Sharks have captured the imagination of humans for centuries with their sleek, powerful bodies and rows of razor-sharp teeth. But where did these aquatic predators come from? Are they similar in anatomy and evolutionary history to land animals with four limbs like amphibians, reptiles, birds and mammals?
Read on to uncover the answer.
What Are Tetrapods?
Definition and Key Characteristics
Tetrapods are a highly diverse group of vertebrate animals that have four limbs. The key defining anatomical feature of tetrapods is having four limbs, with each limb having a central bone surrounded by two pairs of bones.
This gives tetrapods a distinctive skeletal structure different from fish and other vertebrates.
There are over 30,000 species of tetrapods, which are divided into four major groups: amphibians, reptiles, birds, and mammals. All tetrapods share common anatomical features in their limbs, including having four load-bearing limbs, a humerus bone connected to radia and ulna bones, a femur bone connected to the tibia and fibula bones, and digits at the ends of each limb.
Major Tetrapod Groups
Amphibians are cold-blooded tetrapods that live both on land and in water at some point during their life cycle. There are over 8,000 amphibian species including frogs, toads, salamanders, and caecilians.
They have moist skin without scales and go through metamorphosis, transitioning from an aquatic larval stage to a terrestrial adult stage.
Reptiles are cold-blooded tetrapods with dry, scaly skin that lays soft-shelled eggs on land. There are over 10,000 reptile species including snakes, lizards, turtles, crocodiles, and tuatara. They have an advanced respiratory system with lungs, allowing them to live entirely on land, and most species have a three-chambered heart.Birds are warm-blooded tetrapods that have wings, feathers, a four-chambered heart and a lightweight skeleton. With over 10,000 living species, birds have the most diversity in characteristics like size, color, behavior and habitat.
All birds have beaks, lay hard-shelled eggs, and have very high metabolisms to support flight.
Finally, mammals are warm-blooded tetrapods with hair that nourish their young with milk from mammary glands. There are around 6,400 mammal species including humans that inhabit all continents. Mammals have specialized teeth, three middle ear bones, and true sweat glands allowing temperature regulation.
Shark Anatomy and Evolution
Skeletal and muscular systems
The skeletal system of sharks is composed of cartilage rather than bone. This cartilage is flexible yet strong, allowing sharks to be very agile swimmers. Their muscular system is designed for continuous swimming – sharks have red aerobic muscles for cruising long distances as well as white anaerobic muscles for short bursts of speed to capture prey.
One amazing shark muscle is the rectus abdomicus, which functions like a rubber band – stretching as sharks feed to allow their stomach to expand by over 300%!
Origins and evolution
Sharks have a long evolutionary history spanning over 400 million years. Their ancient anatomical designs, like the cartilaginous skeleton, have served them well – modern sharks look very similar to fossils of sharks from 150 million years ago.
Based on fossil evidence, sharks likely evolved from an extinct group of fish called placoderms somewhere around 420 million years ago during the Silurian period. Sharks faced competition from bony fish and marine reptiles throughout history but have shown tremendous resilience – successfully adapting to changes and finding their niche as efficient ocean predators.
Differences Between Sharks and Tetrapods
Limb anatomy
One of the most striking differences between sharks and tetrapods like amphibians, reptiles, birds and mammals is in their limb anatomy. Sharks have pectoral and pelvic fins which aid in lift and thrust through the water.
Their paddle-shaped fins contain flexible cartilage and are very different from the jointed limbs used for walking by tetrapods. While tetrapod limbs have evolved for weight-bearing and locomotion on land, shark fins are adapted for aquatic life.
Respiratory systems
Sharks and tetrapods also differ in their respiratory systems. As fish, sharks have gills that extract oxygen from water as it flows over them. In contrast, tetrapods breathe air using lungs connected to nostrils or a trachea.
While sharks drown in air, most adult tetrapods would suffocate without access to atmospheric oxygen. Some amphibians have neotenic gills in adulthood, but all terrestrial tetrapods rely predominantly on lungs for gas exchange.
Reproductive strategies
Sharks utilize internal fertilization and give birth to live young, while tetrapods display incredible diversity in their reproductive strategies. Most mammals are viviparous like sharks, but egg-laying is also very common, seen in monotremes, reptiles, birds and some fish.
Shark pups are relatively mature and independent at birth compared to altricial tetrapod offspring needing extensive parental care. According to museum experts, sharks may only reproduce every two years while small tetrapods like rodents can breed prolifically year-round.
Are Sharks Descended From Tetrapods?
While sharks and tetrapods like amphibians, reptiles, birds, and mammals share some similarities, sharks are not descended from tetrapods. Sharks belong to the class Chondrichthyes, which includes fish with cartilaginous skeletons, while tetrapods belong to the classes Amphibia, Reptilia, Aves, and Mammalia, which have bony skeletons.
Here’s a closer look at the evidence:
Anatomical Differences
Sharks and tetrapods have very different anatomical structures:
- Sharks have gills to breathe underwater, while tetrapods have lungs and breathe air.
- Sharks have scales made of dentine, while tetrapods have skin covered in keratin.
- Sharks have jaws not attached to their cranium, while tetrapods have jaws attached to the skull.
- Sharks have cartilaginous skeletons, while tetrapod skeletons contain bone.
These major structural differences suggest sharks and tetrapods evolved independently to adapt to different environments.
Evolutionary History
Sharks and tetrapods also have very different evolutionary origins:
- The earliest sharks arose about 420 million years ago during the Silurian period.
- The earliest tetrapods evolved from lobe-finned fish about 390 million years ago during the Devonian period.
The 30 million year gap between the appearance of sharks and tetrapods, along with their vast anatomical differences, clearly shows they evolved separately along different lineages and are not directly related.
Genetic Evidence
Studies of mitochondrial DNA provide further proof that sharks did not descend from tetrapods. Molecular analysis consistently places sharks and tetrapods in completely different taxonomic classes.
Conclusion
As we have seen, sharks and tetrapods have very different anatomical structures and evolutionary lineages. While both groups contain vertebrates, sharks are decisively fish rather than four-limbed creatures.
Their cartilaginous skeletons, gills and methods of reproduction mark them as distinct from amphibians, reptiles, birds and mammals – the true tetrapods. So next time you marvel at a shark’s powerful form gliding through the water, remember – you’re watching a highly successful fish, not a scaly tetrapod!
