Therizinosaurs --"reaping lizards"--were some of the strangest dinosaurs ever to roam the earth. Technically part of the theropod family--the bipedal, carnivorous dinosaurs also represented by raptors, tyrannosaurs and "dino-birds"--therizinosaurs had an unusually goofy appearance, sporting feathers, pot bellies, gangly limbs, and unusually long, scythe-like claws on their long front hands. Even more bizarrely, there's ample evidence that these dinosaurs pursued a herbivorous (or at least omnivorous) diet, a sharp contrast to their strictly meat-eating theropod cousins. (See a gallery of therizinosaur pictures.)
Adding to their mystery, only a few species of therizinosaur have been identified, most of them hailing from eastern and central Asia (Nothronychus was the first example to be discovered on the North American continent). The most famous genus--and the one that gave this family of dinosaurs its name--is Therizinosaurus, which was discovered in Mongolia a few years after World War II. In the absence of other bones, which were only discovered later, the joint Soviet/Mongolian excavation team that unearthed the partial fossil of this dinosaur barely knew what to make of its three-foot-long claws, wondering if they'd stumbled on an ancient killer turtle! (Some earlier texts reference therizinosaurs as "segnosaurs," after the equally mysterious genus Segnosaurus, but this is no longer the case.)
Part of what makes therizinosaurs so baffling to scientists is that they can't comfortably be assigned to any existing dinosaur family, although theropods are certainly the closest fit. Based on some anatomical similarities, it was once thought that these dinosaurs were closely related to prosauropods, the sometimes bipedal, sometimes quadrupedal herbivores that preceded the sauropods of the late Jurassic period. That all changed with the discovery of the middle Cretaceous Alxasaurus, a primitive therizinosaur with some distinctly theropod-like characteristics, which helped put the evolutionary relationships of the entire breed in sharper focus. The consensus now is that therizinosaurs evolved in their unusual direction from a primitive branch of the theropod family.
From a biologist's perspective, the oddest thing about therizinosaurs wasn't their appearance, but their diet. There's a convincing case to be made that these dinosaurs a) used their long front claws to slice and dice vegetable matter (since they would have been too ungainly to slash fellow dinosaurs), and b) harbored an extensive network of intestines in their prominent pot bellies, an adaptation that would only have been needed in order to digest tough vegetable matter. The inescapable conclusion is that therizinosaurs (distant relatives of the prototypically carnivorous Tyrannosaurus Rex) were largely herbivorous, in much the same way that prosauropods (distant relatives of the protopically plant-eating Brachiosaurus) supplemented their diets with meat.
The following is a list of the major therizinosaur genera; just click on the links for more information.
Alxasaurus An early relative of the bizarre Therizinosaurus.
Beipiaosaurus The only known feathered therizinosaur.
Enigmosaurus This "puzzle lizard" was closely related to Therizinosaurus.
Erlikosaurus This late therizinosaur roamed the Mongolian forests.
Falcarius A bizarre, feathered theropod from North America.
Nanshiungosaurus Yet another bizarre therizinosaur from Asia.
Neimongosaurus A rare therizinosaur from Inner Mongolia.
Nothronychus The first therizinosaur to be found outside Asia.
Segnosaurus This was once considered the prototypical therizinosaur.
Suzhousaurus A large therizinosaur from the early Cretaceous.
Therizinosaurus The genus that gave therizinosaurs their name.