Are Crocodiles Lizards? A Detailed Look At Crocodilian Classification

Crocodiles, alligators, caimans, and gharials—these aquatic reptiles with fearsome teeth and powerful jaws capture our imagination and fascinate nature enthusiasts. But there is an enduring question around their biological classification—are crocodiles just big lizards?

If you’re short on time, here’s a quick answer: No, crocodiles are not lizards. Crocodilians belong to a separate order of reptiles called Crocodilia, while lizards belong to the order Squamata.

In this comprehensive guide, we will analyze the evolution, anatomy, behavior, and taxonomy of crocodilians and lizards to understand their similarities and key differences. We will review recent scientific evidence on crocodilian genetics and phylogeny to conclusively answer whether the two groups are closely related or distant cousins in the reptile family tree.

The Distinct Features and Behaviors of Crocodilians

Physical Attributes: Scales, Teeth and Bones

Crocodilians have unique scales, teeth, and skeletal structures that differentiate them from other reptiles like lizards.

• Their scales are made of keratin and serve as external armor. The belly scales are small and rectangular while the back scales are larger and reinforced with bone for extra protection.
• They possess conical, pointed teeth designed to grasp slippery prey like fish or tear flesh. These teeth are continually replaced throughout a crocodilian’s life.
• Their skeletons contain lightweight yet durable bone tissue reinforced by collagen fibers and calcium phosphate crystals. This helps support massive body weight while allowing them to float, dive, and lunge swiftly.

Hunting and Feeding Habits

Crocodilians are ambush apex predators that wait patiently with only their eyes and nostrils above water to attack unsuspecting prey.

• Using bursts of speed and the element of surprise, they can capture mammals, birds, fish, turtles, and even sharks for food.
• Their jaws deliver bone-crushing bites, often disabling prey immediately. They then drag animal carcasses underwater to drown before consuming them.
• Crocodilians can go months between major meals. After eating up to half their body weight, their metabolism slows while food digests over weeks or months.

Reproduction and Nesting

Crocodilians demonstrate unique nurturing behaviors unlike other reptiles.

• Females dig nests of vegetation, sand, soil and clay before laying 20 to 80 eggs in breeding seasons dictated by climate.
• Mother crocodilians guard their nests for up to three months while embryos develop hard shells. The temperature of the nest determines offspring gender.
• After calling to babies at hatching time, mothers may even transport hatchlings to water in their mouths and watch over offspring for an extended period as they mature.

To learn more, see the Encyclopaedia Britannica crocodilian overview.

How Lizards Compare to Crocodiles

The Great Diversity Among Lizards

Lizards exhibit an astounding diversity in terms of habitats, sizes, colors, and more. Over 6,000 lizard species have been identified so far. They range from small geckos just a few centimeters long to the mighty Komodo dragon reaching over 3 meters.

Lizards inhabit diverse environments including deserts, forests, grasslands and even the polar regions. Their skin colors and patterns are equally diverse, allowing effective camouflage. Clearly, lizards have adapted remarkably well to fill a variety of ecological niches.

In contrast, there are only 23 crocodilian species alive today. Though crocodilians once exhibited greater diversity, competition from early mammals and environmental changes drastically reduced their varieties.

Modern crocodiles and alligators are relatively large, aquatic ambush predators concentrated mostly in the subtropics and tropics. They lack the phenotypic plasticity seen in lizards.

Differences in Anatomy and Physiology

Lizards and crocodiles have distinct anatomical and physiological differences reflecting their unique evolutionary paths. Lizards generally have light, agile bodies with long tails and well-developed limbs for climbing and running. Most have movable eyelids and external ear openings.

Crocodiles, meanwhile, are bulkier with flat snouts, webbed hind feet, and powerful tails that propel them in water. They have slit-like nostrils, ears and eyes placed higher on their heads as adaptations for their semi-aquatic lifestyle.

Many lizards are diurnal and fast-moving with high metabolic rates. Crocodiles tend to be nocturnal ambush predators with slower metabolisms. Generally, lizards and crocodiles have different cardiovascular and respiratory systems reflecting their respective active vs sit-and-wait strategies.

Varied Mating and Reproductive Strategies

Reflecting their diversity, lizards exhibit myriad mating rituals and reproductive strategies. Many engage in elaborate courtship behaviors before mating while others have simpler rituals. Fertilization can be internal or external.

While most lizards lay eggs, some species are viviparous, giving birth to live young. Litter sizes range from just 2-3 offspring in some species to over 80 in others!

Crocodiles generally have similar reproductive strategies across species. Breeding and egg-laying are usually timed with seasonal rainfalls or temperature changes. Females lay 20-80 eggs in holes or mounds of rotting vegetation to incubate.

Baby crocodiles receive no parental care and are vulnerable to predators until reaching 1-2 meters long when their thicker skin develops.

So while lizards and crocodiles belong to the same phylogenetic class of reptiles, millions of years of evolution along different ecological lines have resulted in profound differences between these groups today in anatomy, physiology and reproductive strategies.

The Evolutionary Origins of Crocodilians

Archosaur Ancestors and Crocodilian Lineage

Crocodilians belong to a group of ancient reptiles called archosaurs, which also includes dinosaurs, pterosaurs, and birds. The earliest archosaur ancestors emerged during the Late Permian period over 250 million years ago.

These primitive archosaurs were small, agile bipeds that likely resembled miniature versions of Velociraptor.

By the start of the Triassic period around 245 million years ago, archosaurs had diversified into several major lineages. One of these were the crocodylomorphs, the evolutionary precursors to true crocodilians.

Early crocodylomorphs like Sphenosuchus and Hesperosuchus still retained some ancestral bipedal characteristics. However, over the course of the Triassic and into the Jurassic, they evolved to become larger quadrupedal semi-aquatic predators.

The first true crocodilians emerged during the Late Jurassic around 150 million years ago. These included primitive forms like Protosuchus, which already showed classic crocodilian traits like armored skin and elongated snouts.

From these early beginnings, crocodilians diversified into a variety of forms throughout the Cretaceous period. Some lineages became enormous in size, like the massive Deinosuchus which grew up to 15 meters long!

When and How They Diverged from Dinosaurs and Pterosaurs

Crocodilians, dinosaurs, and pterosaurs all belong to a subgroup of archosaurs called Avemetatarsalia. However, crocodilians split off from the lineage leading to dinosaurs and pterosaurs quite early on, back in the Middle Triassic around 245 million years ago.

At the time, most archosaurs were still small, gracile bipeds. But crocodylomorphs began adapting to life around the water’s edge, evolving stockier builds, armor plating, and other aquatic adaptations. Meanwhile, the dinosaur ancestors retained more terrestrial bipedal forms.

So the crocodilian and dinosaur lineages diverged as they adapted to different ecological niches.

Interestingly, some analyses suggest crocodilians are actually more closely related to dinosaurs than to many of their fellow archosaurs. This reflects their shared avemetatarsalian ancestry. However, crocs had already embarked on a semi-aquatic path by the time of their divergence, so they did not acquire the specialized hip structure that characterizes dinosaurs and allowed them to become so diverse and successful.

The Taxonomic Classification of Crocodilians and Lizards

Order Crocodilia vs Order Squamata

Crocodilians and lizards both belong to the reptile class, but they are classified into different taxonomic orders. Crocodilians belong to the order Crocodilia, while lizards belong to the order Squamata. The main differences between the two orders are:

• Crocodilians have a more primitive skull structure compared to lizards, with the snout firmly fused to the braincase.
• The hearts of crocodilians contain four chambers, while lizards have three-chambered hearts.
• Crocodilians have integumentary sense organs called dome pressure receptors distributed on their skin, which lizards lack.
• The ankles in crocodilians have a crurotarsal joint that allows for more sprawling locomotion, while most lizards have a more upright parasagittal gait.

These morphological differences reflect the fact that crocodilians and lizards belong to distinct archosaur lineages that diverged back in the Triassic period some 245-200 million years ago.

Recent Advances in Molecular Phylogenetics

Molecular phylogenetics looks at evolutionary relationships by comparing DNA and protein sequences across species. Recent phylogenetic studies using advanced sequencing techniques have greatly clarified our understanding of crocodilian and lizard relationships:

• Analysis of mitochondrial DNA shows that crocodilians are most closely related to birds, forming a group called Archosauria. Lizards are more distant relatives within the diapsid reptiles.
• Nuclear DNA analysis supports the Archosauria grouping. Crocodilians and birds share unique insertions in the B2M, ND2 and FXN genes that are not found in lizards and other reptiles.
• Some crocodilian proteins like C-type lysozyme have closer sequence homology to bird proteins than lizard proteins.

The evidence overwhelmingly supports crocodilians and lizards belonging to distinct clades that diverged deep in the evolutionary tree. Despite some superficial resemblances, they are only distant cousins.

Why Crocodilians Are Not Big Lizards

Calling crocodiles or alligators “big lizards” is inaccurate from a taxonomic perspective. The salient differences include:

Crocodilians also tend to be much larger than lizards, with some species reaching over 5 meters (16 feet) long. The largest lizard species max out around 3 meters (10 feet). This considerable size difference is tied to different evolutionary pressures and ecological roles.

Conclusion

In conclusion, while crocodilians share some superficial similarities with lizards in having scales and laying eggs, they possess many unique specializations related to their aquatic lifestyle.

Modern taxonomic classification clearly differentiates crocodilians at the order level from lizards and snakes, which together constitute the order Squamata. Extensive genetic evidence conclusively shows they belong to distinct branches of the reptile family tree that diverged over 200 million years ago.