Despite new fossil discoveries, new sources of data, and new methods of analysis, several important issues concerning the origin and phylogeny of Primates remain unresolved. One currently controversial issue is the time of origin of the Order Primates. Both the analysis of molecular data (Springer et al.) and mathematical modeling (Soligo et al.) suggest a time of origin in the middle of the Cretaceous period (80-90 MYA), while the earliest fossil record of primates is only 55 MYA. The fossil record can only provide a minimum age for the origin of any taxon, while these other approaches may be measuring the initial divergence between a taxon and its sister group—an event that may be not marked by any morphological differentiation. Soligo et al. discount this latter possibility, since the molecular estimate for the Strepsirhine-Haplorhine split is 80 MYA. They calculate, therefore, that there is a 25-MY gap between the origin of identifiable primates and their first appearance in the fossil record.
The supraordinal relationships among mammals have been an area of intense interest among paleontologists, and primates are no exception. Although primatologists have reached some consensus about the content of the Order, there is still little agreement as to which living or fossil group is the sister taxon of Primates. The molecular analysis of nuclear and mitochondrial genes by Springer et al. provides support for the clade Euarchontoglires, consisting of Primates, Dermopterans, Scandentia, Rodentia, and Lagomorpha. Within this clade, Primates are most closely related to Dermoptera and Scandentia (=clade Euarchonta). Neither tree shrews nor flying lemurs are the exclusive sister group of Primates, but form a clade with each other. Unfortunately, this analysis does not include Ptilocercus, a tree shrew that may be the most primitive of its clade and thus may have particular relevance to primate origins (Sargis, Godinot). Nor can the relationships of fossil taxa be addressed. The morphological analysis of the postcranium by Sargis, which does include Ptilocercus, finds, like the molecular analyses, that Scandentia and Dermoptera form a group (but only if Chiroptera is excluded). On the other hand, Godinot's analysis, which includes cranial, dental, and postcranial characters, makes a strong case for a special relationship between tree shrews, particularly Ptilocercus, and primates. Plesiadapiformes, the Paleogene fossil group that has been traditionally most closely linked to Primates, were too incomplete to be analyzed effectively in these analyses. Silcox, by using the more ubiquitous dental characters (as well as cranial and postcranial features) supports a sister-group relationship between Plesiadapiformes and Primates to the exclusion of tree shrews or flying lemurs. Therefore she, like Bloch and Boyer (Section III), supports the assignment of this fossil group within the Order Primates following the conventional paleontological interpretation.
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