why do the three species have similarities and differences in their body structures, and how can Xiao use the structures to determine if the new fossil is more closely related to hippopotamus or green iguanas?

Xiao can use these structures to create a similarity matrix that enables to differentiate between synapomorphies and homoplasies.

Explanation:

A synapomorphy is a trait that has been inherited from the same ancestor, this trait enables to establish a relation of homology between two or more species; while a homoplasy is an analog structure that doesn't have homology.