Crocodiles and alligators are not dinosaurs, though they are closely related. This common misconception stems from the fact that both groups are ancient, large reptiles that dominated the Earth during the Mesozoic Era. While they share a deep evolutionary history, their lineages diverged millions of years before the appearance of the first true dinosaurs.
Not Dinosaurs, But Archosaurs
Modern crocodiles and alligators belong to the Crocodylomorpha lineage, which is part of a much larger and more ancient group of reptiles known as Archosauria. The name Archosauria means “ruling reptiles,” and this clade encompasses the last common ancestor of living crocodilians and birds, along with all of its descendants. This grouping includes the crocodilians, the dinosaurs, and the extinct flying reptiles called pterosaurs.
The Archosauria clade split into two primary branches early in its history. The crocodile-line archosaurs are called Pseudosuchia, which includes alligators, crocodiles, and all other archosaurs more closely related to them than to birds. The other branch, which led to dinosaurs and birds, is called Avemetatarsalia, or the bird-line archosaurs. Therefore, crocodiles are cousins to dinosaurs, having shared a common archosaur ancestor, but they are not members of the Dinosauria clade itself.
Defining the Dinosauria Clade
The scientific definition of a dinosaur is based on a specific set of anatomical features that distinguish them from all other Archosaurs. The most defining characteristics relate to their hip and limb structure, which allowed for a fully erect, upright posture. Unlike the sprawling or semi-erect posture of most reptiles, including crocodilians, dinosaurs held their legs directly beneath their bodies, an adaptation for efficient terrestrial locomotion.
A major feature of the dinosaur hip is the presence of a perforate acetabulum, a hole in the center of the hip socket where the femur connected. This unique structure allowed the leg to swing in a straight, forward-and-back motion. Dinosaurs also featured a sacrum, or fused hip vertebrae, composed of at least three vertebrae, and a modified ankle structure. Crocodilians lack this specialized hip and maintain a more traditional reptilian gait, even when moving in a raised “high walk.”
The Evolutionary Split and Shared Traits
The divergence between the crocodile-line (Pseudosuchia) and the bird-line (Avemetatarsalia) occurred early in the Triassic Period, approximately 248 million years ago. The shared common ancestor lived around this time. In the early-to-mid Triassic, the Pseudosuchia were the more dominant and diverse group, with many species that were large, terrestrial predators.
By the Late Triassic, the evolutionary landscape shifted, and the early Avemetatarsalia, including the first true dinosaurs, began to diversify. Despite their separate paths, both groups retained several features inherited from their common archosaur ancestor. These shared traits include teeth set in sockets and two extra openings in the skull—the antorbital fenestra and the mandibular fenestra. These anatomical details contribute to the superficial resemblance that leads to the common misclassification of crocodilians as dinosaurs.
Evolutionary Persistence: Surviving the Extinction Event
The crocodilian lineage, a subset of the Pseudosuchia, demonstrated remarkable evolutionary resilience, surviving the Cretaceous-Paleogene (K-Pg) extinction event 66 million years ago that wiped out all non-avian dinosaurs. Their survival is attributed to a combination of physiological and ecological factors that provided protection against the drastic environmental changes. The semi-aquatic lifestyle of the ancestral crocodilians was a major advantage, as freshwater habitats were less severely impacted than terrestrial and marine ecosystems.
Crocodilians are ectothermic, meaning they have a low metabolic rate and require significantly less food than the larger, endothermic dinosaurs. This ability to survive for months without a meal allowed them to weather the prolonged period of food scarcity and ecosystem collapse following the asteroid impact. Furthermore, their young were small, grew slowly, and could sustain themselves on small, varied prey such as insects and invertebrates, ensuring the continuation of the species through the mass extinction event.