Maybe you can picture a little squirrel zipping around under a T. rex’s feet, but that’s not quite how it happened. The squirrels you see today didn’t show up until long after non-avian dinosaurs vanished, so they never actually shared the prehistoric stage.
Still, some small, squirrel-like mammal relatives did scurry around while dinosaurs ruled the planet. Let’s dig in a bit—early mammals had some surprising habits, and fossils like euharamiyids suggest they climbed trees and maybe even nibbled on nuts, kind of like today’s squirrels.
Curious what the fossils really show? Let’s see how scientists fit these ancient puzzle pieces together.
Did Squirrels Coexist With Dinosaurs?
Nope, true squirrels didn’t scamper around with non-avian dinosaurs. Early mammal relatives did live at the same time as dinosaurs, but they looked and acted pretty differently.
Key Differences Between Squirrels and Dinosaurs
Squirrels are rodents. They have specialized teeth for gnawing and a skull shape that only pops up tens of millions of years after dinosaurs disappeared.
Modern squirrels belong to Rodentia, and that group doesn’t show up in the fossil record until well after the big dinosaur extinction.
Dinosaurs, on the other hand, came in all shapes and sizes—from tiny feathered hunters to massive, long-necked sauropods. Their hips, skulls, and limbs look nothing like what you’d find in mammals.
Early mammal relatives sometimes reached squirrel size, but their teeth, ear bones, and jaw joints set them apart. Some of these mammals had fur and could climb trees, but their ear and jaw bones still looked pretty primitive.
Want a wild example? There’s a Jurassic fossil that was about the size of a squirrel but had a weird mix of old and new traits. You can read about it here: Jurassic squirrel’s secret is out after 165m years.
Timeline of the Age of Dinosaurs Versus Squirrel Evolution
Dinosaurs stomped around from about 230 million years ago until the end of the Cretaceous, which wrapped up with the big extinction event 66 million years ago. That’s when most non-avian dinosaurs vanished.
True squirrels (the Sciuridae family) didn’t appear until much later, during the Cenozoic Era. Rodents really took off after the dinosaurs left, filling up all sorts of empty ecological spaces.
Fossils of early mammaliforms go back around 160 million years, showing that small, furry creatures did live with dinosaurs, but they weren’t squirrels.
Here are some dates worth remembering:
- About 160 million years ago: small mammal relatives scurried around while dinosaurs ruled.
- 66 million years ago: the Cretaceous–Paleogene extinction wiped out most non-avian dinosaurs.
- After 66 million years ago: modern mammal groups, including squirrel ancestors, started to diversify in the Cenozoic.
The Evolution and Fossil Record of Early Squirrel Relatives
Let’s wander back through time and see how squirrel-like mammals evolved—from their tiny Mesozoic ancestors to the first true squirrels in the Eocene.
The fossil record lays out a pretty interesting timeline, with key fossils marking the way.
Ancestral Mammals of the Mesozoic Era
During the Mesozoic, small mammaliforms lived right alongside dinosaurs. Most were mouse-sized, with teeth adapted for munching insects and seeds.
Their skeletons had limb and claw shapes that let them climb, hop, and sometimes even glide. That’s pretty cool, right?
But here’s the thing: these creatures weren’t rodents or squirrels. They sat on early branches of the mammal family tree, and over time, their descendants split off into many modern groups.
Fossils from the Jurassic and Cretaceous often turn up as teeth and jaw bits, hinting that some of these little mammals spent time in the trees.
Traits like sharp, ever-growing incisors and nimble limbs didn’t really show up until later, when rodents and squirrels started to evolve.
Some of the most important fossils come from North America and Asia. While these early mammals didn’t look much like today’s squirrels, they laid the groundwork for future tree-dwelling mammals.
Jurassic Gliders: Maiopatagium furculiferum and Other Mammaliforms
Maiopatagium furculiferum lived about 160 million years ago in what’s now China. Fossils show it had long limb bones and a flap of skin (a patagium), which means it probably glided from tree to tree.
That’s a pretty wild adaptation for the Jurassic! Some early mammals really went for it when it came to aerial tricks.
Other Jurassic mammaliforms had tiny skeletons with grasping hands and bendy spines. Their teeth suggest they ate a mix of seeds, insects, and fruit—pretty similar to what modern tree-dwellers snack on.
These fossils aren’t from the squirrel family, but they show that gliding and tree-climbing evolved more than once.
If you compare Maiopatagium with later gliding mammals, you’ll spot the same kinds of changes: longer forelimbs, flexible shoulders, and finger proportions that help support skin membranes.
Those shifts happened long before true squirrels ever showed up. Funny how nature repeats itself sometimes, isn’t it?
Rise of True Squirrels in the Eocene Epoch
True squirrel relatives show up in the fossil record during the Eocene epoch, about 40 to 34 million years ago. Paleontologists have uncovered more complete skeletons from this era in North America and Eurasia, and these fossils really look a lot like modern squirrels—think long tails, strong hind legs, and those ever-growing incisors perfect for gnawing seeds.
During the Eocene, squirrels started branching out into both tree-dwelling and ground-dwelling forms. Fossils from this period include some of the earliest members of the Sciuridae family and their rodent relatives.
Their teeth and jaw structure make it obvious they belong to the rodent group that eventually led to today’s squirrels. By the late Eocene, squirrel-like animals had begun spreading across different continents.
You can spot features in these fossils that feel familiar: tails for balance, claws made for gripping, and those distinctive tooth patterns. It’s kind of amazing to see the rise of true squirrels from their much smaller mammal ancestors.
