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Lets Learn About Sharks!
Sharks are a group of elasmobranch fish with a cartilaginous skeleton. They have five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Sharks have been classified as a sister group to rays. The earliest known sharks date back more than 420 million years ago. There are estimated over 500 species of sharks. They do not live in freshwater although there are a few known exceptions, such as the bull shark which can be found in saltwater and freshwater.
Black-tip reef shark
Sharks are covered with dermal denticles that protect their skin from damage and parasites. Shark teeth are embedded in the gums rather than attached to the jaw.
Great White Shark
Some sharks lose up to 30,000 teeth in their lifetime. Their tooth shape depends on the sharks diet. Sharks that feed on mollusks and crustaceans have dense and flattened teeth for crushing. Those who feed on fish have needle-like teeth for gripping. Sharks that feed on larger prey have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting.
Tails help sharks with speed and acceleration depending on their tail shape. Tiger sharks have a large upper lobe, that allows them slow cruise and sudden bursts of speed. Tiger sharks must be able to twist and turn easily when they are hunting.
Most sharks are "cold-blooded" or, poikilothermic, this means that their internal body temperature matches what their surrounding water is. Members of the family Lamnidae such as the shortfin mako shark and the great white shark are homeothermic, meaning their body temperature is higher than their surrounding water. Homeothermic sharks have an aerobic red muscle that is located near the center of the body that generates heat, which the body retains.
Shortfin mako shark
Sharks have a heterocercal caudal fin where the dorsal portion is usually noticeably larger than the ventral portion. This provides a surface area for muscle attachment and This allows more efficient locomotion. Most fish have a homocercal tail, where the fin appears symmetric.
Types Of Tails
Heterocercal: When vertebrae extend to the upper lobe of the tail, making it longer.
Reversed heterocercal: When the vertebrae extend to the lower lobe of the tail, making it longer.
Diphycercal: When the vertebrae go to the tip of the tail and the tail is symmetrical.
Homocercal tail: where the fin appears symmetric but the vertebrae extend to a short distance into the upper lobe of the fin.
WMNH paleontologists research the Permian period at 287 million years ago. We study Orthacanthus and Xenacanthus, both Permian freshwater sharks. They thrived in swamps and ponds. They possessed small, pickle-fork-shaped teeth, that supported a fish-eating lifestyle. They have a long serrated venom-filled spine on the back of the skull.
Xenacathus Shark Tooth
Orthacanthus Head Spine
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