Final answer:
Yes, it is true that strepsirrhines have only a postorbital bar while haplorhines also have a postorbital plate, denoting a distinct evolutionary trait between the two primate groups.
Step-by-step explanation:
The statement that in strepsirrhines, such as lemurs and lorises, the bony enclosure takes the form of a postorbital bar only, while haplorhines, which include tarsiers, monkeys, apes, and humans, also possess a postorbital plate is True. Strepsirrhines have unique characteristics compared to haplorhines, including a grooming claw and a toothcomb. Haplorhines, which have more reliance on vision and include both diurnal and larger-brained primates compared to the generally nocturnal strepsirrhines with a more developed sense of smell, have a fully enclosed bony eye socket, providing greater eye protection and structural support.