What is labeled A in this diagram? 
Definitions:
Tetrapod
A vertebrate animal that has four limbs or limb-like appendages, including amphibians, reptiles, birds, and mammals.
Convergent Evolution
The process where organisms not closely related independently evolve similar traits as a result of having to adapt to similar environments or ecological niches.
Monotreme
A mammal that lays eggs instead of giving birth to live young, such as the platypus and echidnas, belonging to the most primitive order of mammals.
Therian
A subgroup of mammals that includes marsupials and placental mammals, distinguished from monotremes.
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