What characteristic is shared by birds, salamanders, and sharks?

Jaws and paired appendages

Explanation:

Two pairs of appendages are found in sharks, birds and  salamanders. In the shark (fish) these appendages are fins, while in birds and  salamanders ( vertebrates) these appendages are limbs.  

Sharks, birds and  salamanders also have jaws. The jaw is an opposable articulated structure, typically used for picking and masticating food. Most vertebrates have jaws.

the dna molecules have a negative charge because of the phosphate groups in their sugar-phosphate backbone, so they start moving through the matrix of the gel towards the positive pole. power is turned on and dna fragments migrate through gel (towards the positive electrode).

explanation:

if someone had the list of traits you provided