The scaly skin and reptilian appearance of lizards often lead to questions about their direct evolutionary relationship with dinosaurs. Exploring the evolutionary history of reptiles clarifies that these groups followed distinct evolutionary paths within the broader tree of life.
The Broader Reptile Lineage
The evolutionary journey of reptiles began approximately 310 to 320 million years ago, during the late Carboniferous period, when the first reptiles evolved from advanced reptiliomorphs. These early amniotes, which resembled small lizards, developed the amniotic egg, allowing them to reproduce on land and spread across terrestrial environments. Soon after their emergence, the amniotes diverged into two primary lineages: Synapsida, which eventually led to mammals, and Sauropsida, which encompasses all modern reptiles and birds.
Within the Sauropsida, a key split occurred based on skull structure, specifically the presence of temporal fenestrae, openings behind the eye socket. Early reptiles, known as anapsids, had solid skulls without these openings. However, Sauropsida soon gave rise to diapsids, characterized by two temporal fenestrae on each side, allowing for stronger jaw muscle attachment. This diapsid lineage further divided into two major groups: Lepidosauromorpha and Archosauromorpha, leading to the distinct evolutionary paths of lizards and dinosaurs.
Distinct Paths of Lizards and Dinosaurs
Modern lizards, snakes, and tuataras belong to the Lepidosauria, a subgroup within the Lepidosauromorpha. Lepidosaurs are distinguished by characteristics like periodically shed scaly skin and a kinetic skull with movable joints. Their evolutionary history dates back to the early Jurassic or late Triassic period, with the oldest squamate fossils appearing in the Middle Jurassic.
In contrast, dinosaurs are part of the Archosauria, a group within the Archosauromorpha, which also includes crocodilians and pterosaurs. Archosaurs evolved approximately 250 million years ago in the late Permian period, exhibiting distinct anatomical features. Unlike lepidosaurs, archosaurs possess a more rigid skull and an upright ankle joint that enabled more efficient movement. This fundamental divergence in their early reptilian ancestry means lizards and dinosaurs followed separate evolutionary trajectories.
Dinosaurs’ Closest Living Relatives
While lizards share a distant common ancestor with dinosaurs, they are not the closest living relatives. Instead, birds and crocodilians hold this distinction, both being the sole surviving members of the Archosauria clade. Birds are direct descendants of theropod dinosaurs. Fossil evidence, including shared features like hollow bones, supports this direct lineage.
Crocodilians also share a common archosaur ancestor with dinosaurs, diverging around 240 million years ago. Although not direct descendants of dinosaurs, their shared ancestry means they are more closely related to birds and dinosaurs than to other reptiles like lizards. Genomic studies comparing birds and crocodilians have provided insights into their evolutionary relationships.