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Why are some Paleocene fossils no longer considered true primates?

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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).

Some fossils from the Paleocene Epoch, such as those belonging to the group Plesiadapiformes (or Paromomyiformes), are no longer widely considered true primates due to reevaluations of their anatomical features and phylogenetic relationships.1

While these fossils were traditionally classified as primates, experts like Robert D. Martin have argued that their connection to authenticated primates is tenuous. Crucially, the discovery of limb bones—specifically finger bones—in genera such as Ignacius has provided strong evidence suggesting that these animals may actually belong to the order Dermoptera, which includes the modern colugo or "flying lemur".12

Furthermore, the application of phylogenetic systematics and cladistic methodology has led researchers to refine the definition of the order Primates to ensure that taxonomic groups are monophyletic. Because these Paleocene fossils lack the definitive characteristics of true primates and share closer affinities with other mammalian groups, they are increasingly excluded from the primate order in modern classifications.132