Any fossilized remnants of a living form that shares features with both its ancestral and developed descendant groups is referred to as a transitional fossil.
This is especially essential when the descendent group differs significantly from the ancestral group in terms of physical anatomy and style of life.
In other words, it possesses characteristics that are shared by an ancestral group and its descendants.
Given the fact that speciation can occur extremely quickly in small groups, such fossils are understandably rare.