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
three kinds of ray-finned fish alive today:
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.
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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