Therizinosaurus cheloniformis is a dinosaur known for its extraordinarily long claws and unusual body plan, making it one of the most enigmatic creatures from the Late Cretaceous period. The genus name translates to “scythe lizard,” a direct reference to its most striking feature. The initial discovery began a decades-long scientific mystery.
The Initial Discovery and Misidentification
The first fossils of Therizinosaurus were unearthed in 1948 during a Soviet-Mongolian field expedition in the Nemegt Formation of the Gobi Desert in Mongolia. The team recovered a set of massive, blade-like manual unguals—the bones that anchored the creature’s claws.
Russian paleontologist Evgeny Maleev formally described the remains in 1954, but the scarcity of the material led to a profound misinterpretation. He concluded that the fossils belonged to a gigantic, turtle-like reptile, perhaps an aquatic species. This initial error is preserved in the species name, cheloniformis, which means “turtle-shaped.”
The sheer size of the claws, which could reach lengths of over 50 centimeters (nearly 20 inches) in bone length alone, contributed to the confusion. Maleev even created a new family, Therizinosauridae, to classify what he believed was a unique lineage of giant turtles.
Solving the Classification Puzzle
The scientific consensus regarding Therizinosaurus remained uncertain for decades, though some paleontologists suggested it might be an unusual theropod dinosaur as early as the 1970s. This idea gained traction with the discovery of more complete, related fossils in the late 1970s and 1980s.
The new genus Segnosaurus was described in 1979, and while its features were unusual, it was tentatively placed within the Theropoda clade. The true identity of the “scythe lizard” and its relatives was not confirmed until the discovery of the more complete skeleton of Alxasaurus in 1993, followed by Beipiaosaurus in 1999.
These new finds provided the missing anatomical links, confirming that this group of animals, then called “segnosaurs,” possessed features consistent with Theropods. Key features like a backward-pointing pubis bone in the pelvis and certain wrist structures solidified their place within the Maniraptora subgroup of coelurosaurs.
The discovery of these intermediate species proved that the Therizinosauridae were, in fact, highly specialized herbivorous theropod dinosaurs that had evolved a unique body plan. The earlier name, Therizinosaurus, was kept for the group’s family name due to the rules of priority in scientific nomenclature.
Defining Features of Therizinosaurus
The most famous characteristic is its enormous forelimb claws, which were the longest of any known terrestrial animal, potentially reaching up to a meter (about three feet) in life with their keratin sheaths. These claws were likely used for pulling branches and foliage toward its mouth for feeding, rather than for active predation.
The body of Therizinosaurus was equally distinctive, contrasting sharply with its predatory theropod cousins. It was an obligate biped, but its pelvis was broad and wide, suggesting a heavy, pot-bellied build. This build was necessary to accommodate a large digestive system for processing tough plant material.
The animal is also inferred to have possessed a long neck topped by a relatively small skull ending in a horny beak, which is another adaptation for an herbivorous diet. While only partial remains of the type species exist, the features shared with its more complete relatives paint a picture of a massive, slow-moving creature with a feathered coat, standing perhaps four to five meters tall.