The first reptiles appeared roughly 310 to 320 million years ago, during the Carboniferous period. The oldest confirmed reptile fossils date to around 318 million years old and already show two distinct lineages, one leading to modern reptiles and birds, the other leading to mammals. A newer and more controversial discovery, fossilized footprints from Australia dated to about 355 million years ago, could push that origin back even further.
The Oldest Reptile Fossils
For over a century, a small lizard-like creature called Hylonomus has held the title of earliest known reptile. Its fossils come from the Joggins Fossil Cliffs in Nova Scotia, Canada, and are approximately 300 million years old. Hylonomus was tiny, likely no more than 20 centimeters long, with a slender body, small sharp teeth suited for eating insects, and a long tail with distinctive bone structures on its underside.
The Joggins cliffs preserve an unusual snapshot of Carboniferous life. Rivers periodically flooded low-lying swampy forests, burying trees in sediment. As the large tree trunks decayed and hollowed out, they became natural pit traps. Small animals crawling across the forest floor would fall inside and become entombed as sediment filled the stumps. That’s where Hylonomus skeletons were found, preserved inside fossilized tree trunks alongside ancient amphibians.
Older fossils have since pushed the timeline back. Fossils from around 318 million years ago already include representatives of both major branches of the amniote family tree. And in 2024, researchers reported fossilized footprints from an Australian sandstone slab dated to roughly 355 million years old. The tracks show long toes with claw marks, a combination that suggests they were made by a primitive reptile. If confirmed, this discovery would push the origin of reptiles back by about 35 million years, to the very beginning of the Carboniferous.
Another contender, a fossil called Westlothiana found in Scotland, was once thought to be a very early reptile. It dates to the Viséan age, making it older than Hylonomus. However, detailed analysis of its skull and skeleton placed it just outside the true reptile group. Scientists now classify it as a “stem amniote,” meaning it sits on the evolutionary branch leading toward reptiles but hadn’t quite crossed the line.
What Made Reptiles Different
The key innovation that separated reptiles (and their relatives) from amphibians was the amniotic egg. Amphibians lay soft, jellylike eggs that dry out quickly on land, tying them to water for reproduction. The amniotic egg solved this problem with a set of internal membranes that retain moisture, allow oxygen and carbon dioxide to pass through, store nutrients, and collect waste. An outer shell, either hard or leathery, provided physical protection.
This was a dramatic leap in vertebrate evolution. It meant reptiles could reproduce entirely on land, without needing a pond or stream for their eggs. That freedom opened up vast stretches of dry habitat that amphibians couldn’t colonize.
There’s an ongoing debate about how the amniotic egg actually evolved. The traditional view is that early ancestors laid eggs on land and gradually developed the protective membranes to cope with dry conditions. A competing hypothesis suggests the membranes first evolved inside the mother’s body, as a way to retain embryos longer before laying them. Supporting this idea, fossils of amniotic eggs are almost completely absent before the Late Triassic period (around 230 million years ago), while evidence of live birth in ancient reptile relatives appears as early as the Early Permian, roughly 290 million years ago. This suggests that for the first 100 million years of their existence, many early reptiles may have given live birth or laid soft, poorly mineralized eggs.
Another physical development was scaly, waterproof skin. Recent research on body impressions from early reptiles shows that keratinized scales, the tough, protein-rich outer covering seen on modern reptiles, were already present in late Carboniferous and earliest Permian species. These earliest scaly skin impressions predate the major waves of global warming and drying that reshaped the planet later in the Permian, suggesting that waterproof skin evolved before it was strictly necessary and then gave reptiles a head start when conditions changed.
The World They Evolved In
The Carboniferous period gets its name from carbon, specifically the massive coal deposits that formed during this era. Much of the land near the equator was covered in dense, swampy forests with a warm, tropical, wet climate and little seasonal variation. Giant ferns, towering club mosses, and early seed plants created thick canopies. The air was oxygen-rich, fueling the growth of enormous insects and other arthropods.
Early reptiles were small creatures in this world, scurrying through the undergrowth and likely feeding on insects and other invertebrates. They were far from dominant. The swamps belonged to large amphibians and the towering plant life that would eventually become coal seams.
The Split Into Two Great Lineages
Almost as soon as reptiles appeared, their family tree forked into two branches that would shape the rest of vertebrate history. By around 318 million years ago, fossils already represent both groups: Synapsida, the lineage that would eventually produce mammals, and Reptilia (also called Sauropsida), the lineage leading to lizards, snakes, turtles, crocodiles, dinosaurs, and birds.
The earliest members of both groups looked superficially similar, small four-legged animals with generalized body plans. The differences were subtle, mainly in the skull. Early reptiles in the strict sense typically had solid skull roofs without openings behind the eye sockets. Over time, different lineages independently evolved openings in the skull’s temporal region, likely because bone thinned and broke away in areas that bore less mechanical stress during biting. These openings became a defining feature: the mammal lineage developed one pair, while many reptile lineages developed two. Some groups, like turtles, later lost their openings entirely.
How Reptiles Took Over
During the Permian period (299 to 252 million years ago), the planet changed dramatically. The vast swamps of the Carboniferous dried up as the supercontinent Pangaea assembled, creating enormous inland deserts and replacing tropical wetlands with drier, conifer-dominated forests. Reptiles were relatively rare during this time compared to their synapsid cousins, which dominated land ecosystems through much of the Permian.
But reptiles were already changing fast. Research from Harvard’s Museum of Comparative Zoology found that rapid evolution in reptile body plans began around 270 million years ago, tens of millions of years before the catastrophic Permian-Triassic extinction that wiped out roughly 90% of species 252 million years ago. Rising global temperatures drove this acceleration. As climates shifted, most reptile lineages evolved at exceptionally high rates, reshaping their bodies to adapt to new conditions. Larger-bodied groups like the ancestors of crocodiles, turtles, and dinosaurs couldn’t shed heat as easily and had to change their body plans more radically.
One lineage bucked the trend. The lepidosaurs, ancestors of modern lizards and tuataras, evolved very slowly during this period, their body plans held in check by natural selection rather than diversifying wildly like their relatives.
After the Permian-Triassic extinction cleared the field, reptiles exploded in diversity during the Triassic period (252 to 200 million years ago). This is when most major modern reptile groups, including the ancestors of crocodiles, lizards, and turtles, first appeared, alongside now-extinct groups like the dinosaurs’ earliest relatives. The “Age of Reptiles” had begun, and it would last for over 180 million years.