Final answer:
Scientists infer species assignments in fossils using morphological analysis, genetic comparisons where possible, relative dating, and by identifying common ancestors. These methods provide evidence to distinguish between interspecific and intraspecific variation.
Step-by-step explanation:
When researchers discover fossils and want to make species assignments without evidence of interbreeding, they rely on a variety of methods. Since they can't directly test if two fossilized organisms were capable of mating, scientists must infer relationships based on other evidence. Analyzing the physical characteristics of fossils is one part of this process; they look for features that might indicate whether individuals belong to different species (interspecific variation) or are variations within the same species (intraspecific variation).
Additionally, when possible, scientists compare DNA sequences from fossils to living or well-preserved species. This genetic information can help reveal evolutionary relationships. Other methods include examining the geological context of the fossils using relative dating techniques to estimate the time period in which they lived, which can provide insight into the rate of speciation.
Ultimately, if scientists can identify a common ancestor in the fossil record, this can further clarify whether different fossils represent separate species or variations within one species. Each piece of evidence from morphology to genetic data contributes to our understanding of species assignments in the fossil record.