How are extinct organisms assigned to species when interbreeding cannot be tested?

Because extinct organisms cannot be tested for reproductive isolation, paleontologists typically assign them to species based on their morphology—the observable physical characteristics preserved in the fossil record. These are often referred to as morphospecies, which are defined strictly by their structural differences from related forms.12

When classifying these remains, taxonomists may use the following approaches:

• Form Groups: For fragmented remains (such as isolated leaves, stems, or roots), researchers construct "form groups" by assigning a genus or species name based solely on the structure of the fossilized material.3
• Comparative Anatomy: Taxonomists analyze physical traits to distinguish between homologies (inherited from a common ancestor) and analogies (similarities resulting from convergent evolution).4
• Cladistics: This method classifies species based on shared derived characters, or synapomorphies, to infer descent from a single common ancestor.5

Because the fossil record is often discontinuous, these classifications can involve subjective decisions or "phyletic splitting," where researchers must balance the need to show evolutionary lines with the reality that fossil evidence is frequently incomplete.31