THERIZINOSAURUS: CRETACEOUS SCYTHE LIZARD

The Scythe Lizard Therizinosaurus

The slow-moving but massive fellow you see here is Therizinosaurus. He belonged to a genus of sizable therizinosaurid that lived during the Late Cretaceous, 70 million years ago. 

These big beasties reached up to 10 metres in length and likely weighed over 3,000 kg. They lumbered along with their unusually long arms pulling down and munching on vegetation in what is now the Nemegt Formation in the Nemegt Valley of Asia. 

While there are several species of Therizinosauridae, including several from North America, the massive Asian therizinosaurids are known from a single type species Therizinosaurus cheloniformis or scythe lizard found in the Gobi Desert.

In 1918, a very fortuitous Palaeontological Field Expedition to the Mongolian Gobi Desert by the USSR Academy of Sciences found the remains of a giant, turtle-like reptile near the remains of a large carnivorous dinosaur. 

Later, In 1948, another expedition was launched to retrace the work of the USSR expedition. More of the fossil bones were collected and together with those originally collected in 1918, were written up as the type specimen of Therizinosaurus described by Soviet palaeontologist Evgeny Aleksandrovich Maleev in 1954.  You may recall that Maleeve was the fellow who described the ankylosaur Talarurus and the theropod Tarbosaurus.

The genus is best known for its gigantic manual unguals or claw bones, from which it gets its name. 

Therizinosaurus was a colossal therizinosaur that could grow up to 9–10 m (30–33 ft) long and weigh possibly over 3 t (3,000 kg). Like other therizinosaurs, it would have been a slow-moving, long-necked high browser equipped with a rhamphotheca (horny beak) and a wide torso for food processing. 

The forelimbs were particularly robust and had three fingers that bore unguals which, unlike other relatives, were very stiffened, elongated, and only had significant curvatures at the tips.

Therizinosaurus had the longest known manual unguals of any land animal, reaching above 50 cm (500 mm) in length. Its hindlimbs ended in four functionally weight-bearing toes differing from other theropod groups in which the first toe was reduced to a dewclaw and also resembling the unrelated sauropodomorphs.

It was one of the last and the largest representative of its unique group, the Therizinosauria (formerly known as Segnosauria; the segnosaurs). During and after its original description in 1954, Therizinosaurus had rather complex relationships due to the lack of complete specimens and relatives at the time. 

Maleev thought the remains of Therizinosaurus to belong to a large turtle-like reptile, and also named a separate family for the genus: Therizinosauridae. Later on, with the discovery of more complete relatives, Therizinosaurus and kin were thought to represent some kind of Late Cretaceous sauropodomorphs or transitional ornithischians, even though at some point it was suggested that it may have been a theropod. 

After years of taxonomic debate, nevertheless, they are now placed in one of the major dinosaur clades, Theropoda, specifically as maniraptorans. Therizinosaurus is widely recovered within Therizinosauridae by most analyses and with more recent specimens found in Utah and New Mexico.

The unusual arms and body anatomy (extrapolated after relatives) of Therizinosaurus have been cited as an example of convergent evolution with chalicotheriines and other primarily herbivorous mammals, suggesting similar feeding habits. 

Their elongated hand claws were more useful when pulling vegetation within reach rather than being used for active attack or defence because of their fragility, however, they may have had some role for intimidation. Its arms also were particularly resistant to stress, which suggests a robust use of these limbs. Therizinosaurus was a very tall animal, likely having reduced competition over the foliage in its habitat and outmatching predators like Tarbosaurus.