Sarcopterygians and actinopterygians form the two known lineages of bony fishes. The former are distinguished from the latter by several characteristics, the most notable of which is their lobed fins. In contrast to actinopterygian fins, which are supported by thin rod-like bones known as rays, sarcopterygians sport thick and fleshy pectoral fins that join to the trunk by a single bone (the humerus) and are lined with muscles that help adjust the body position. The name sarcopterygian means “fleshy fin”, from the Greek words sarx and pterygon.

The earliest sarcopterygians had additional distinguishing features. Their jaws were lined with strong muscles and their skull was divided into two halves (front and rear) held together by an intracranial joint. They had two dorsal fins, an epicercal tail, and a body covered with a film of cosmine, a substance similar to dentine. Another innovation was the hard enamel coating on their teeth.

Sarcopterygians appeared in the Upper Silurian at the same time as many groups of bony fish. They diversified during the Devonian when the Read More
Sarcopterygians and actinopterygians form the two known lineages of bony fishes. The former are distinguished from the latter by several characteristics, the most notable of which is their lobed fins. In contrast to actinopterygian fins, which are supported by thin rod-like bones known as rays, sarcopterygians sport thick and fleshy pectoral fins that join to the trunk by a single bone (the humerus) and are lined with muscles that help adjust the body position. The name sarcopterygian means “fleshy fin”, from the Greek words sarx and pterygon.

The earliest sarcopterygians had additional distinguishing features. Their jaws were lined with strong muscles and their skull was divided into two halves (front and rear) held together by an intracranial joint. They had two dorsal fins, an epicercal tail, and a body covered with a film of cosmine, a substance similar to dentine. Another innovation was the hard enamel coating on their teeth.

Sarcopterygians appeared in the Upper Silurian at the same time as many groups of bony fish. They diversified during the Devonian when they became more abundant and varied than the actinopterygians.

There are six main groups of sarcopterygians: onychodontiforms, rhizodontiforms, actinistians, osteolepiforms, elpistostegalians and dipnomorphs (this latter group comprising the dipnoi and porolepiforms). All but the first two groups are represented among the Miguasha fossils.

The sarcopterygians were relatively large fish that prospered in fresh, salty or brackish water. Many became extinct during the mass extinction event at the end of the Devonian Period. The only ocean dwellers to survive were the coelacanths, an actinistian subgroup. Those that lived in fresh or brackish water, like the rhizodontiforms and dipnoi, were not overly affected by the event.

During the Mesozoic Era, coelacanths became increasingly abundant in the sea, whereas only a few dipnoi survived in continental environments. Today, the once diverse sarcopterygians have been reduced to a mere eight species.

This apparently dramatic demise is somewhat deceptive. After all, the sarcopterygians left behind many successful descendants in the form of tetrapods. Land-dwelling vertebrates, including humans, are all “sarcopterygian offspring”, descendants of the elpistostegalians whose highly specialized adaptations for aquatic life proved very useful for conquering land about 365 million years ago. For example, the type and relative positions of the elpistostegalian fin bones, which include the humerus, ulna and radius, are directly comparable to those of tetrapod limbs.

With the discovery of at least nine sarcopterygian species, but only one actinopterygian, it seems that the waters of the Miguasha estuary were representative of the biodiversity during this part of the Devonian Period.

© Miguasha National Park 2007

Pectoral fin of a modern coelacanth

A thick fleshy lobe from the coelacanth Latimeria chalumnae, one of the rare modern-day sarcopterygians.

Miguasha National Park
2007
© Miguasha National Park


Actinistians, also called coelacanths, are a group of sarcopterygians recognizable by their stocky form and three-lobed tail, with the exception of the very earliest representatives of the group. They are also distinguished by the presence of a rostral gland in the snout, which allows them to detect tiny organisms in sediments, a second dorsal fin, and an anal fin with joints and a bone structure reminiscent of paired fins. The first dorsal fin is also placed farther forward than in other sarcopterygians.

Actinistians appeared toward the end of the Devonian. From an anatomical perspective, they seem to have been highly varied at the beginning of their evolutionary history. Living mainly in seas, they prospered for 215 million years until the end of the Cretaceous when they were they were not spared the effects of the mass extinction that hit the dinosaurs and numerous other groups. All actinistian fossils date from before the end of the Cretaceous.

In 1938, fishermen caught a strange 1.5-metre long fish at the mouth of the Chalumna River in South Africa. This stunning discovery brought the coelacanth out of the depths of time and into the spotlight of biology Read More
Actinistians, also called coelacanths, are a group of sarcopterygians recognizable by their stocky form and three-lobed tail, with the exception of the very earliest representatives of the group. They are also distinguished by the presence of a rostral gland in the snout, which allows them to detect tiny organisms in sediments, a second dorsal fin, and an anal fin with joints and a bone structure reminiscent of paired fins. The first dorsal fin is also placed farther forward than in other sarcopterygians.

Actinistians appeared toward the end of the Devonian. From an anatomical perspective, they seem to have been highly varied at the beginning of their evolutionary history. Living mainly in seas, they prospered for 215 million years until the end of the Cretaceous when they were they were not spared the effects of the mass extinction that hit the dinosaurs and numerous other groups. All actinistian fossils date from before the end of the Cretaceous.

In 1938, fishermen caught a strange 1.5-metre long fish at the mouth of the Chalumna River in South Africa. This stunning discovery brought the coelacanth out of the depths of time and into the spotlight of biology and paleontology. The shock was felt among specialists around the world; after all, they now had the incredible and completely unexpected chance to unravel many of the mysteries surrounding actinistians, which were known only as fossils up to that time.

The identification of the first coelacanth catch was made by South African ichthyologist James Leonard Brierley Smith. Smith had been alerted to its discovery by Marjorie Courtenay-Latimer, who oversaw the unloading of the creature at the dock in East London, South Africa.

Since that time, more than 200 specimens have been caught, mostly off-shore of the Comoros Islands. Baptized Latimeria chalumnae in honour of the lady and the river, the species enjoys widespread recognition and has enlightened paleontologists on sarcopterygian anatomy. Fifty years after the capture of the first specimen, another species was discovered in Indonesia, 10,000 km from South Africa, and was named Latimeria menadoensis. Both species are threatened by extinction and are protected.

The existence of these “living fossils”, despite their absence in the fossil records for the past 65 Ma, could be explained by the stable environment at the bottom of the ocean where they live. During the extinction event at the end of the Cretaceous, species living near the surface were wiped out, while others, evolving in the ocean depths, were less affected by the event.

Miguasha sediments have yielded specimens of the most primitive of all actinistians, the ancestor of the two living species and all those that preceded them.

© Miguasha National Park 2007

<i>Latimeria chalumnae</i>

The modern-day coelacanth Latimeria chalumnae was considered a “living fossil” when it was caught in 1938.

Miguasha National Park
2002
© Miguasha National Park


Porolepiforms were very large marine predators that appeared at the beginning of the Devonian Period. They subsequently colonized brackish and fresh water environments, becoming quite common during the Upper Devonian. The fossil record, however, reveals few genera. Their distinctive scales are used as index fossils for the relative dating of rock layers.

Porolepiforms had long, tapered pectoral fins, a short, thick head and small eyes. As with the dipnoi, porolepiforms had lost their intracranial joint and the skull was a single unit. Their features, including a tail that could reach up to two metres long, lead some researchers to conclude that porolepiforms were ambush predators. Propulsion fins set far back on their bodies allowed for good acceleration, and their cranial morphology suggests strong musculature in the head region.

The similarity of the long, thin pectoral fin in porolepiforms and dipnoi, along with other features, has prompted specialists to place these two groups into a new, larger group: the dipnomorphs.

Of all the porolepiforms, the widespread genus Holoptychius is one of the best known. Read More
Porolepiforms were very large marine predators that appeared at the beginning of the Devonian Period. They subsequently colonized brackish and fresh water environments, becoming quite common during the Upper Devonian. The fossil record, however, reveals few genera. Their distinctive scales are used as index fossils for the relative dating of rock layers.

Porolepiforms had long, tapered pectoral fins, a short, thick head and small eyes. As with the dipnoi, porolepiforms had lost their intracranial joint and the skull was a single unit. Their features, including a tail that could reach up to two metres long, lead some researchers to conclude that porolepiforms were ambush predators. Propulsion fins set far back on their bodies allowed for good acceleration, and their cranial morphology suggests strong musculature in the head region.

The similarity of the long, thin pectoral fin in porolepiforms and dipnoi, along with other features, has prompted specialists to place these two groups into a new, larger group: the dipnomorphs.

Of all the porolepiforms, the widespread genus Holoptychius is one of the best known. It is also the last genus of this group to be wiped out at the end of the Devonian. Although it both appeared and then disappeared during Devonian time, it nonetheless prospered for 50 million years. Two porolepiform species have been described at Miguasha, and a possible third species has not been yet described.

© Miguasha National Park 2007

Reconstruction of the porolepiform Holoptychius

The porolepiform Holoptychius, a classic Devonian fish.

Illustration by François Miville-Deschênes

© Miguasha National Park


Strange creatures called dipnoi (lungfish) fascinated the first naturalists to study the few species still alive in modern times. Although their appearance might indicate otherwise, these are fish with lungs, not finned amphibians!

These curious sarcopterygian fish first appeared in Lower Devonian seas and reached maximum diversity by Upper Devonian time. Their diversity was revived during the Triassic Period before gradually declining once again. Only three genera exist today: Neoceratodus in Australia, Lepidosiren in Amazonia, and four Protopterus species in tropical Africa.

The name “dipnoi” name means “double breather”, because in addition to gills, they all have one or two functional lungs, allowing them to breathe air. Both the American and African species of lungfish are dependant on air for survival, their gills having degenerated so much that they will drown if they cannot reach the surface.

Such strange features explain why researchers once suggested this group of fish was ancestral to all terr Read More
Strange creatures called dipnoi (lungfish) fascinated the first naturalists to study the few species still alive in modern times. Although their appearance might indicate otherwise, these are fish with lungs, not finned amphibians!

These curious sarcopterygian fish first appeared in Lower Devonian seas and reached maximum diversity by Upper Devonian time. Their diversity was revived during the Triassic Period before gradually declining once again. Only three genera exist today: Neoceratodus in Australia, Lepidosiren in Amazonia, and four Protopterus species in tropical Africa.

The name “dipnoi” name means “double breather”, because in addition to gills, they all have one or two functional lungs, allowing them to breathe air. Both the American and African species of lungfish are dependant on air for survival, their gills having degenerated so much that they will drown if they cannot reach the surface.

Such strange features explain why researchers once suggested this group of fish was ancestral to all terrestrial vertebrates. For example, the Australian species still has fins strong enough to raise its body onto shore and struggle towards another water hole. But we now know, thanks to paleontology, that dipnoan fish are a lineage of sarcopterygians that developed these evolutionary adaptations separately from other groups – an example of convergent evolution – and they never evolved into terrestrial forms.

Various traits can be used to recognize dipnoi, but their skull anatomy is particularly distinctive. The skull and jaw apparatus tend to display a certain degree of consolidation, and there are no marginal teeth along the lower mandible. Food was ground between dental plates in the palate, which look somewhat like partly-fused teeth. These dense, hard plates fossilize well and are commonly all that is preserved from the original fish. In fact, almost half of the 300 known fossil dipnoi species have been described by their dental plates alone.

Fossils that preserved the entire body display a set of several separate fins towards the back of the body, which must have provided a powerful push through the water. In contrast, the dorsal, caudal and anal fins in living lungfish are fused into a single fin that surrounds the rear half of the animal.

The modern lungfish of Africa display a very unusual summer dormancy (estivation) behaviour. During a drought, they dig a burrow in the mud where they remain buried for several months. They stay in the ground, breathing with their lungs, even after the water has completely disappeared and the mud is bone dry. When the rains finally return, they emerge and resume swimming.

Fossilized burrows containing hibernating dipnoi have been found in layers dating back to the beginning of the Permian Period. It is thought that estivation behaviour, which implies the ability to breathe air, developed by the end of the Devonian Period, or during the Carboniferous, when this group invaded freshwater ecosystems.

Two dipnoi genera, Scaumenacia and Fleurantia, are found at Miguasha. It is unlikely that they dug burrows, although anatomical details suggest that they could gulp air like today’s lungfish. But as with all the Devonian dipnoi, they depended primarily on breathing through their gills to obtain most of their oxygen.

© Miguasha National Park 2007

Reconstruction of the Miguasha dipnoi <i>Scaumenacia</i>

Miguasha’s dipnoi displays some similarities in physiognomy with its distant offspring Neoceratodus – a lungfish found in Australia. These rare dipnoi representatives also survived in Africa and South America, and their ancestry goes as far back as the Devonian Period.

Miguasha National Park
2003
© Miguasha National Park


Of all the extinct sarcopterygians, the osteolepiforms are probably the best known. Highly diversified during the Devonian, they include some stars of paleoichthyology, including Osteolepis with its long streamlined body, large median fins and epicercal tail, and Eusthenopteron, which reached more than a metre long and sported a three-pointed symmetrical tail. More than any other species, Eusthenopteron provided paleontologists with the most information about this group.

The osteolepiform shape, and by deduction their way of life, is sometimes compared to that of the modern day pike. Long and slender, with posterior fins set far back and a long mouth lined with strong teeth, these fish were without a doubt quick and efficient predators. Their skull moved in a peculiar manner and included an intracranial joint that allowed for a very large gape.

Osteolepiforms had single nasal openings on either side of their head, both of which were attached to a large opening in the animal’s palate. This orifice is equivalent to the choana in primi Read More
Of all the extinct sarcopterygians, the osteolepiforms are probably the best known. Highly diversified during the Devonian, they include some stars of paleoichthyology, including Osteolepis with its long streamlined body, large median fins and epicercal tail, and Eusthenopteron, which reached more than a metre long and sported a three-pointed symmetrical tail. More than any other species, Eusthenopteron provided paleontologists with the most information about this group.

The osteolepiform shape, and by deduction their way of life, is sometimes compared to that of the modern day pike. Long and slender, with posterior fins set far back and a long mouth lined with strong teeth, these fish were without a doubt quick and efficient predators. Their skull moved in a peculiar manner and included an intracranial joint that allowed for a very large gape.

Osteolepiforms had single nasal openings on either side of their head, both of which were attached to a large opening in the animal’s palate. This orifice is equivalent to the choana in primitive tetrapods, a feature that enabled tetrapods to breathe through their nostrils with their mouth closed.

The structure of the humerus is very similar to the humerus in basal tetrapods, despite having a different shape. In most osteolepiforms, the base of the paired and median fins was adorned on both sides by a large, thickened scale known as the basal scute.

Appearing in the Middle Devonian, osteolepiforms survived the extinction at the end of the Devonian, only to be wiped out at the beginning of the Permian. The famous osteolepiform Eusthenopteron, along with its lesser known cousin Callistiopterus, are found in the cliffs at Miguasha.

© Miguasha National Park 2007

Reconstruction of <i>Eusthenopteron foordi</i>

The Devonian osteolepiform Eusthenopteron foordi, a swift predator with a hydrodynamic body.

Illustration by François Miville-Deschênes
2003
© Miguasha National Park


Elpistostegalians are the closest ancestors of tetrapods, displaying characteristics of both fish and tetrapods. Similar to fish, they have a body covered in scales, gills, rays at the tips of their fins, and a pair of jugal plate under the lower jaw. They resemble tetrapods with their large skull size, the flattened skull shape from back to front, a long snout with respect to total skull length, eyes on top of the skull, and no dorsal or anal fins. Moreover, the skull bone pattern includes two frontal bones, which did not exist in other sarcopterygians, as well as enlarged laterally-oriented ribs.

Elpistostegalians lived within a very narrow timeframe of just a few million years during the Upper Devonian. Four species are known: Elpistostege in Miguasha, which gave its name to the group; two Panderichthys species in Latvia; and Tiktaalik, found recently on Ellesmere Island in the Canadian Arctic. When alive, these animals swam in deltas and estuaries near the continental shores of Euramerica, which straddled the equator at the time. It was along th Read More
Elpistostegalians are the closest ancestors of tetrapods, displaying characteristics of both fish and tetrapods. Similar to fish, they have a body covered in scales, gills, rays at the tips of their fins, and a pair of jugal plate under the lower jaw. They resemble tetrapods with their large skull size, the flattened skull shape from back to front, a long snout with respect to total skull length, eyes on top of the skull, and no dorsal or anal fins. Moreover, the skull bone pattern includes two frontal bones, which did not exist in other sarcopterygians, as well as enlarged laterally-oriented ribs.

Elpistostegalians lived within a very narrow timeframe of just a few million years during the Upper Devonian. Four species are known: Elpistostege in Miguasha, which gave its name to the group; two Panderichthys species in Latvia; and Tiktaalik, found recently on Ellesmere Island in the Canadian Arctic. When alive, these animals swam in deltas and estuaries near the continental shores of Euramerica, which straddled the equator at the time. It was along these shores that the first true tetrapods, equipped with digits, appeared at the end of the Frasnian Age during Upper Devonian time.

Elpistostegalians and tetrapods were not only similar in morphology, but the periods and habitats in which they lived also coincided. Vaguely resembling little crocodiles with fins, these fish lived in shallow water, breathed air into their lungs, and could peek above the surface with their prominent eyes.

The fins of Tiktaalik are sufficiently well-preserved to be studied in detail. In addition to having the classic sarcopterygian structure (humerus, cubitus and radius), the fin could be turned downward and bent slightly forward at its “elbow”. It wouldn’t be surprising if this limb not only helped Tiktaalik rest on the bottom, but also allowed it to crawl on land when needed.

© Miguasha National Park 2007

<i>Elpistostege</i>

Elpistostege from Miguasha gave its name to the elpistostegalians, a group of Devonian fish that represent a transitional phase toward the first tetrapods.

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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