The Cretaceous aged Hell Creek Formation of Montana is best known for the discovery over a century ago of Tyrannosaurus rex and for some of the best fossils ever found at the end of the age of the dinosaurs about 65 million years ago.  At that time the planet experienced a mass extinction that wiped out many species including non-avian dinosaurs, but for many mammals including our own ancestors this marked a beginning, not the end.  Teeth of the oldest primate (called Purgatorius) have been known from rocks just above the Hell Creek Formation for decades, but the rest of the skeleton has remained elusive. A new study just presented at the 72nd Annual Meeting of the Society of Vertebrate Paleontology in Raleigh, North Carolina, describes the first ankle bones of Purgatorius from northeastern Montana. This new discovery provides new evidence for the ecology of the first primate.

Analysis of these new fossils, has led paleontologists Stephen Chester, Yale University, Jonathan Bloch, Florida Museum of Natural History, and William Clemens, University of California Museum of Paleontology, to conclude that Purgatorius had an ankle joint with a fairly wide range of motion. This type of ankle is characteristic of animals that live in trees and adjust their feet to uneven branches and trunks when climbing. “The way in which the earliest primates lived has been a subject of great debate for many years, but these fossils are the first direct evidence that show that these primates spent most of their time in the trees following their divergence from other mammals in the earliest Paleocene” said Chester.

 The authors of this study believe that the specialized ankle bones of Purgatorius likely played a key role in the evolutionary success of early primates. “These new fossils support the idea that the first 10 million years of primate evolution happened in the context of an intense period of similar diversification in flowering plants, including the ability to climb in branches and collect fruits and other products of the trees at the very beginning.” said Bloch. Many other mammals living in the earliest Paleocene were land-dwelling and by living in the trees, Purgatorius was able to exploit new resources in its environment.

Ongoing expeditions to northeastern Montana led by Clemens, Greg Wilson from the University of Washington, and colleagues from other institutions continue to yield important fossils and geological information documenting the pattern of change of the fauna and flora some 65 million years ago. “The new ankle bones are an extraordinary discovery.  We plan to continue research in Montana in order to address other critical questions about causes of the mass extinction as well as the origin of primates and other animals that characterize the beginning of the age of mammals” said Clemens.

For additional information, see recent summary in Nature.

Abstract from SVP Meeting.

TARSAL MORPHOLOGY OF THE OLDEST PLESIADAPIFORM PURGATORIUS INDICATES ARBOREALITY IN THE EARLIEST PRIMATES

CHESTER, Stephen G. B., Yale University, New Haven, CT, USA; BLOCH, Jonathan I., Florida Museum of Natural History, University of Florida, Gainesville, FL, USA; CLEMENS, William A., University of California Museum of Paleontology, Berkeley, CA, USA.

The origin of primates has long been thought to relate in part to arboreality, yet direct fossil evidence of positional behaviors of the earliest primates has been lacking as these taxa are only represented by fragmentary dentitions. Though plesiadapiforms (stem primates) are generally considered to have been arboreal based on partial skeletons from the late Paleocene and early Eocene, it is unclear whether this substrate preference evolved independently or was characteristic of the first primates in the early Paleocene. The Garbani Channel fauna localities in Garfield County, northeastern Montana, are thought to represent the late Puercan (Pu3; ~65MYA) and have yielded hundreds of dental specimens of Purgatorius, the oldest and most primitive plesiadapiform known. Several isolated astragali and calcanea were recovered from four Garbani Channel localities and are referred to Purgatorius based on size, abundance, and diagnostic similarities to dentally associated tarsals of euarchontans in general, and plesiadapiforms specifically. In the astragalus, these similarities include a dorsoventrally deep fibular facet relative to the medial tibial facet, a medial edge of the trochlea that extends onto the neck, and a helical sustentacular facet clearly confluent with the navicular facet. Within plesiadapiforms, these astragali are most similar to those of micromomyids in having a body with a relatively high medial ridge and a large flexor fibularis groove. Several similarly sized calcanea are also diagnostically similar to those of other plesiadapiforms in having an ectal facet fairly aligned with the long axis of the calcaneum, a prominent sustentaculum with a helical sustentacular facet that extends distally onto the body, a large peroneal tuberosity, a round and concave cuboid facet, a distinct plantar pit, and lacking a fibular facet. These characteristics indicate a mobile ankle that would allow pedal inversion in order to adjust to an uneven substrate, typical of euarchontan mammals. While results from recent phylogenetic analyses failed to support primate or placental affinities of Purgatorius, this new tarsal evidence strongly suggests that Purgatorius is a plesiadapiform that lies near the ancestry of all primates within Euarchonta. These specimens are the first to demonstrate that the earliest known plesiadapiforms possessed postcranial modifications for arboreality compared to other mammals in the earliest Paleocene, and these specializations likely played a key role in the evolutionary success of the earliest primate radiation.