-armoured body parts. But when such parts are discovered, as is sometimes the case at Miguasha, it is possible to discern an epicercal tail with a main lobe pointing upwards, a single dorsal fin, and sometimes the attachment points for pelvic fins.

Specialists have identified five or six groups of placoderms worldwide, including more than 200 genera. For 50 million years, placoderms dominated the aquatic world of the Devonian, living in saltwater, brackish and freshwater bodies. It was only when they disappeared that sharks began to diversify, eventually becoming the great predators of the sea.

Two of the placoderm groups are present at Miguasha: antiarchs and arthrodires. True to form, the Escuminac Formation delivered exceptionally well preserved specimens in both cases.

© Miguasha National Park 2007

<i>Bothriolepis canadensis</i>

Thoracic plates from a Bothriolepis canadensis specimen. It is quite easy to see why this fish was once mistaken for a tortoise!

Miguasha National Park
2002
© Miguasha National Park


Antiarchs were the second most diverse placoderm group after the arthrodires. The plates of armour covering the head and thorax joined together to form a heavy closed box with a small anterior opening for the nostrils and two prominent eyes. Another opening on the ventral side near the front served as a mouth. Each pectoral fin was completely clad in bone. In some genera, these bone-covered fins articulated in a manner that is more reminiscent of arthropods, such as crabs, than vertebrates.

A benthic mode of living was traditionally attributed to antiarchs, given their flattened profile, the position of the eyes on top of the head, the ventral mouth, and the lumpy appearance that does not suggest an agile swimmer. While probing the bottom sediments for small invertebrates or plant matter, they may have used their “arms” to cover themselves in mud when they needed to hide. These “arms” may also have served as stabilizers, like a type of rudder, while swimming.

But recent discoveries of intact Bothriolepis specimens in Miguasha rocks have shown that their bodies were not as flat as previously thought, and that they had forward-facing eye Read More
Antiarchs were the second most diverse placoderm group after the arthrodires. The plates of armour covering the head and thorax joined together to form a heavy closed box with a small anterior opening for the nostrils and two prominent eyes. Another opening on the ventral side near the front served as a mouth. Each pectoral fin was completely clad in bone. In some genera, these bone-covered fins articulated in a manner that is more reminiscent of arthropods, such as crabs, than vertebrates.

A benthic mode of living was traditionally attributed to antiarchs, given their flattened profile, the position of the eyes on top of the head, the ventral mouth, and the lumpy appearance that does not suggest an agile swimmer. While probing the bottom sediments for small invertebrates or plant matter, they may have used their “arms” to cover themselves in mud when they needed to hide. These “arms” may also have served as stabilizers, like a type of rudder, while swimming.

But recent discoveries of intact Bothriolepis specimens in Miguasha rocks have shown that their bodies were not as flat as previously thought, and that they had forward-facing eyes.

© Miguasha National Park 2007

Reconstruction of <i>Bothriolepis canadensis</i>

In 1842, Bothriolepis was the first fish discovered at Miguasha. Although originally mistaken for a tortoise, paleontologists realized it was a primitive fish known as a placoderm, which belongs to the antiarch group. Its head, thorax and even its anterior fins were covered by bony plates.

Illustration by François Miville-Deschênes
2003
© Miguasha National Park


Here is Bothriolepis canadensis, one of Miguasha’s most famous fossils. Occurring by the thousands in the famous cliff, this “tortoise-like fossil” is easily recognized by specialists and amateurs alike, young or old. It was also the first fossil vertebrate mentioned by Abraham Gesner when he announced the discovery of the Miguasha site in his 1843 report. Not surprisingly, he confused this species with a tortoise...

About a hundred different fossil species of Bothriolepis have been found around the world, but B. canadensis at Miguasha is the best known due to the large number of specimens and their excellent preservation. In fact, Miguasha’s species serves as something of a model for all Bothriolepis.

Bothriolepis fossils generally consist of the robust plates that surround its head and thorax, as well as the unusual streamlined fins attached to the back of its skull. This heavily boned front Read More
Here is Bothriolepis canadensis, one of Miguasha’s most famous fossils. Occurring by the thousands in the famous cliff, this “tortoise-like fossil” is easily recognized by specialists and amateurs alike, young or old. It was also the first fossil vertebrate mentioned by Abraham Gesner when he announced the discovery of the Miguasha site in his 1843 report. Not surprisingly, he confused this species with a tortoise...

About a hundred different fossil species of Bothriolepis have been found around the world, but B. canadensis at Miguasha is the best known due to the large number of specimens and their excellent preservation. In fact, Miguasha’s species serves as something of a model for all Bothriolepis.

Bothriolepis fossils generally consist of the robust plates that surround its head and thorax, as well as the unusual streamlined fins attached to the back of its skull. This heavily boned front part of the animal is typically all that is preserved.

Most of these bony “carapaces” have been somewhat crushed by the weight of the sediments in which they were buried. The discovey of some undeformed, three-dimensionally preserved specimens led to a review of this fish’s morphology. It appears that Bothriolepis had a much more rounded shape than previously thought, and as a direct consequence of the latest reconstructions, it is now believed that its eyes faced forward instead of upward. Of all the Bothrolepis species, only B. canadensis has been found with the unmineralized hind region preserved.

Some fossilized individuals almost seem to be swimming in the rock, as if groups of them were taken by surprise – buried alive by a sudden surge of sediments following some small-scale catastrophic event. This rapid burial, combined with certain physiochemical conditions that promote preservation, resulted in a number of exceptional specimens that reveal many details of their internal anatomy. The sheer abundance of specimens also allows paleontologists to document all the growth stages, from juvenile individuals less than 2 cm long, to larger, older individuals in which the bony “carapace” covering the head and thorax alone reached 23 cm in length.

At the beginning of the 20th century, American paleontologist William Patten discovered a bed that was very rich in Bothriolepis specimens, most of them facing in the same direction. It is an excellent example of a snapshot in time: a fish population caught momentarily fighting a fatal current. In the early 1940’s, cross-sections of many of these specimens revealed the traces of internal organs preserved as casts. In some cases, the organs were forcefully filled with the same sediment that engulfed the animal. In others, the organs were filled with sediment ingested by the fish while feeding on the muddy bottom.

The excellent condition of the Patten specimens made it possible for paleontologists to trace the position of the animal’s main internal cavities, from the pharyngeal cavity in the head to the intestinal system backward. It turns out that the internal structures are strongly suggestive of a spiral valve intestine, very similar to that of sharks today.

The slicing work also inspired a stunning hypothesis, which proposed that Bothriolepis was equipped with lungs. This idea is based on the presence of two internal sacs that appear to be connected to the pharyngeal cavity. Highly controversial, the presence of these sacs, or presumed “lungs”, has recently been confirmed in a three-dimensional specimen that was opened lengthwise. We also know that the walls of these mysterious organs were fed by a network of blood vessels that converged toward the centre of the animal: the place where its heart would have been. The delicate traces of these blood vessels can be seen directly on the ventral surface of the thorax interior in two specimens. It may be that following the death of the animal, the iron in its hemoglobin oxidized, staining the adjacent sediments.

How many Bothriolepis species are there at Miguasha? It seems fairly certain that there are at least two, with the second species discovered and described in 1924. Named B. Traquairi, after Scottish paleontologist Ramsey Heatly Traquair, the one and only specimen officially assigned to this species has a more slender body than B. canadensis. A population of Bothriolepis, whose long shapes are reminiscent of B. Traquairi, was discovered in a bed of red sandstone in the Escuminac Formation. Although the contents of this layer have not yet been described, the discovery appears to confirm the presence of at least two species of Bothriolepis at Miguasha.

© Miguasha National Park 2007

Complete specimen of <i>Bothriolepis canadensis</i>

An example of a complete specimen of Bothriolepis canadensis showing the unmineralized rear part of the body. It is a rare find because the lack of mineralization makes it difficult to preserve this part of the animal.

Miguasha National Park
2006
© Miguasha National Park


Their name means “jointed neck”, reflecting the fact that mobile joints connecting platy armour covering arthrodire’s head and thorax allowed some vertical movement. Powerfully equipped for active swimming and endowed with large jaws lined with teeth or sharp edges, these fish were efficient predators. The group made up nearly two thirds of all placoderms.

When its jaw opened downward, the upper part of arthrodire’s skull moved upward to maximize gape size and engulf the largest prey possible. A recent study modelled the musculature and jaw movements of Dunkleosteus, an arthrodire fossil found in Ohio. Dunkleosteus was the largest placoderm ever, measuring up to 9 metres long and weighing several tonnes. It could open its mouth extremely quickly, like modern day fish that use suction to capture their prey. When Dunkleosteus closed its mouth, its muscles created an extraordinarily strong bite with a force of about 5,000 newtons! This was the most powerful bite of any fish, and a force equalled only by the famous Read More
Their name means “jointed neck”, reflecting the fact that mobile joints connecting platy armour covering arthrodire’s head and thorax allowed some vertical movement. Powerfully equipped for active swimming and endowed with large jaws lined with teeth or sharp edges, these fish were efficient predators. The group made up nearly two thirds of all placoderms.

When its jaw opened downward, the upper part of arthrodire’s skull moved upward to maximize gape size and engulf the largest prey possible. A recent study modelled the musculature and jaw movements of Dunkleosteus, an arthrodire fossil found in Ohio. Dunkleosteus was the largest placoderm ever, measuring up to 9 metres long and weighing several tonnes. It could open its mouth extremely quickly, like modern day fish that use suction to capture their prey. When Dunkleosteus closed its mouth, its muscles created an extraordinarily strong bite with a force of about 5,000 newtons! This was the most powerful bite of any fish, and a force equalled only by the famous Tyrannosaurus Rex, a dinosaur that appeared much later.

Arthrodires were not all monsters, with some only reaching a dozen centimetres in length. Miguasha fossils include the arthrodire Plourdosteus, which grew up to a metre long.

© Miguasha National Park 2007

<i>Dunkleosteus</i>

The arthrodire Dunkleosteus from Ohio was one of the giants in Devonian seas. Growing 7 to 9 metres long and equipped with sharp jaws, it was a formidable predator indeed. Plourdosteus – the arthrodire found at Miguasha – was basically a smaller version of Dunkleosteus.

Miguasha National Park
2002
© Miguasha National Park


“Small but voracious” probably best describes the arthrodire Plourdosteus canadensis. Although dwarfed in comparison to its American cousin, the giant Dunkleosteus of Ohio, Plourdosteus canadensis nonetheless ruled the Miguasha paleoestuary where its size was on par with other large fish. Its svelte body and the solid carapace covering its head and trunk made for a swift predator and a tough prey for any fish that attacked it from the front.

Its jaws consisted of two lower mandibles with a partly mobile symphysis (central junction). The jaws closed behind four smaller upper plates along the front of the skull. In juveniles, small distinct teeth are seen on each of these structures, but wear over the life of the animal caused the jaws to become one continuous cutting edge.

Despite the absence of three-dimensional specimens, wear marks left on the jaw structures enabled paleontologists to understand how this fish chewed its food. Results show that the movements were mainly abduction and adduction, simple opening and closing, and did not seem to involve propulsion or retropulsion, forward and backward movements.

Arthrodires normally have a Read More
“Small but voracious” probably best describes the arthrodire Plourdosteus canadensis. Although dwarfed in comparison to its American cousin, the giant Dunkleosteus of Ohio, Plourdosteus canadensis nonetheless ruled the Miguasha paleoestuary where its size was on par with other large fish. Its svelte body and the solid carapace covering its head and trunk made for a swift predator and a tough prey for any fish that attacked it from the front.

Its jaws consisted of two lower mandibles with a partly mobile symphysis (central junction). The jaws closed behind four smaller upper plates along the front of the skull. In juveniles, small distinct teeth are seen on each of these structures, but wear over the life of the animal caused the jaws to become one continuous cutting edge.

Despite the absence of three-dimensional specimens, wear marks left on the jaw structures enabled paleontologists to understand how this fish chewed its food. Results show that the movements were mainly abduction and adduction, simple opening and closing, and did not seem to involve propulsion or retropulsion, forward and backward movements.

Arthrodires normally have an internal cartilaginous skeleton, but that of Plourdosteus reveals a partially ossified spinal column, which must have helped it swim more efficiently.

Baptized Coccosteus canadensis when it was first discovered, due to its resemblance to a species in the Old Red Sandstone of Scotland, differences in the skull bone pattern caused it to be renamed Plourdosteus in 1951. Its name renders homage to the Plourde family at Miguasha, who were well known among paleontologists visiting the area. From one generation to the next, the Plourde family came to know more and more about the Escuminac Formation and its fossils, helping to enrich the collections of many paleontological institutions in Quebec and around the world.

© Miguasha National Park 2007

<i>Plourdosteus canadensis</i>

This jaw plate from Plourdosteus canadensis was removed from the encasing rock using acid.

Illustration by Philippe Janvier
2003
© Miguasha National Park


Reconstruction of <i>Plourdosteus canadensis</i>

The placoderm Plourdosteus canadensis was a predator in the Devonian ecosystem at Miguasha. The name Plourdosteus commemorates the important role that the Plourde family played in the history of the Miguasha site. Members of the Plourde family were active for more than a century, taking part in many fossil digs.

Illustration by François Miville-Deschênes
2003
© Miguasha National Park


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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