Acanthodians are one of the large groups of fish that did not survive the passage of time, having disappeared during the Permian Period. Nonetheless, their presence was hardly fleeting. Appearing at the end of the Ordovician Period and the beginning of the Silurian, acanthodians swam the world’s waters for 150 million years.

Like the placoderms, we only know acanthodians in their fossilized form. The name comes from the Greek acanthus, meaning spine, for the hard spine in front of each of their fins. The fins of an acanthodian are quite distinctive because they lack rays, consisting instead of small membranes of skin covered with the same scales that cover its body. Acanthodians are also distinguished by their very small diamond-shaped scales, somewhat similar to those of sharks.

Acanthodians might well be the oldest gnathostomes in the fossil record. Acanthodian-like scales were found in Ordovician layers, and fragmentary fish fossils are present in rocks from early Silurian time. They prospered mainly in the Devonian Period, during which time they attained worldwide distribution, although never becoming the predominant group in any given region.
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Acanthodians are one of the large groups of fish that did not survive the passage of time, having disappeared during the Permian Period. Nonetheless, their presence was hardly fleeting. Appearing at the end of the Ordovician Period and the beginning of the Silurian, acanthodians swam the world’s waters for 150 million years.

Like the placoderms, we only know acanthodians in their fossilized form. The name comes from the Greek acanthus, meaning spine, for the hard spine in front of each of their fins. The fins of an acanthodian are quite distinctive because they lack rays, consisting instead of small membranes of skin covered with the same scales that cover its body. Acanthodians are also distinguished by their very small diamond-shaped scales, somewhat similar to those of sharks.

Acanthodians might well be the oldest gnathostomes in the fossil record. Acanthodian-like scales were found in Ordovician layers, and fragmentary fish fossils are present in rocks from early Silurian time. They prospered mainly in the Devonian Period, during which time they attained worldwide distribution, although never becoming the predominant group in any given region.

Researchers recognize three major groups of acanthodians divided into roughly sixty genera for a total of 150 different species. Many of these species, however, are known only from their spines, teeth or small scales because fossilization of their skeletons, made of fragile cartilage, is quite rare and often incomplete if it occurs, hence the great significance of the four Miguasha specimens.

Most acanthodians grew no longer than 25 cm, although Gyracanthus reached up to two meters during the Carboniferous Period. Their bodies were long and trapezoidal, their mouths subterminal, their tail heterocercal, and their eyes quite large and positioned far forward on their heads. This morphology suggests a well developed sense of sight and a lifestyle characterized by active swimming. Some were carnivores, with many small teeth. Those without teeth, including the four Miguasha species, filtered their food while swimming. Their spines must have served as a good defence against predators, although their remains are sometimes found in the abdomens and coprolites of other species.

© Miguasha National Park 2007

<i>Diplacanthus</i>

Diplacanthus, a Miguasha acanthodian.

Illustration by François Miville-Deschênes
2002
© Miguasha National Park


As tiny fish that swam in schools, Triazeugacanthus affinis would have suited the nickname “Devonian anchovies”. The presence of spines, however, tells us that these fish were really acanthodians, not actinopterygians like anchovies.

Triazeugacanthus ranged from half a centimetre to six centimetres long, and each fin – with the exception of the caudal fin – contains a stout spine, which is the diagnostic feature of the group. The dorsal fin is set very far back, and the anal fin a little bit more forward. It had pelvic and pectoral fins, and between them a small pair of spines. Like all acanthodians, Triazeugacanthus had an epicercal tail.

Triazeugacanthus was the most abundant acanthodian species in the Miguasha paleoestuary. Large concentrations of up to 600 fish per square meter along some laminite surfaces imply very high mortality rates provoked by as yet unknown phenomena.

The species had relatively large scales for its size, and a lateral line – typic Read More
As tiny fish that swam in schools, Triazeugacanthus affinis would have suited the nickname “Devonian anchovies”. The presence of spines, however, tells us that these fish were really acanthodians, not actinopterygians like anchovies.

Triazeugacanthus ranged from half a centimetre to six centimetres long, and each fin – with the exception of the caudal fin – contains a stout spine, which is the diagnostic feature of the group. The dorsal fin is set very far back, and the anal fin a little bit more forward. It had pelvic and pectoral fins, and between them a small pair of spines. Like all acanthodians, Triazeugacanthus had an epicercal tail.

Triazeugacanthus was the most abundant acanthodian species in the Miguasha paleoestuary. Large concentrations of up to 600 fish per square meter along some laminite surfaces imply very high mortality rates provoked by as yet unknown phenomena.

The species had relatively large scales for its size, and a lateral line – typically the sensory organ of a fish – can be seen through the scales, running along the entire body length. A simple magnifying glass is often enough to see the otoliths (ear bones). Located in the inner ear of most fish, these small bones are responsible for equilibrium. There are six bones in Triazeugacanthus, three in each ear, indicating they had three semi-circular canals. This would have given these fish a refined sense of movement and three-dimensional position like all fish that are good swimmers. The particular characteristic suggests that acanthodians were closely related to bony fish.

In 1935, the British paleontologist W. Graham-Smith published his descriptions of a small enigmatic Miguasha species he named Scaumenella mesacanthi. This species seemed closely related to Triazeugacanthus because both were found in equal abundance within some sedimentary layers. The fossils consisted of thin traces of carbon, which led Smith to assume that Scaumenella was a very primitive vertebrate. Other researchers suggested alternative interpretations for the animal, such as ostracoderm larvae, prochordata, or other invertebrates with morphologies similar to vertebrates.

In 1985, a detailed study confirmed that Scaumenella were, in fact, small Triazeugacanthus at various stages of decomposition. One of the features that helped solve the mystery was the identical otoliths. The name Scaumenella was withdrawn, but the term “scaumenellization” is still used to describe this type of decay in Miguasha fish.

© Miguasha National Park 2007

<i>Triazeugacanthus affinis</i>

This small species was very abundant in the ancient Miguasha estuary.

Illustration by François Miville-Deschênes
2003
© Miguasha National Park


Homalacanthus concinnus is the largest acanthodian to have swum in the waters of the Miguasha estuary. At 30 centimetres long, it was a giant version of Triazeugacanthus affinis. It was distinguished from its little cousin by the absence of a pair of intermediate spines between the pectoral and pelvic fins.

The species is common in some laminite layers, but is rarely found in limestone concretions. A number of specimens have been preserved well enough to provide information about the digestive system, which was extremely simple. Barely a quarter the length of the fish, it formed a line that started at the pectoral girdle and ran just below the lateral line before dropping down to the level of the pelvic fins. Some specimens have been discovered with this digestive tube filled with the little crustaceans Asmusia membranacea.

A specimen of Eusthenopteron was even found with a Homalacanthus down its throat. This direct evidence of the feeding relationship betwe Read More
Homalacanthus concinnus is the largest acanthodian to have swum in the waters of the Miguasha estuary. At 30 centimetres long, it was a giant version of Triazeugacanthus affinis. It was distinguished from its little cousin by the absence of a pair of intermediate spines between the pectoral and pelvic fins.

The species is common in some laminite layers, but is rarely found in limestone concretions. A number of specimens have been preserved well enough to provide information about the digestive system, which was extremely simple. Barely a quarter the length of the fish, it formed a line that started at the pectoral girdle and ran just below the lateral line before dropping down to the level of the pelvic fins. Some specimens have been discovered with this digestive tube filled with the little crustaceans Asmusia membranacea.

A specimen of Eusthenopteron was even found with a Homalacanthus down its throat. This direct evidence of the feeding relationship between two Miguasha estuarine species was, in 1982, the subject of the first scientific article on Miguasha written by a Quebecer, former park director Marius Arsenault. After 100 years of study by specialists from elsewhere, a wave of research by local scientists was finally underway and continues in earnest to this day.

© Miguasha National Park 2007

<i>Homalacanthus concinnus</i>

Homalacanthus concinnus was the biggest acanthodian to swim in the waters of the ancient Miguasha estuary. It grew up to 30 centimetres long.

Illustration by François Miville-Deschênes
2003
© Miguasha National Park


<i>Homalacanthus concinnus</i>

Homalacanthus concinnus was the biggest acanthodian to swim in the waters of the ancient Miguasha estuary. It grew up to 30 centimetres long.

Illustration by François Miville-Deschênes
2002
© Miguasha National Park


Of the four known acanthodian species at Miguasha, two are of the same genus and are very similar. Diplacanthus horridus and Diplacanthus ellsi both had two large dorsal spines supporting triangular membranous veils. These fins were quite tall with respect to the length of the fish, giving it the distinctive appearance of a caravel, a small 15th century Spanish sailboat.

These two rare species were up to fifteen centimetres long, and sported very pronounced spines with deep grooves along their entire length. The name Diplacanthus reflects the fact that the pectoral spine was paired with a small spine on its underbelly.

The two Diplacanthus species at Miguasha were identified based on the appearance of their spines, with the grooves being more numerous and deeper in D. ellsi. This species was only recently named in honour of R. W. Ells, the Geological Survey of Canada geologist who discovered the first specimen during an expedition to Miguasha in 1881.
Of the four known acanthodian species at Miguasha, two are of the same genus and are very similar. Diplacanthus horridus and Diplacanthus ellsi both had two large dorsal spines supporting triangular membranous veils. These fins were quite tall with respect to the length of the fish, giving it the distinctive appearance of a caravel, a small 15th century Spanish sailboat.

These two rare species were up to fifteen centimetres long, and sported very pronounced spines with deep grooves along their entire length. The name Diplacanthus reflects the fact that the pectoral spine was paired with a small spine on its underbelly.

The two Diplacanthus species at Miguasha were identified based on the appearance of their spines, with the grooves being more numerous and deeper in D. ellsi. This species was only recently named in honour of R. W. Ells, the Geological Survey of Canada geologist who discovered the first specimen during an expedition to Miguasha in 1881.

© Miguasha National Park 2007

<i>Diplacanthus horridus</i>

The spectacular spines of this rare species were rooted deep inside its body.

Miguasha National Park
2002
© 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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