Final answer:
An ancestral trait is known as a shared ancestral character, indicating all members of a taxonomic group inherit the trait from a common ancestor and helping to reveal evolutionary paths.
Step-by-step explanation:
The ancestral trait is known as a shared ancestral character because it is a trait that is found in the ancestor of a group. This means all of the organisms within the clade or taxonomic group inherited that trait from a common ancestor.
For example, having a vertebrate is a shared ancestral character that indicates a common ancestor among all vertebrates. When a new trait emerges that is not shared by all members of a group, it is called a shared derived character.
Such traits help scientists understand evolutionary relatedness and construct phylogenetic trees that reveal the paths of evolutionary change.