While modern crocodiles are known for their sprawling, four-legged gait and aquatic lifestyle, their ancient relatives present a surprising contrast. Millions of years ago, certain ancestors of today’s crocodilians walked upright on two legs. This highlights the diverse evolutionary paths of the lineage that led to today’s powerful predators.
Ancient Relatives That Walked on Two Legs
Many prehistoric animals related to modern crocodiles, called crurotarsans, exhibited bipedal locomotion. These creatures thrived during the Triassic period, from 252 to 201 million years ago, long before true crocodiles emerged. Fossils from North and South America provide evidence.
One example is Poposaurus, a large predator from approximately 220 million years ago in the southwestern United States. Skeletal reconstructions indicate Poposaurus walked exclusively on two legs, similar to some dinosaurs. Its robust leg bones and specialized hip structure supported this upright posture.
Effigia, discovered in New Mexico, was another bipedal animal in this group. Around 205 million years ago, this toothless herbivore had a bird-like skull. Its post-cranial skeleton and fossil trackways confirm its two-legged stride.
Postosuchus, a predator from the late Triassic, also demonstrated bipedal tendencies, especially as an adult. While it might have occasionally dropped to all fours, its powerful hind limbs and smaller forelimbs suggest a primary two-legged movement. These features distinguish them from modern, quadrupedal descendants.
The Evolution of Bipedalism in Crocodile Ancestors
Bipedalism developed within the archosaur lineage, which includes crocodile ancestors and dinosaurs, during the Triassic period. Scientists propose hypotheses for why these early relatives adopted an upright stance. One idea suggests bipedalism facilitated faster running to pursue prey or escape predators.
An upright posture also offered advantages for display behaviors or navigating dense vegetation, providing a better vantage point. This strategy allowed animals to exploit new ecological niches. However, not all lineages maintained this bipedal form; some later reverted to a quadrupedal stance as environments changed.
This shift back to four legs often coincided with adaptations for different lifestyles, such as ambush predation or a more aquatic existence. Early archosaur diversification showcased diverse body plans and locomotory styles, allowing groups to adapt to the changing Triassic world.
Why Modern Crocodiles Are Different
Modern crocodiles differ from their bipedal ancestors due to millions of years of adaptation to a semi-aquatic environment. Their sprawling gait, with limbs extending outwards, is effective for terrestrial movement and maneuvering in water. This contrasts with their bipedal relatives’ upright limbs.
A powerful, laterally flattened tail is key for modern crocodilian swimming propulsion. This tail, along with webbed feet, allows speed and agility in water, facilitating ambush predation. Their streamlined bodies are suited for life largely submerged.
Their quadrupedal, semi-aquatic lifestyle reflects specialization for hunting in freshwater and estuarine habitats. Modern crocodiles lie in wait, submerging most of their bodies to lunge at prey from the water’s edge. This ambush tactic, combined with powerful jaws and robust build, makes them successful apex predators. Their current form represents a successful evolutionary path distinct from their bipedal past.