Evolution of Fish Skeletons

• Cartilaginous fish have a skeleton made of cartilage, while bony fish have a skeleton made of bone.
• Cartilage evolved first in the ancestry of vertebrates, followed by bone.
• Bone formation occurs through a process called endochondral ossification, where cartilage is replaced by bone.
• Both types of fish have similar cranial structures, but bony fish have more bony structures throughout their bodies.

Bony Fish

Bony fish have a skeleton made of bone, unlike cartilaginous fish. They are the most diverse group of fish, with over 28,000 species. Some interesting examples include salmon, tuna, and goldfish. Bony fish have a swim bladder that helps them control their buoyancy, allowing them to stay at different depths in the water. They also have gills to extract oxygen from the water.

Evolution of Fish Skeleton

• Fish skeletons evolved from cartilage to bone.
• Bony rays in fins provide structure and support.
• Dorsal fins are attached to the vertebra, while pectoral and pelvic fins are attached to the skull and pelvic girdle respectively.
• Gill arches and operculum differ between fish groups.
• Bony fish have swim bladders, while chondrichthyes do not.
• Scales cover the epidermis, but differ in structure.

From Cartilage to Bone

• Chondrichthyes: Have placoid scales that resemble teeth

• Bony fish: Have overlapping cycloid scales or comb-like ctenoid scales

• Chondrichthyes skeleton: Made of cartilage, not mineralized like bone

• Ossification: Process of bone formation through endochondral ossification

• Mesenchymal cells: Differentiate into osteoblasts to lay down bone matrix protein

• Calcium binding: Calcium phosphate binds to the bone matrix protein

Chondrichthyes

Trivia: Chondrichthyes, also known as cartilaginous fish, have placoid scales that resemble teeth. These scales are made of dentin and are covered in enamel, just like our teeth! Sharks, rays, and skates are examples of Chondrichthyes. Fun Fact: The scales of Chondrichthyes are so sharp that they were historically used as sandpaper!

From Cartilage to Bone

• Ossification is the process of bone formation
• Chondrocytes create the cartilage structure
• Osteoclasts remove material, while osteoblasts deposit material to create bone
• Osteocytes are living cells within the bone
• Intramembranous ossification occurs in flat bones like the skull
• Endochondral ossification occurs in long bones like the limbs

Ossification

Ossification is the process of bone formation. It involves the transformation of cartilage into bone tissue. Osteoblasts are responsible for depositing new bone, while osteoclasts break down old bone. This process is crucial for bone growth, repair, and remodeling.