Evolutionary History Of Microsyopoidea (mammalia, Primates?) And The Relationship Of Plesiadapiformes To Primates. (volumes I And Ii) (north America).

Abstract

The suborder or infraorder Plesiadapiformes is the first group of primate-like mammals known in the fossil record. Plesiadapiformes first appear in the early Paleocene, some 65 million years ago in sediments of the Western Interior of North America. The relationship of Paleocene plesiadapiforms to Eocene primates of modern aspect and relationships among various families, genera, and species of plesiadapiforms are uncertain. In particular, the relationship of Microsyopoidea to plesiadapiforms has been questioned. Morphological and functional studies of dental, cranial, and postcranial remains of plesiadapiforms summarized here suggest retention of plesiadapiforms within the order Primates. However there is no evidence to support a direct phylogenetic relationship between plesiadapiforms and primates of modern aspect. The origins of plesiadapiforms and primates remain unknown. Four distinct superfamilies are recognized within Plesiadapiformes. Microsyopoidea are characterized by the retention of generalized primitive (relative to Purgatorius) dental and cranial adaptations. Microsyopoids are represented by two families: Paleocene Palaechthonidae and Eocene Microsyopidae. Available evidence indicates that microsyopids are most closely related to palaechthonids and can best be viewed as having descended from that Paleocene family, not from the families Mixodectidae or Leptictidae as previously suggested. The other three superfamilies of Plesiadapiformes are Plesiadapoidea, Mixodectoidea, and Apatemyoidea. Of these, plesiadapoids are characterized by specialized dental and cranial adaptations. Mixodectoids and apatemyoids are distinctly more primitive than either of the other two superfamilies, remaining more insectivore-like. Two families, Microsyopidae and Paromomyidae, survived well into the Eocene, while other plesiadapiform families disappeared by the earliest Eocene. Dental characteristics suggest that these two families specialized on diets different from those of adapids and omomyids and thus avoided direct competition with primates of modern aspect. Geographic distributions and climatic reconstructions indicate that warming temperatures into and through the Eocene contributed to the extinction of most plesiadapiforms.