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Why is the fossil Rooneyia considered a transitional primate form?

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Illustration comparing the body plans of representative anthropoids, showing a male western lowland gorilla, a male human, a male bonobo, a female Bornean orangutan, and male and female northern white-cheeked gibbons, all labeled and depicted to scale. — primate Representative apes (superfamily Hominoidea).

Rooneyia is considered a transitional primate form because its skull exhibits a distinct combination of both primitive and advanced features. Discovered in Texas and dating to the Oligocene Epoch, this omomyid fossil represents the type of morphological mixture expected in a form bridging the evolutionary gap between lower and higher primates.12

Beyond its transitional status, the discovery of Rooneyia is significant because it challenged the previous belief that primates had vanished from North America by the late Eocene.2