Ancient Reptiles Evolve from Four Legs to Two During Adolescence: Unveiling Evolutionary Mysteries

Ancient crocodilian relatives navigated their early years on all fours before transitioning to a bipedal stance as they matured.

This evolutionary adaptation involved differing growth rates in their arm and leg bones. “The forelimb length starts at about 75% of the hindlimb length and ends up being about 50%,” explains Elliot Armor Smith from the University of Washington in Seattle.

This discovery contributes to a growing body of evidence indicating that crocodile-like ancestors from the early dinosaur era showcased remarkable diversity, with some developing body shapes and lifestyles akin to modern ostriches.

Christian Sidor, Armor Smith’s collaborator, excavated the Cay Quarry, located in Arizona’s Petrified Forest National Park. This site comprises mudstone and sandstone laid down by ancient rivers during the Triassic period, around 215 million years ago.

Through their excavation, Armor-Smith and Sidor unearthed over 3,000 bones from early crocodilian relatives known as Shubosauridae. “It’s a jumble of individual limb bones that aren’t necessarily related,” notes Armor-Smith.

They successfully identified a new species within the Shubosaur family, naming it Sonselasuchus cedrus. Over 950 bones belong to this species, which diverges significantly from modern crocodiles, resembling more of a flightless bird or theropod dinosaur with short arms and a toothless beak instead of a jagged mouth full of teeth.

Other Shubosaurs share similar body features. “Shubosaurids are unique creatures that existed in the late Triassic period,” asserts Michelle Stocker from Virginia Tech. “They bear a striking resemblance to dinosaurs.” Most closely, they resemble ornithomimids, ostrich-like dinosaurs from the late Cretaceous period, occurring more than 100 million years after the Shubosaurids.

Sonselasuchus cedrus marks the beginning of a four-legged life. Fossils from young individuals exhibit similar sizes in forelimbs and hindlimbs, while older specimens showed larger hind limbs capable of supporting greater weight. “The robust femur indicates strength,” Armor-Smith remarks, “though even the largest humerus is relatively delicate.”

This is unusual, yet not entirely surprising. A 2019 study uncovered evidence of two dinosaur species transitioning from quadrupedal to bipedal locomotion as they matured. One was a sauropod, an ancestor of colossal sauropods like Brachiosaurus, while the other was a distantly related early ceratopsian akin to Triceratops.

Both juvenile and adult S. cedrus likely lived somewhat distinct lives, even adopting different diets, according to Stocker.

Common narratives of the dinosaur age often imply that dinosaurs, especially avians, evolved in remarkable ways, while crocodiles have largely remained unchanged. This perception misrepresents the extensive diversity of pseudosuchians, the lineage that includes crocodiles. Stocker clarifies, “They engaged in numerous unique and radical adaptations first, paving the way for dinosaurs.”

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