Final answer:
Mystery fossil genus A with bilophodont molars and a larger body size is an Old World monkey, while mystery fossil genus B, being much smaller, is likely a New World monkey. Key traits used for these identifications include the type of molars and body size.
Step-by-step explanation:
Based on the provided descriptions of the mystery fossil genera, we can infer which is a fossilized Old World monkey and which is a fossilized New World monkey using certain traits. First, let's look at mystery fossil genus A, which has been dated to the late Pliocene and early Pleistocene.
Its canines were sexually dimorphic, and it had bilophodont molars. Its postcranial traits indicate that it was a terrestrial quadruped, and its overall body size was over 55 lb (25 kg). These characteristics, particularly the bilophodont molars and terrestrial lifestyle, are consistent with those often found in Old World monkeys.
On the other hand, mystery fossil genus B, which has been dated to the early Miocene and estimated to have adults that weighed around 2 lb (1 kg), suggests that it may be a New World monkey due to its lighter weight, which is common in some small-sized New World monkeys.
To differentiate between Old World and New World monkeys, dental structure and body size can be significant. Old World monkeys generally have bilophodont molars, which are molars that have two ridges, and a sturdier build. New World monkeys, in contrast, can range widely in size but typically include smaller species.
Sexual dimorphism in both canines and body weight may occur in many primate species, so these traits alone are not definitive. However, combining them with other physical traits aids in more accurate identification.