Therizinosaurus: The Panda bear of the Cretaceous

Therizinosaurus by Ivan Iofrida

With long arms and sharp claws, Therizinosaurus has almost all the character traits of a predator, however, this dinosaur was an herbivore.

Therizinosaurus is a theropod, the dinosaur family that includes T. rex, Allosaurus, dromeosaurs, and even birds. But unlike most of its non-avian cousins, Therizinosaurus evolved to have a mostly plant-based diet.

A lot like panda bears, which have traded a predator lifestyle for bamboo leaves while retaining most of the characteristics that make them part of the bear family, Therizinosaurus kept a set of strong arms ending in very long and sharp claws.

This made this dinosaur one of the most bizarre and also one of the most capable of defending itself against a predator. Bumping into a Therizinosaurus in the middle of a dark forest would most likely be as terrifying as stumbling into a T. rex’s nest.

No longer a turtle

Therizinosaurus fossils were first found in 1948 by a team of soviet paleontologists exploring the Nemegt Formation of the Gobi Desert in Mongolia.  

This expedition found a great deal of dinosaur and turtle fossil remains, but the most noteworthy find was three sizeable incomplete claw bones in combination with other objects including a metacarpal fragment and multiple rib pieces in close proximity to a big theropod's skeleton.

The fossils were given the specimen number PIN 551-483 and later used as the basis for the new genus and type species Therizinosaurus cheloniformis, which became the holotype specimen, described by the Russian paleontologist Evgeny Maleev in 1954.

The name Therizinosaurus, which refers to this animal’s enormous claws, is derived from the Greek words therzo, which means scythe, reap, or cut, and sauros, which means lizard. The specific name cheloniformis, which refers to the remains, is derived from the Greek word chelóni, which means turtle, and the Latin formis, since the remains were thought to belong to a turtle-like reptile.

However, in 1970 another Russian paleontologist named Anatoly K. Rozhdestvensky, had a better look into these fossils and suggested that Therizinosaurus was in fact a theropod dinosaur and not a turtle, by comparing its claw bones with those of other meat-eating dinosaurs.

New fossil findings of this animal and of other species closely related to Therizinosaurus, like Segnosaurus and Nothronychus, further supported this claim, making this dinosaur the first of a new family called Therizinosauridae.

A not-so-gentle giant

Mounted forelimbs of specimen MPC-D 100/15 at Nagoya City Science Museum

Although new fossil remains have been found, Therizinosaurus is to this day one of the most incomplete members of this group. The most complete remains only include bones of its forelimbs, claws, some ribs, and its hindlimbs, and we have yet to find its skull.

Taking this data into account, and looking at other members of this group, paleontologists have managed to determine that Therizinosaurus would have reached 9 to 10 meters (30 to 33 ft) in length with an estimated height of 4 to 5 meters (13 to 16 ft) and a weight from 3 to 5 metric tons (3.3 to 5.5 short tons).

But one of the most fascinating things about this animal was its diet. In 1993, Canadian paleontologists Dale and Donald Russell compared Therizinosaurus and Chalicotherium, an extinct large ungulate mammal known for walking on its knuckles. This team identified some similarities in both animals’ body plans.

Both Therizinosaurus and Chalicotherium had large, well-developed, and relatively strong arms, a robust pelvic girdle suited for a sitting behavior, and robust and shortened hindlimbs.

Dale and Donald considered these adaptations to represent an example of convergent evolution, which happens when different organisms evolve similar traits without being related. An example of this is sharks, dolphins, and ichthyosaurs, evolving a very similar body plan in spite of being separated by hundreds of millions of years of evolution, and belonging to different clades.

Since animals with this type of body plan are known to represent herbivores, the authors suggested this lifestyle for Therizinosaurus. Dale and Donald Russell reconstructed the feeding behavior of Therizinosaurus as being able to sit while consuming foliage from large shrubs and trees, using its arms to pull the branches towards itself, and its long neck to feed on the leaves without having to stand.

When browsing in a bipedal stance, Therizinosaurus may have been able to reach even higher vegetation supported by its short and robust feet. Whereas Chalicotherium was more suited to hook branches, Therizinosaurus was better at pushing large clumps of foliage because of its long claws.

In 2018, paleontologist Anthony R. Fiorillo suggested that Therizinosaurus had a reduced bite force that may have been useful for cropping vegetation or foraging. This hypothesis was suggested based on an analysis of other therizinosaurids such as Erlikosaurus and Segnosaurus.

A Jurassic star is born

Claire Dearing (Bryce Dallas Howard) is being chased by a Therizinosaurus in Jurassic World Dominion

Therizinosaurus has recently risen to fame, being one of the new dinosaur stars of Jurassic World Dominion. But how accurate was this dinosaur’s depiction in the new movie?

Although the fossil remains of Therizinosaurus are relatively incomplete, its physical characteristics can be inferred through more complete and related therizinosaurids.

Paleontologists believe that like other members of its family, Therizinosaurus had a proportionally small skull bearing a horny beak atop its long neck, a bipedal gait, a large belly to process its plant-based diet, and it was most likely covered in feathers.

Jurassic World Dominion’s interpretation of this animal is inspired by most of these traits.

It’s a bit larger than the real animal, having 12 meters (40 ft) in length and 6 meters (20 ft) in height, which is not unusual for this franchise.

This animal was in fact covered in feathers and it’s not unlikely that it might have acted aggressively towards an unknown threat.

Where Jurassic World Dominion seems to have dropped the ball is with this dinosaur’s head sculpt. The beak is too birdlike and its snout is too short, but overall it is one of the most accurate dinosaur designs in the latest installment of this franchise.

Being a theropod, Therizinosaurus most likely would have had acute eyesight, a trait that this animal would’ve needed to detect predators and other threats and to find food and other members of its species.

In this movie this dinosaur is portrayed as being blind, however, this seems to be due to an incident that happened off-screen involving that particular animal, and not a characteristic common to all in-world Therizinosaurus.

Therizinosaurus was an odd and fascinating dinosaur that questioned our understanding of theropod evolution. Its discovery opened up a new wave of research for paleontologists and other scientists interested in the ecology of this long-lost world.