Final answer:
The statement that scientists use chemical signatures of fossil teeth to determine the diet of ancient creatures is true. Fossil teeth retain isotopic compositions that reflect the diet of the individual, providing insights into whether the being was herbivorous, carnivorous, or omnivorous. Evidence such as unique anatomy and dating techniques suggest these creatures may be extinct.
Step-by-step explanation:
Using chemical signatures of fossil teeth, scientists indeed try to determine the diet of ancient creatures following the principle "you are what you eat". This is true because the isotopic composition of the teeth can reflect the diet since different foods have distinctive ratios of isotopes. For example, carbon isotopes can distinguish between plants that use different photosynthetic pathways and therefore can give clues about whether an animal was primarily a herbivore, carnivore, or omnivore. The tooth enamel is especially valuable because it does not remodel after it is formed, and so it keeps a chemical record of the individual's diet through its lifespan.
The evidence from these fossils that suggests they may be organisms that are now extinct includes the incomplete nature of the fossil record, which often shows organisms with no living counterparts. Also, certain traits observed in the fossils, such as the teeth structure of carnivorous dinosaurs like Tyrannosaurus rex, can indicate specific dietary choices that are not observed in contemporary fauna. Moreover, relative dating techniques that determine the geological ages can show that these creatures lived in periods from which no living species descended directly.