Lizards are one of the most successful and diverse groups of land vertebrates, inhabiting nearly every environment on Earth except the high Arctic and Antarctica. They belong to the order Squamata, a grouping that includes all lizards, snakes, and amphisbaenians (worm lizards). With over 11,991 described species, Squamata is the largest order of reptiles. Tracing the evolutionary origins of this diverse group requires following a lineage that stretches back hundreds of millions of years, beginning with the first animals that broke their dependence on water.
The Ancient Ancestors: From Amphibians to Reptiles
The distant ancestors of all lizards were ancient four-legged creatures that relied on water for reproduction, much like modern amphibians. This changed approximately 340 million years ago during the Carboniferous period with the evolution of the amniotic egg. This biological innovation contained specialized membranes to protect the developing embryo, allowing it to grow entirely on land, free from aquatic nurseries. This adaptation separated the first Amniotes from their amphibian relatives, granting them access to drier terrestrial habitats.
The Amniotes soon diverged into two major evolutionary lines: the Synapsids (which led to modern mammals) and the Sauropsids. Lizards belong to the Sauropsid line, which gave rise to all modern reptiles and birds. The earliest definitive reptiles, such as the small, lizard-like Hylonomus, date to around 300 million years ago.
The Sauropsid lineage diversified, with the ancestors of lizards separating from the ancestors of turtles. This initial split set the stage for the anatomical distinctions that would define later reptile groups. The key to understanding the lizard lineage lies in a specific modification of the skull structure that provided new anchor points for jaw muscles.
The Diapsid Split: Paving the Way for Lizards
The crucial anatomical feature defining the lizard’s immediate ancestry is the diapsid skull, a term meaning “two arches.” This skull type is characterized by two temporal fenestrae, or openings, located behind each eye socket. These openings evolved to accommodate larger, stronger jaw muscles, enabling a more powerful bite and wider gape. The earliest recognized diapsids appeared around 300 million years ago, marking a major turning point in reptile evolution.
The Diapsida group split into two branches: the Archosauromorpha and the Lepidosauromorpha. The Archosauromorpha includes all modern crocodiles, extinct dinosaurs, and birds. The Lepidosauromorpha, meaning “scaly lizard forms,” is the direct line that leads to lizards and snakes.
Lizards are firmly rooted in this Lepidosaur branch, which also includes the tuataras of New Zealand. The last common ancestor of all modern lizards and the tuatara belongs to the Superorder Lepidosauria. This places the divergence of the lizard lineage from the line leading to crocodiles and birds no later than the Middle Permian period, approximately 259 million years ago. This separation established the fundamental body plan of modern scaly reptiles, characterized by a unique shedding process of the skin and a trend toward a lighter, more flexible skull structure.
Identifying the First Squamates
The transition to the first true member of the Order Squamata is marked by the evolution of a highly specialized skull. While many early lepidosauromorphs are known, the oldest fossils definitively placed on the evolutionary path to Squamata are rare and often fragmentary. One important early fossil is Paliguana whitei, an enigmatic species from the Early Triassic of South Africa, considered the earliest known stem-lepidosauromorph, dating back over 250 million years.
A later, more telling fossil is Megachirella wachtleri, a small reptile from the Middle Triassic of the Italian Alps, dating to about 240 million years ago. Phylogenetic analysis of this specimen places it as the oldest known relative of the Squamata lineage, pushing the group’s origins back by tens of millions of years. The defining feature that separates Squamata from its closest relatives is the evolution of a kinetic skull.
This kinetic skull allows for movement between the upper jaw and the braincase. This flexibility enables a wider gape for catching prey and is the anatomical trademark of the group.
The presence of a nearly modern, or “crown,” squamate was recently confirmed by the discovery of Cryptovaranoides from the Late Triassic of England, dating to approximately 202 million years ago. This fossil possesses numerous traits found in modern lizards, confirming that the diversification of the lizard lineage was well underway in the Triassic period. The ancestors of modern lizards were already established and evolving alongside their archosaurian relatives.