Plesiadapiformes represent an extinct order of mammals that emerged shortly after the mass extinction event that ended the age of dinosaurs. These ancient creatures lived during the Paleocene and Eocene epochs, making them some of the earliest placental mammals to diversify in the Cenozoic Era. They hold a significant place in paleontological studies because of their debated evolutionary relationship to primates, including humans. Their unique combination of features has made them a focal point in understanding the origins of our own mammalian lineage.
Physical Characteristics and Lifestyle
Plesiadapiformes were a diverse group, with at least 11 families and over 140 species, showing diverse adaptations. Most species were relatively small, ranging from 10 grams to 3 kilograms (marmot-sized). Their appearance varied, with some resembling modern squirrels.
Plesiadapiformes had specialized dentition, including enlarged, rodent-like incisors that were slender and pointed. Their molars, however, typically featured multiple cusps and basins, suggesting a varied diet that likely included insects, fruits, and possibly leaves or tree exudates. Skeletal evidence from their limbs indicates they were adept, largely arboreal climbers. Unlike modern primates, they possessed claws rather than flattened nails. Their eyes were positioned laterally, unlike the forward-facing eyes of true primates.
Geographic Distribution and Timeline
Plesiadapiformes thrived from approximately 66 to 47 million years ago, spanning the Paleocene and into the middle Eocene epochs. Fossil remains have been discovered across the Northern Hemisphere.
Fossil evidence for Plesiadapiformes has been primarily found in North America and Europe, with some notable genera like Plesiadapis found on both continents. This suggests the presence of ancient land bridges, possibly via Greenland, that facilitated their dispersal. More recently, representatives of the group have also been found in Asia, further expanding their known distribution.
The Primate Connection Debate
The classification of Plesiadapiformes and their relationship to true primates has been a long-standing subject of scientific discussion. Early paleontologists often placed them within the Order Primates due to some shared dental characteristics, particularly in their molar teeth. Certain skeletal elements, like ankle bones in Purgatorius, suggested an arboreal lifestyle similar to later primates. Studies involving CT scans of skulls, such as that of Carpolestes simpsoni, have indicated some shared cranial features with primates, including the petrosal bone forming the auditory bulla, a trait seen in all living primates.
However, significant anatomical differences also present arguments against their direct classification as true primates. They lacked several defining features of modern primates, such as a postorbital bar (a bony ring around the eye socket). Additionally, many Plesiadapiformes possessed unusual, enlarged anterior teeth that differ from those of true primates.
Despite these distinctions, the current consensus among many researchers is that Plesiadapiformes are a “stem group” or “sister group” to true primates, also known as Euprimates. This means they represent very close relatives that branched off before the evolution of the full suite of defining primate traits, rather than being direct ancestors of all primates. Their study remains central to understanding the complex evolutionary tree leading to modern primates.
Extinction of the Plesiadapiformes
Plesiadapiformes declined and became extinct towards the end of the Eocene epoch. This process was likely influenced by a combination of environmental changes and increasing competition from other evolving mammalian groups. One contributing factor was the shifting global climate, with a return to warmer, more subtropical conditions after the Paleocene. While the exact impact of climate change on their decline is still being investigated, changes in available food resources due to these shifts may have played a role.
A more prominent factor in their disappearance appears to have been competition from other rapidly diversifying mammals. The emergence of the first “true primates,” or Euprimates, during the early Eocene presented a new challenge for Plesiadapiformes. These Euprimates were likely more adapted to arboreal niches, possessing features like forward-facing eyes and grasping hands and feet with nails, which offered advantages in tree-dwelling environments. Additionally, the rapid diversification of rodents, which also occupied similar ecological niches and competed for resources, further contributed to the decline of many Plesiadapiform groups.