The superficial resemblance between snakes and legless lizards often leads to misidentification. Both reptiles evolved a long, cylindrical, and limbless body plan, an adaptation effective for specific environments like dense grass or burrows. Despite this shared external form, snakes and legless lizards represent distinct evolutionary branches within the order Squamata. Numerous anatomical differences, extending from the head structure to the tail mechanics, provide clear ways to distinguish these two groups.
Observable Head Features
Distinguishing between a snake and most legless lizards often involves examining the eyes. Legless lizards, such as the Common Glass Lizard, possess movable eyelids, allowing them to blink and close their eyes for protection and moisture retention. Snakes lack true eyelids entirely; instead, their eyes are covered by a transparent, fixed scale called a brille or spectacle, meaning they cannot blink and appear to stare constantly.
Another difference lies in their auditory anatomy, specifically the presence or absence of external ear openings. Legless lizards typically retain small, visible openings on the sides of their heads, allowing them to hear airborne sounds much like their legged relatives. Snakes have no external ear openings, relying instead on internal structures to detect vibrations transmitted through the ground and their jawbones. While a few species of legless lizards have also lost these external ear openings, the combination of eyelids and external ear holes generally serves as a strong indicator of a lizard.
Cranial Flexibility and Jaw Structure
A fundamental anatomical divergence between the two groups is found in the structure and flexibility of their skulls, which directly relates to their feeding strategies. Snakes possess a highly kinetic skull with an elastic arrangement of bones, particularly in the lower jaw. Their two lower jawbones, or mandibles, are not fused at the chin but are connected by flexible ligaments, allowing them to separate and move independently.
This specialized cranial flexibility permits snakes to ingest prey that is significantly wider than their own head. In contrast, the skulls of legless lizards are far more rigid and closely resemble those of other lizards, with the lower jawbones typically fused at the chin. This rigid structure restricts legless lizards to consuming prey items that are smaller than the size of their head, such as insects and earthworms.
Scale Patterns and Tail Anatomy
The scales covering the body also present distinct patterns, particularly on the underside. Snakes rely on a single row of large, broad, rectangular scales, known as ventral scales or scutes, that span the entire width of their belly. These wide scales are crucial for gripping the substrate and facilitating their serpentine locomotion.
Legless lizards, however, typically have smaller, more numerous scales on their belly that are similar in size and shape to the scales on their back. They do not possess the single, wide ventral plates found on a snake. The structure and proportion of the tail offer another dependable method of separation. In many species of legless lizards, the tail constitutes a significant portion of the total body length, often making up half or even two-thirds of the animal. Legless lizards retain the defense mechanism of caudal autotomy, the ability to voluntarily drop or shed their tail when seized by a predator. Snakes, by comparison, have a proportionally much shorter tail and are incapable of autotomy.
Convergent Evolution and Common Examples
The visual similarity between snakes and legless lizards is a textbook example of convergent evolution, where unrelated organisms independently evolve similar traits due to adapting to comparable ecological niches. The loss of limbs developed multiple times in different lizard lineages, likely as an advantage for burrowing or navigating through dense, subterranean environments. This adaptation resulted in a snake-like body form, even though the two groups separated millions of years ago.
Common examples of legless lizards that cause frequent confusion include the European Slow Worm (Anguis fragilis) and the various species of Glass Lizards (Ophisaurus species). These animals belong to different lizard families, such as Anguidae and Pygopodidae, demonstrating that the limbless body plan has evolved independently in numerous instances. Snakes, classified under the suborder Serpentes, encompass families like Colubridae and Viperidae, and their unique anatomical traits confirm their separate evolutionary history.