Fossil Remains: The Plesiadapiformes and the Origins of the Primate Order
While there are many physical traits that characterize most living primates, and other traits which characterize some of the primates to the exclusion of all other mammals, there are very few which characterize all primates to the exclusion of all other mammals. The cladistic approach to animal classification that mandates groupings by shared and derived traits has therefore posed problems with respect to defining the primate order.
Ventral view of a Phenacolemur jepseni specimen inspected at the American Museum of Natural History in New York City.
|Close-up view of the exposed middle ear section of the Phenacolemur jepseni specimen shown left. This view shows a ridge on the petrosal bulla which had been thought to represent a suture mark, although a closer examination of this area suggests a post-depositional fracture unrelated to the structural anatomy of the specimen.|
The petrosal bulla is the skeletal casing of the middle ear and remains as the single identified trait which occurs in all living primates to the exclusion of all other mammals and which can be simultaneously detected in fossil primates. This minute trait is a protective bony covering (bulla) ontogenetically connected to a bony plate (petrosal) which separates the middle ear region from the interior of the skull. Non-primates often have an inflated bulla that is either extended from an element other than the petrosal, or independently developed. The bullae of adult non-primates may post-natally fuse with the petrosal, however, rendering the classification of fossil specimens thought to be closely related to the earliest representatives of the primate order difficult for species not represented by fossils of juveniles.
The Plesiadapiformes are thought by many researchers to be the earliest representatives of the primate order, evolving over 60 million years ago. This insectivorous suborder contains members which exhibit a striking continuity of traits with some of the earliest prosimians. However, specimens representing two of the Plesiadapiformes, Cynodontomys latidens and Ignacius graybullianus, bear strong traces of suture marks in the respective petrosals that are indicative of independent bullae. Thus their placement in the primate order, or alternatively the assignment of Plesiadapiformes as the cladistic origin of the primate order, remains questionable.
A director of ACS (Gregory F. Walwer) conducted a study designed to identify traces of resorbed sutures in bone which may help to reveal possible independent bullae in other Plesiadapiform fossil species for which juveniles are not available. Additionally, a visit to the American Museum of Natural History was made in order to inspect a Phenacolemur jepseni specimen bearing faint traces of possible suture marks, thus calling into question the classification of a third Plesiadapiform as a primate. While these last traces were determined to be post-depositional fractures rather than partially resorbed sutures, thereby tentatively linking P. jepseni to the origins of the primate order, the growing body of evidence indicates that at least a portion of the Plesiadapiform suborder does not exhibit the single characteristic definitively uniting all primates and poses interesting questions concerning the origins of the primate order.