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Evolution

Prehistoric Fish

Sturgeon are often called a 'prehistoric' looking fish. Sturgeon are far more primitive than most of the other fish that we are used to. In fact the only freshwater fish that is more primitive than sturgeon is the lamprey.

The oldest sturgeon fossils in North America were found in Alberta. They are estimated to be over 60 million years old. This fossil would have been formed at the end of the Cretaceous Period, about the time that the great dinosaur extinction occurred. The oldest lake sturgeon fossil (the sturgeon species that we find in Manitoba) is estimated to be about two million years old. In fact sturgeons were around for a long time before these fossils were formed.

The First Vertebrates

Fish were the first vertebrates to appear. Vertebrates are animals with backbones and include fish, amphibians, reptiles, birds and mammals. Before fish, there were only animals without backbones (invertebrates). Invertebrates include animals such as insects, shellfish, and worms. The first known vertebrates were jawless fishes that appeared 500 million years ago. The living descendants of these jawless fish, lampreys and hagfish, are the most primitive fish living today. Besides having no jaws, these fish also lacked vertebrae. Instead they had a notochord (a cartilage rod), which acted as a backbone. Sturgeon are the only other freshwater fish still living, which have a notochord.



An ostracoderm, an extinct jawless fish that existed from about 510 million to 350 million years ago. The name means "shell-skinned," and this fish was typically armored with scales and bony plates. (Copyright 1995 by Grolier Electronic Publishing, Inc
Fish with Jaws

The first fish with jaws appeared about 425 million years ago, 25 million years after the first fish. After another 25 million years, the 'bony fishes' appeared and became the dominant kind of fish. There were three

Kinds of bony fishes

• lung fish
• lobe-finned fish
• ray-finned fish

All three kinds of bony fish still exist, however lung fish and lobe-finned fish are quite rare. The lobe-finned fish were actually thought to be extinct until a coelacanth was caught in the Indian Ocean in 1939. Both lung-fish and the lobe-finned fish had many features that make it apparent how fish could leave the water and eventually develop into amphibians. The ray-finned fish remained in the water and have gone on to become the most successful vertebrates on the planet.

•The lungfish is a member of an ancient group of fishes characterized by functional lungs, a direct circulatory connection from the lung to the heart (as in mammals). Lungfishes were often classified with the lobe-fin fishes

Modern Fish

There are three kinds of ray-finned fish alive today:
• sturgeons
• gars and bowfins
• modern fish (teleosts)

Sturgeons appeared first, and were the dominant type of fish until they were almost completely replaced by gars and bowfins about 225 million years ago. When modern fish appeared, they then almost completely replaced the gars and bowfins.

Today there are an estimated 30,000 species of modern fish, making them the most successful group of vertebrates in existence. They include all of the fish that most people are familiar with, such as pike, walleye (pickerel), whitefish, trout and every other fish you would find in Manitoba, except sturgeon and lampreys.

Sturgeon

After they were almost completely replaced, first by gars and bowfins and later by modern fish, the only sturgeons remaining were quite different from the ones that had dominated before then. Modern sturgeons are considered a degenerative form of the ancient sturgeons. The ancient sturgeons had a heavy, armour-like covering of hard, shiny, interlocking scales. They had a bony skeleton and strong jaws. Their modern descendants have lost their covering of scales and retain only a bony plate over their skull and five rows of bony plates called scutes that run the length of their body. The bony skeleton is also gone, replaced with one that is almost entirely cartilage (including the skull). The strong jaws are also gone, replaced with a feeble jaw, which supports a tube-like mouth with which they feed off the bottom.

One of the most obvious differences between sturgeon and modern fish is their tail. Modern fish have a symmetrical (homocercal) tail. The top and bottom lobes are the same size and shape. Sturgeon have a more primitive asymmetric (heterocercal) tail, similar in shape to a shark's tail. Other differences include:

- the presence of a notochord, a feature that is not found in modern fish (only the even more primitive lamprey still has a notochord).

- the lack of scales. Instead, sturgeon have a bony skull plate and five rows of bony scutes.

- a spiral valve in the gut. This distinctive feature is also found in sharks. While most animals have an intestine which folds back on itself to increase its length and absorptive surface, sturgeon have a spiral shaped valve inside their intestine, which performs the same purpose in a gut, which is quite short compared to other fish.

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