Evolving the Musculoskeletal System

Excess cartilage: osteochondroma, chondosarcoma, acromegaly
Excess joint fluid: pigmented villonodular synovitis (pvns), arthrocentesis, rheumatoid arthritis
Excess ligaments: Madelung's deformity, scoliosis

Tetrapods occasionally produced small, unpaired bones between the paired nasals which articulated with the premaxilla. The appearance of internasals is an example of random genetic drift of a minor mutation which results in a benefit (an additional tooth bearing bone).The development of the rib serves as another example. The earliest tetrapod ribs were independent of the vertebral column. A mutation of the Hox gene (an increase in the number of copies of the gene) led to fusion of these ribs to the spine. This fusion disengaged the rib cage from the musculature of the body and allowed for greater ventilation (the animal can breathe more easily).