What Are Snakes Classified As in the Animal Kingdom?

Snakes belong to the class Reptilia (reptiles), the order Squamata (the same order that includes all lizards), and the suborder Serpentes. That placement surprises many people who think of snakes and lizards as fundamentally different kinds of animals. In reality, molecular evidence has shown that snakes are deeply nested within the broader lizard family tree, making them, in a sense, highly specialized limbless lizards with a suite of unique adaptations.

The Full Taxonomic Ladder

If you trace snakes from the broadest grouping down to the most specific, the classification looks like this:

• Kingdom Animalia: snakes are animals, as opposed to plants, fungi, or microorganisms.
• Phylum Chordata: they have a spinal cord and, at some stage of development, a notochord.
• Class Reptilia: cold-blooded vertebrates with scales and (typically) shelled eggs, alongside turtles, crocodilians, and tuataras.
• Order Squamata: the scaled reptiles, which also contains all lizards and the worm-like amphisbaenians.
• Suborder Serpentes: the snakes themselves, sometimes referred to in older literature as Ophidia.

What makes this hierarchy interesting is the Squamata level. Squamata is by far the largest order of living reptiles, containing roughly 11,000 described species. Snakes account for around 3,900 of those. The rest are lizards and amphisbaenians. The fact that snakes share an order with lizards is not just a bureaucratic convenience; it reflects genuine evolutionary kinship confirmed by decades of genetic work.

Why Snakes Are Essentially Specialized Lizards

For much of the twentieth century, herpetologists divided squamates into two neat camps: Iguania (iguanas, chameleons, anoles, and their relatives) and Scleroglossa (geckos, skinks, monitors, snakes, and everything else). Snakes sat inside Scleroglossa, which was treated as the sister group of Iguania. A large molecular phylogenetic study using nuclear and mitochondrial DNA from 69 squamate species overturned that tidy picture. The analysis found strong support for most recognized squamate families as real evolutionary groups, but Scleroglossa itself did not hold together. Iguania, far from being the sister group of Scleroglossa, turned out to be deeply nested within it.¹

The practical upshot is that “lizard” is not a single natural group unless you include snakes in it. Biologists sometimes use the informal term “non-snake squamates” when they need to talk about lizards as a unit, because the lizard label on its own is paraphyletic. Snakes are one branch among many within the squamate radiation, not a separate lineage that diverged before lizards diversified. They just happen to have gone through dramatic changes in body plan that make them look radically different.

What Makes a Snake a Snake

Even though snakes are phylogenetically lizards, they do have a suite of traits that sets them apart from their legged relatives. The most obvious is limblessness, but that alone is not diagnostic since several lizard groups have also reduced or lost their limbs independently. What distinguishes snakes is a combination of features working together.

One of the most recognizable is the spectacle, a fused transparent scale that covers each eye in place of movable eyelids. This structure, shared with most geckos, is one of the most visually distinctive traits of snakes and functions as a permanent protective window over the cornea.² Unlike geckos, though, snakes combine this with a highly flexible skull, no external ear openings, a single functional lung in most species, and dramatically elongated internal organs stacked in a line.

The skull deserves special mention. Most snakes, particularly the “typical” snakes people encounter, have loosely connected skull bones that allow the upper and lower jaws to spread apart and swallow prey much wider than the snake’s own head. This cranial kinesis is not universal across all snakes, though. Blindsnakes, a group of tiny burrowing species, have compact, rigid skulls with no kinetic joints, adapted instead for pushing through soil.³ That variation is a reminder that “snake” covers an enormous range of body plans, from thread-thin burrowers the size of an earthworm to six-meter constrictors.

How Snakes Lost Their Limbs

Limb loss in snakes was not a sudden event. Fossil evidence from the Cretaceous shows that early snakes retained well-developed hindlimbs, complete with a pelvis and recognizable leg bones. New fossil material of the Cretaceous snake Najash has shown robust support for an extensive early radiation of snakes that still had hindlimbs, demonstrating that this “intermediate” body plan was not just a brief evolutionary stopover on the way to full limblessness but a stable condition that persisted for a long time.⁴

The genetic mechanism behind limb loss has been traced in remarkable detail. Researchers studying python embryos found that hindlimb development stalls because of mutations that knocked out key binding sites in a limb-specific regulatory switch for the gene Sonic hedgehog. Without that switch working properly, the gene’s activity in developing limb buds is weak and short-lived, and the genetic circuit that normally drives limbs to grow outward shuts down early.⁵

A separate study zeroed in on the same regulatory region from a different angle. When scientists replaced the mouse version of this genetic switch with the snake version, the resulting mice developed severely reduced limbs. Remarkably, restoring just a single lost binding site in the snake version was enough to bring the switch back to full function, producing normal limbs in mice.⁶ The finding is striking because it means the difference between having legs and not having them can come down to a handful of changes in regulatory DNA rather than a wholesale rewriting of anatomy genes. The genes for building limbs are still present in the snake genome; they just lack the instructions to turn on in the right place at the right time.

Where Did Snakes Come From

The ecological origin of snakes has been debated for well over a century, with three competing scenarios: they evolved underground as burrowers, they evolved in the ocean, or they evolved on the surface as terrestrial predators. Each hypothesis has had passionate advocates, and the evidence has swung back and forth.

Analysis of the inner-ear anatomy of both fossil and living snakes supports a burrowing origin for the lineage leading to modern (crown) snakes. A distinctive spherical vestibule in the inner ear, shared by the early fossil snake Dinilysia patagonica and living burrowing squamates but absent in swimmers and surface generalists, suggests that burrowing was the dominant lifestyle in the lineages leading to today’s snakes.⁷

A large-scale study of skull shape evolution across squamates added an important wrinkle, however. Using geometric analysis of skull form and size, the researchers confirmed that the most recent common ancestor of crown snakes had a small skull clearly adapted for burrowing. But when they looked one step further back, at the common ancestor of all snakes plus their closest lizard relatives, that ancestor appeared to be a surface-dwelling terrestrial animal with no burrowing adaptations.⁸ In other words, the ancestral snake lineage probably started on the surface, transitioned to an underground lifestyle (during which the elongated, limbless body plan was refined), and then radiated back out into surface, arboreal, aquatic, and marine habitats. It is a more nuanced picture than any single “origin story” captures.

The Major Groups Within Serpentes

Once you are inside the suborder Serpentes, the diversity is enormous. Herpetologists traditionally split living snakes into two broad groups based on body plan and ecology, though the exact boundaries keep shifting as new genetic data come in.

The first division is between the blindsnakes (historically grouped as Scolecophidia) and the rest (Alethinophidia). Blindsnakes are small, burrowing, worm-like snakes with vestigial eyes covered by head scales. They feed almost exclusively on ant and termite larvae. Their skulls are compact and rigid, and they lack the jaw flexibility that most people associate with snakes. Whether Scolecophidia is a single natural group or several independently derived lineages of burrowing snakes is still debated, but the ecological type is unmistakable.

Alethinophidia contains everything else: boas, pythons, and the massive radiation of “advanced” snakes called Caenophidia. The advanced snakes are by far the most species-rich group, encompassing colubrids, elapids (cobras, mambas, sea snakes), and viperids (vipers and pit vipers), among others. Venom has been described as a key evolutionary innovation underlying the diversification of advanced snakes, and the sophisticated venom-delivery systems found in elapids and vipers are elaborations of a toxin-producing apparatus that appears to have much deeper roots in the squamate tree.⁹

Venom and the Toxicofera Hypothesis

One of the more surprising findings in squamate classification over the past two decades is that venom is not unique to snakes. Molecular phylogenetic work has established that a venom system is ancestral to snakes, meaning it was present before snakes diverged from their lizard relatives. Several lizard groups, including monitors and some iguanians, possess venom-producing glands, though their delivery systems are far less specialized than what you see in a viper or a cobra.¹⁰

This broader group of venom-bearing squamates has been given the name Toxicofera, and it includes snakes, anguimorph lizards (monitors, Gila monsters, and relatives), and iguanians. Snakes just happen to have taken the ancestral venom toolkit and diversified it far beyond what any lizard lineage has done. Some snake lineages have even secondarily lost their venom, which makes sense if venom is metabolically expensive to produce and you have evolved a different predatory strategy, such as constriction.

Within venomous snakes, the position and size of fangs correlate with how venom is used. Vipers and elapids have front-mounted fangs optimized for injecting venom into prey quickly. Many colubrid snakes, by contrast, have rear-mounted fangs. Research into fang evolution across these groups has found that rear-fanged colubrids that shifted their fangs forward did so not by growing new teeth up front but by shortening the entire upper jawbone, effectively repositioning existing structures.¹¹ It is an elegant example of how evolution repurposes anatomy rather than inventing it from scratch.

How Snakes Move Without Legs

Classification is not just about ancestry; it also captures something about how an organism lives. Snakes have evolved an extraordinary range of movement strategies to compensate for their lack of limbs. For decades, textbooks taught that snakes use four modes of locomotion: lateral undulation (the classic S-shaped slithering), rectilinear movement (a slow, straight-line crawl using belly scales), concertina locomotion (accordion-like bunching and extending used in tight spaces), and sidewinding (the diagonal rolling motion used by desert species on loose sand).

Recent biomechanical work has shown that this four-mode scheme is too simple. Detailed analysis of how snakes actually move reveals at least five distinct types of lateral undulation and four distinct types of concertina locomotion, bringing the total to roughly eleven recognized gaits.¹² Some of these distinctions matter to the animals in practical ways: the concertina variant used to climb a vertical tree trunk involves different muscle activation patterns than the concertina used to navigate a narrow tunnel, even though both look superficially similar to a human observer.

This locomotor diversity is part of what makes snakes so ecologically successful. Without legs, they have colonized deserts, rainforest canopies, freshwater rivers, and open oceans. Some species glide between trees. Others swim with the efficiency of an eel. The limbless body plan, far from being a constraint, opened up ecological niches that limbed lizards never reached.

Common Misconceptions About Snake Classification

A few misunderstandings crop up regularly when people try to place snakes in the animal kingdom. The first is calling them amphibians. Snakes are not amphibians. Amphibians (frogs, salamanders, caecilians) have moist, permeable skin and typically go through a larval aquatic stage. Snakes have dry, scaly skin, breathe air exclusively through lungs, and lay shelled eggs or give live birth. The confusion probably arises because some snakes live near water or swim well, but habitat alone does not determine classification.

A second misconception is that snakes are their own separate lineage of reptiles, as distinct from lizards as crocodiles are from turtles. As the molecular phylogenetics covered earlier makes clear, snakes are nested within the lizard radiation. If you drew a family tree of all squamates and removed the labels, you would not be able to draw a clean line separating “lizards” on one side and “snakes” on the other without cutting through several branches that include both. Legless lizards, such as glass lizards and slow worms, add to the confusion by looking superficially snake-like despite being only distantly related to actual snakes. You can usually tell them apart by checking for eyelids and external ear openings, both of which legless lizards retain and snakes lack.

A third persistent idea is that all snakes are venomous. In reality, of the roughly 3,900 known snake species, only about 600 are considered medically significant to humans. Many lineages, including boas, pythons, and most blindsnakes, use constriction or simply swallow prey without venom playing a role in subduing it. The Toxicofera framework suggests many “non-venomous” snakes still produce trace amounts of toxin-like compounds in oral glands, but these are not medically relevant and do not function as venom in any practical sense.

Snakes Compared With Other Limbless Reptiles

Limblessness has evolved independently in squamates at least two dozen times. Amphisbaenians (worm lizards) are burrowing squamates with reduced or absent limbs that superficially resemble snakes but belong to a separate lineage within Squamata. Several skink lineages have independently reduced their limbs to tiny stubs or lost them entirely. Even within geckos, a few lineages show limb reduction. What separates true snakes from these other limbless squamates is the full package of traits described earlier: the spectacle over the eye, the absence of ear openings, the highly modified skull architecture (in most species), the elongated body with hundreds of vertebrae, and the internal organ rearrangement that comes with extreme elongation.

Amphisbaenians are particularly easy to confuse with snakes because some species are large enough to look the part. But their skulls are built for head-first burrowing in a fundamentally different way, often with a shovel-shaped or keel-shaped snout designed to compact soil. They also have ring-like body segments that give them an annulated, earthworm-like texture you will never see on a snake. Despite the superficial similarity, the molecular evidence places amphisbaenians on a completely different branch of the squamate tree from snakes, confirming that the two groups arrived at limblessness independently.

Why Classification Keeps Changing

If you pick up a herpetology textbook from the 1990s and compare it with a current one, the higher-level arrangement of snake families will look different. New genetic sequencing techniques keep reshuffling the branches. Scolecophidia, once treated as a coherent suborder of primitive snakes, may turn out to be several independent lineages. Some families that were lumped together under Colubridae, the enormous “wastebasket” family of typical snakes, have been split into separate families as relationships became clearer. Even the boundaries of well-known families like Elapidae and Viperidae have been tweaked as new species are described and old ones are re-examined genetically.

This instability is not a sign of confusion; it is a sign that the field is actively improving. Morphology alone can be misleading in a group where convergent evolution is rampant. Two unrelated snake lineages living in similar environments often evolve strikingly similar body shapes, head proportions, and color patterns, leading earlier taxonomists to group them together. Molecular data cut through that noise and reveal the actual branching pattern, which is why so many rearrangements have happened in the past 25 years. For a casual observer, the important takeaway is that the broad strokes are stable: snakes are squamate reptiles, closely related to lizards, subdivided into blindsnakes and the rest, with advanced snakes forming the most species-rich branch. The fine print within those categories is where the action is.