As the ‘Age of the Dinosaurs’ came to an abrupt end 66 million years ago, another evolutionary story began, the ‘Rise of the Primates’, and it starts with this mouse-sized mammal known as Purgatorius says Will Newton.
What is Purgatorius?
Purgatorius is widely regarded as the earliest example of a primate - a group that includes monkeys, apes, and humans, and is defined by its members’ relatively large brains, front-facing eyes, grasping hands, and tendency to form large and complex social structures.
How big was Purgatorius?
Purgatorius is a lot smaller than most of today’s primates, measuring just 15cm in length and weighing in at roughly 35 grams.
What did Purgatorius look like?
As the oldest primate, Purgatorius doesn’t display all of the same, characteristic features as today’s primates. In fact, Purgatorius looked a lot more like a squirrel than a monkey, with a pointed nose, a long, low-slung body, and a bushy tail.
These features are thought to be leftovers from the last common ancestor between rodents and primates, which likely lived just a few million years before Purgatorius.
Despite its rodent-like appearance, Purgatorius is a primate and perhaps the first in a long lineage that, after many millenia, gave rise to us - anatomically modern Homo sapiens.
There are lots of animals in between Purgatorius and us, but in Purgatorius we can already start to see some similarities between the most primitive primate and the most derived primate, particularly in its teeth.
When and where did Purgatorius live?
Purgatorius lived from the Late Cretaceous (66 million years ago) to the Early Palaeocene (63 million years). This was a tumultuous period of Earth’s history, made infamous by the asteroid impact that wiped out not only the non-avian dinosaurs, but roughly 75% of all life on Earth. Purgatorius lived in the shadows of these last surviving dinosaurs and alongside a plethora of other, rodent-like mammals.
A lot of these early mammals, including Purgatorius, avoided extinction at the end of the Cretaceous and prospered in the periods that followed. The disappearance of the non-avian dinosaurs allowed some of these mammals to quickly rise to the top of the food chain and diversify into many of the groups that we recognise today, from dogs to cats, and whales to humans.
The first remains of Purgatorius were discovered in 1965 in eastern Montana at an Early Palaeocene-aged site known as Purgatory Hill. Some Purgatorius remains have also been found in the Early Palaeocene-aged rocks of the nearby Hell Creek Formation, a famous series of rocks that has yielded remains of some of the most well-known dinosaurs, including T.rex, Triceratops, Ankylosaurus, Edmontosaurus, and more.
At the time that Purgatorius was alive, the area that’s now Montana, US, was covered by large, subtropical forests. Based on the discovery of some tiny, well-preserved ankle bones in 2012, researchers think that Purgatorius lived deep in these forests and spent their lives almost entirely in the forests’ canopy. These ankle bones had a very wide range of motion, suggesting Purgatorius were adept climbers capable of jumping from tree to tree in search of food.
What did Purgatorius eat?
Around the same time that Purgatorius emerged and started a lineage that would go on to include all living primates, plants were beginning to bear fruit.
It’s thought that primates and fruit-bearing plants may have co-evolved, with plants producing attractive fruits whose seeds would be gobbled up by primates and then distributed widely as they jumped from tree to tree, pooing as they went.
Purgatorius is also thought to have preyed on insects, amphibians, and a number of other small animals. This ability to eat a variety of different foods is one shared by most living primates and is made possible by their specialised teeth. The first evidence of such specialised teeth is seen in Purgatorius and other early primates.
How is Purgatorius different from other mammals?
A lot of the mammals that lived during the ‘Age of the Dinosaurs’ were small, mousy creatures that spent most of their time hiding in the shadows, avoiding the predatory dinosaurs that dominated their environments.
Purgatorius fits this small, mousy description, but unlike the other mammals they lived alongside, they possessed derived, primate-like teeth. These teeth tell us a lot about Purgatoriusand paint a picture of an omnivorous animal that, as mentioned above, had a very varied diet.
Like us, Purgatorius had incisors, premolars, and molars. Their molars were enlarged, like ours, and they had lingual cusps that helped them grind and chew tough foods, such as the skins of fruits.
Purgatorius’ ankles also set it apart from other mammals and gave it a competitive edge when it came to finding hard-to-reach fruits. It’s this advantage that researchers think contributed to the evolutionary success of early primates and allowed the descendants of Purgatorius to dominate arboreal niches for millenia.
It was only two million years ago that the distant descendants of Purgatorius, Homo erectus, finally left the trees behind and fully adapted to life on the ground.
What came after Purgatorius?
The early evolution of primates is shrouded in mystery due to a lack of fossils; even Purgatoriusis only known from a handful of miniature teeth, fragments of jaw bones, and several tiny ankle bones. That said, from a combination of what little fossils we do have and molecular data, i.e. DNA, researchers have been able to work out that primates underwent a profound split in the Palaeocene and just a few million years after Purgatorius.
This split separated the primates into two distinct groups: the strepsirrhines, or wet-nosed primates, and the haplorhines, or dry-nosed primates. The strepsirrhines are the branch that includes lemurs and aye-ayes, while the haplorhines includes apes and all of the primates that come under the umbrella term of ‘monkey’. Both the strepsirrhines and the haplorhines are diverse, but it's the latter group that ultimately gave rise to humans.
After several million years, haplorhines underwent a split of their own, branching into the platyrrhines (New World monkeys) and the catarrhines (Old World monkeys). The platyrrhines diversified in the Americas after rafting across from Africa on floating mats of vegetation, giving rise to capuchins, marmosets, and spider monkeys, to name just a few modern examples. The catarrhines, on the other hand, stayed put in Africa and gave rise to the apes.
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The Miocene - a period of time that spanned from 23 to 5.3 million years ago - witnessed a great diversification of apes and the emergence of the hominids, a family of great apes that now includes just eight species but during periods of prehistory comprised dozens more, including famous forms such as Gigantopithecus, Australopithecus, and neanderthals.
Then, roughly 300,000 years ago, anatomically modern humans emerged in Africa - the latest chapter in the evolutionary story that started 66 million years ago with Purgatorius.
Why and when did Purgatorius become extinct?
Unlike the non-avian dinosaurs, Purgatorius actually survived the asteroid impact that brought an end to the Cretaceous, 66 million years ago. It’s unclear exactly how Purgatorius made it through this calamitous period, but we know that they did based on the fact that some of their remains have been discovered in rocks that postdate this extinction event.
A forest-dwelling animal like Purgatorius would have likely perished in the wildfires that consumed a lot of Earth’s forests in the aftermath of the asteroid impact, so it’s thought they may have sought refuge on the ground at this time, hiding in burrows they either made themselves, or in burrows abandoned by other animals.
These burrows would have protected Purgatorius during the dark few days after the asteroid impact, but it was their ability to eat a variety of different foods that ultimately kept them alive during the weeks, months, and years that followed. Their forests may have been reduced to ash, but amongst this ash plucky Purgatorius would have found edible seeds, nuts, and even insects.
While Purgatorius may have survived one of, if not the worst day in the entire history of life on Earth, they didn’t live for particularly long and are thought to have faced extinction roughly 63 million years ago.
In the end, this tiny, fruit-munching mammal, that witnessed and survived the asteroid impact that claimed the lives of the dinosaurs, likely fell victim to climate change and the subsequent decrease in the size of its habitat. The emergence of other, better adapted primates may have also contributed to Purgatorius’ extinction.