Description and Classification

Therizinosaurus was a truly bizarre-looking dinosaur, quite unlike the typical predatory theropods such as Tyrannosaurus rex. It is estimated to have reached lengths of up to 10 meters (33 feet) and a height of about 4 to 5 meters (13 to 16 feet), with a body weight potentially around 5 metric tons. It had a long neck, a small skull with a beak-like mouth containing small, leaf-shaped teeth, and a bulky, pot-bellied body, which likely housed a large digestive system for processing plant matter. Therizinosaurus was bipedal, walking on its two hind legs. Unusually for a theropod, its feet had four functional, forward-pointing toes that bore weight, whereas most theropods had three main weight-bearing toes.

Therizinosaurus is the namesake and type genus of the family Therizinosauridae and the larger group Therizinosauria. These dinosaurs are classified as maniraptoran coelurosaurs, a diverse group of theropods that also includes birds, oviraptorosaurs, and dromaeosaurs (like Velociraptor). Despite their evolutionary origins within a primarily carnivorous group, therizinosaurs, including Therizinosaurus, evolved into herbivores. This dietary shift is one of their most fascinating characteristics. Close relatives of Therizinosaurus include Erlikosaurus and Segnosaurus, also found in Mongolia, and the earlier, feathered Beipiaosaurus from China, which provided evidence that therizinosaurs were likely covered in feathers.

Distinguishing Features

Therizinosaurus possessed several unique features that set it apart from other dinosaurs:

• Enormous claws on its forelimbs. Each of the three fingers on its hands bore a massive, sickle-shaped claw. The claw on the first digit was the longest, potentially reaching up to 1 meter (3.3 feet) in length along its outer curve. These are the longest known claws of any terrestrial animal.
• A long neck supporting a relatively small and lightly built skull. The front of its jaws likely had a rhamphotheca (a horny beak).
• Small, tightly packed, leaf-shaped (or peg-like) teeth, which were adapted for stripping leaves from branches rather than tearing flesh.
• A wide, robust pelvis and a distinct pot-bellied appearance, indicating a large gut capacity necessary for digesting tough plant material.
• Four forward-facing, weight-bearing toes on each foot, unlike the typical three-toed condition of most other theropods.
• Though not directly preserved with Therizinosaurus itself, evidence from related species like Beipiaosaurus strongly suggests that Therizinosaurus was covered in a coat of primitive feathers.

Paleoenvironment and Diet

Fossils of Therizinosaurus have been exclusively found in the Nemegt Formation of the Gobi Desert, Mongolia. During the Late Cretaceous, about 70 million years ago, this region was far from the arid desert it is today. Instead, it was a well-watered environment, characterized by meandering rivers, floodplains, and lush forests, offering a rich habitat for a diverse array of dinosaurs. Therizinosaurus shared this ecosystem with other notable dinosaurs, including the giant predatory tyrannosaurid Tarbosaurus, large duck-billed hadrosaurs like Saurolophus and Barsboldia, heavily armored ankylosaurs such as Tarchia, and various sauropods like Nemegtosaurus.

Despite its theropod ancestry and intimidating claws, Therizinosaurus is widely accepted by paleontologists to have been an herbivore. Its small teeth were clearly not suited for hunting large prey but were well-adapted for stripping leaves and other vegetation. The long neck would have allowed it to browse on foliage high off the ground, much like a modern giraffe. The primary function of its enormous claws is still debated; they were likely multi-purpose tools used for pulling down branches to reach leaves, for defense against predators such as Tarbosaurus, and possibly for intra-species display or competition. Its large, pot-bellied physique suggests a capacious digestive tract needed to ferment and break down tough plant fibers.

Significance and Ongoing Research

The discovery and study of Therizinosaurus and its relatives have profoundly impacted our understanding of dinosaur diversity, particularly within the Theropoda. It serves as a striking example of how evolutionary pressures can lead to dramatic shifts in diet and morphology, showing that not all theropods were apex predators. Some lineages, like the therizinosaurs, successfully adapted to an herbivorous lifestyle, filling ecological niches previously thought to be exclusively occupied by other dinosaur groups.

Ongoing research continues to explore various aspects of Therizinosaurus biology. Paleontologists are still working to refine estimates of its body mass, posture, and locomotion. The precise mechanics and primary functions of its extraordinary claws remain a subject of active investigation, with studies comparing them to the claws of other animals and using biomechanical models. Further fossil discoveries of Therizinosaurus or closely related species are eagerly anticipated, as they could provide more direct evidence of its feather covering, skin texture, and more details about its anatomy and behavior. The study of therizinosaurs also contributes to broader research on the evolution of herbivory in dinosaurs and the incredible diversification of maniraptoran theropods, the lineage that ultimately gave rise to modern birds.