Therizinosaurus, a dinosaur from the Late Cretaceous period, is perplexing due to its unusual appearance. Unlike the typical image of a large, predatory dinosaur, this creature possessed features that seem to defy its classification. This has led to questions about why Therizinosaurus is categorized alongside carnivores like Tyrannosaurus rex. The answer lies not in superficial resemblance or diet, but in shared underlying skeletal anatomy that reveals its true evolutionary lineage.
Understanding Theropods
Theropods are a diverse group of bipedal dinosaurs. The name “theropod” translates to “beast-footed,” referring to their characteristic foot structure. While many theropods are known for their carnivorous diets, their classification is based on shared anatomical traits rather than what they ate.
A defining feature of theropods is their hollow, thin-walled bones, which made their skeletons lighter. They typically possessed a three-toed foot structure where the first and fifth digits were reduced. Their hands often featured three main fingers with large claws, while the fourth and fifth digits were reduced or lost.
Therizinosaurus’s Distinctive Appearance
Therizinosaurus lived approximately 70 million years ago, reaching lengths of about 30 to 33 feet and weighing between 3 to 5 tons. Its most striking features were its enormous, scythe-like claws, which could extend up to 3 feet long, making them the longest claws of any known land animal. These claws were initially thought to belong to a giant turtle.
Beyond its impressive claws, Therizinosaurus had a proportionally small head atop a long, flexible neck. Its teeth were small and leaf-shaped, clearly indicating a herbivorous diet, which contrasts sharply with the sharp, recurved teeth of many carnivorous theropods. The dinosaur also possessed a bulky, pot-bellied body shape, likely accommodating a large gut for processing plant matter.
Skeletal Evidence for Theropod Classification
Despite its unique appearance, Therizinosaurus exhibits numerous skeletal similarities that firmly place it within the theropod lineage. Its pelvis, for instance, shows a saurischian hip structure, although in derived forms like Nothronychus, the pubis is retroverted, pointing backward like in ornithischian dinosaurs and birds. This backward-pointing pubis, known as an opisthopubic condition, is a convergent evolution with ornithischians and birds, but the underlying bone structure remains theropodan.
Further evidence comes from shared characteristics in its limb bones and the fundamental structure of its feet. While most theropods have three functional, weight-bearing toes, Therizinosaurus had four functional, weight-bearing toes on its hindlimbs, differing from other theropod groups where the first toe was often reduced to a dewclaw. This unique foot morphology, along with other skeletal elements, links it to the Maniraptora clade, a diverse group of theropods that includes birds and other feathered dinosaurs, such as Velociraptor and oviraptorosaurs.
The Evolution of Herbivorous Theropods
Therizinosaurus is a clear example of evolutionary divergence, illustrating how some theropod lineages adapted to a herbivorous diet. This dietary shift was not an isolated event within the theropod group. Other examples of non-carnivorous theropods include ornithomimids, which had toothless, beaked jaws, and oviraptorosaurs.
The distinctive features of Therizinosaurus, such as its long claws and large gut, are specialized adaptations to its plant-based diet. The long claws were likely used for stripping vegetation from branches, similar to how a sloth might use its claws. The large, pot-bellied body allowed for a more extensive digestive system to process fibrous plant material.