Final answer:
The teeth of Spinosaurus (Option A) lacked serrations, reflecting its adaptations for a diet likely consisting of fish, which aligns with its semiaquatic lifestyle.
Step-by-step explanation:
The teeth of Spinosaurus were not like the teeth of modern carnivorous mammals. Unlike the serrated teeth found in many predatory dinosaurs, Spinosaurus teeth lacked serrations. They were conical, somewhat straight, and pointed, which suggest they were adapted for a diet that includes fish, consistent with other evidence indicating that Spinosaurus had a semiaquatic lifestyle. Therefore, the correct answer to the question 'The teeth of Spinosaurus __________.' is A. lacked serrations.