The earliest known true therizinosaur is Falcarius, from North America. This 3-foot long animal wasn’t very different from other medium-sized theropods, but already had some features that set it apart. It had a relatively small head at the end of a long neck, numerous small and leaf-shaped teeth ideal for feeding on vegetation, a hip structure that provided a slightly more vertical instead of horizontal posture, and robust forelimbs with long claws.
These traits mean that Falcarius likely fed on leaves and fruit keeping its head high off the ground to reach short trees, though recent analysis of its skull show that it still retained high bite forces characteristic of carnivores. So it could have supplemented its diet with eggs, small animals and carrion. In general it was similar to the ostrich-mimic dinosaurs except a bit slower, hinting at a close relationship.
Another early therizinosaur was Beipiaosaurus, from China. This dinosaur was a groundbreaking discovery showing two distinct feather types. Both the downy fibers similar to a baby bird and long, stiff filmaments can be seen in its fossil. Reaching more than 7 feet in length, it was the biggest dinosaur conclusively known to have been feathered before the discovery of the T. rex relative Yutyrannus, also from China.
The shaggy Beipiaosaurus was stockier than Falcarius, with a modified pelvis structure allowing the gut to grow larger for digesting vegetation more efficiently. It had a beak in the front of its jaws and peg-like teeth in the back, probably protected by fleshy cheeks. Its forelimbs and claws were also bigger, and could have been used both for defense and for increasing its reach while feeding on leaves.
Another North American therizinosaur was Nothronychus, the “slothful claw”. With its quite upright posture, Nothronychus could stand at up to 12 feet tall and weigh about a ton, expanding on the tendency to grow bigger and stockier. This is thought to have been a consequence of an enlarged digestive system, since plant matter is much more difficult to break down into nutrients.
Nothronychus by Elia Smaniotto
The 12-inch-long claws allowed Nothronychus to defend itself against predators without needing to run, resulting in a slow and heavy animal akin to the Pleistocene ground sloths from which it got its name. It even had a large fourth toe on each foot to help in supporting its bulk, while almost every other theropod group exhibited very small fourth toes that didn’t reach the ground.
By the end of the Cretaceous Period, therizinosaurs were at their heyday. The Mongolian Erlikosaurus, named after a demon king of the country’s mythology, was a 20-foot long animal and a formidable opponent to any predator. This species was very important to our understanding of therizinosaurs, because its skull was the first known from the group. It served as a model for reconstructing these strange dinosaurs in a time when they were poorly understood. And it also showed us that they had well-developed senses of smell, hearing and balance.
Erlikosaurus by Elia Smaniotto
Finally, 70 million years ago lived Therizinosaurus itself, the main representative of the entire family and its namesake. Therizinosaurus was huge, possibly reaching more than 30 feet in length and up to five tons in weight. Its claws measured just under 3 feet – the longest known from any animal – and were, upon their discovery in 1948, thought to belong to a gigantic sea turtle.
The enigmatic Therizinosaurus remained a paleontological mystery for many years, before the discovery of other, more well-preserved therizinosaurs from other parts of the world. The Freddie Krueger-like claws and long forelimbs made scientists think it was a giant carnivorous theropod unlike any other. It wasn’t until the 1970s that its true identity as a pot-bellied, small-headed herbivore was pieced together.
Therizinosaurs went extinct at the end of the Mesozoic Era along with most dinosaurs except for the small, resilient birds. Their enormous sizes needed a lot of food to function, so they weren’t capable of enduring the hardships brought by the cataclysm. But their fossils have told us a story of how diverse dinosaurs became before their untimely end, and the truly incredible path that a lineage of small, feathered theropods took.