The fossil record is filled with animals that challenge easy categorization, and few exemplify this better than Therizinosaurus. This dinosaur, whose name means “scythe lizard,” presents a biological puzzle. When first studied, its strange collection of features led to significant confusion among paleontologists. Its discovery revealed an animal that defied the established norms of dinosaur evolution, prompting a reevaluation of how certain dinosaur groups lived and behaved.
Distinctive Physical Characteristics
The most distinct feature of Therizinosaurus was the set of three enormous claws attached to each of its forelimbs. These claws, measuring up to 3.3 feet (1 meter) in length, are the longest known of any animal in Earth’s history. These structures were not sharp and curved like those of a predator but were comparatively straight and ended in a duller point, hinting at a purpose other than hunting.
Beyond its famous claws, the rest of its body was an equally strange collection of traits. Therizinosaurus possessed a proportionately small skull at the end of a very long neck, much like some herbivorous sauropods. Its mouth was tipped with a rhamphotheca, or a horny beak, which was suited for stripping vegetation from branches.
The animal’s torso was wide and deep, giving it a pot-bellied appearance that indicated a large digestive system. It stood upright on two powerful hind legs, which ended in four toes each, with three supporting its substantial weight. Evidence from related species, such as Beipiaosaurus, suggests that Therizinosaurus was likely covered in primitive feathers.
An Unexpected Herbivore
Therizinosaurus belongs to Theropoda, a group composed almost entirely of carnivorous dinosaurs like Tyrannosaurus and Velociraptor. This classification created a puzzle, as physical evidence from Therizinosaurus fossils points toward a plant-based diet. Its long neck would have been useful for reaching high foliage, and its large, vessel-like body was necessary to house the extensive gut required to ferment and digest tough plant material.
The structure of its mouth provides further evidence of its herbivorous lifestyle. While a complete skull has not been found, fossils from close relatives show leaf-shaped teeth ideal for shredding plants, not tearing flesh.
This dietary evidence forces a reinterpretation of its most famous feature: the giant claws. Instead of being weapons for predation, scientists now believe they served a function similar to those of a giant ground sloth. Therizinosaurus likely used its long arms and claws to hook onto high branches, pulling them down to its beak to strip away leaves. The claws may have also been used for defense against predators, but their primary role was likely tied to foraging.
Fossil Discoveries and Habitat
The story of Therizinosaurus began in 1948, when a joint Soviet-Mongolian expedition in the Gobi Desert of Mongolia unearthed the first fossils. These initial finds consisted only of the massive claws, which baffled the paleontologists who discovered them. Initially, they were thought to belong to a huge, turtle-like reptile, a theory that persisted until more complete remains of related species were found decades later.
Subsequent discoveries in Mongolia and China helped paleontologists piece together a more complete picture of this animal. These later finds included parts of the forelimbs and hindlimbs, which allowed scientists to correctly identify Therizinosaurus as a theropod. These fossils place the dinosaur in the Late Cretaceous Period, approximately 70 million years ago, in what was then a landscape of lush vegetation and forests.
The habitat of Therizinosaurus was likely rich in plant life, providing the sustenance needed for such a large herbivore. It shared this environment with other dinosaurs, and its unique adaptations would have allowed it to occupy a specific ecological niche as a high browser. The discovery of its fossils in Asia has been important for understanding dinosaur distribution during this period.