Therizinosaurus: Beast of the Week

This week we will be checking out an increasingly popular, unique dinosaur.  Enter Therizinosaurus cheloniformis!  Therizinosaurus was a large plant-eating dinosaur that lived in what is now Mongolia, during the Late Cretaceous, 70 million years ago.  From beak to tail it is estimated to have measured about thirty three feet long and would have had feathers when alive.  The name, Therizinosaurus, translates to "scythe lizard" in reference to its long, scythe-shaped claws.  The species name, cheloniformes, translates to "turtle form" because its remains were originally believed to have belonged to a large turtle when they were discovered in the 1950s. (specifically the claws)

A lazy Therizinosaurus falls asleep while feeding.  I based this scene loosely after a sloth I worked with who I observed doing something similar, dozing off with a piece of food still in his claws.

Therizinosaurus is sadly only known from fragmentary remains, including claws, and arms, and parts of the legs.  Most of what we know about it is based on more complete remains from related dinosaurs from the same family.  Therizinosaurus, along with its family, called therizinosauroidea, all were probably plant eaters, which is unusual for nonavian theropods.  Although the ornithomimosaurs were likely at least plan-eaters part of the time, possibly along with oviraptorosaurs, too.  If Therizinosaurus was like its closest relatives, it would have had a long, thin neck, and a relatively small skull, with a small beak at the tip and small teeth in the back, for shredding plants.

Therizinosaurus hand tipped with gigantic claws on display at the Sauriermuseum Aathal in Switzerland.

The arms and claws were probably Therizinosaurus' most impressive traits.  The arms themselves were relatively long, but would have had a limited range of motion, especially when it comes for reaching upwards.  Each hand ended in three fingers, each tipped with an enormous, scythe-shaped claw.  Each of these claws were about or just over three feet long!  They also were laterally compressed, and blade-like, and at first glance appear to have been really nasty weapons.  However, it is unlikely this was their primary function. (Although I'm sure if you ticked a Therizinosaurus off enough it wouldn't hesitate to use them to hurt you.)  It is more likely they aided Therizinosaurus in either feeding, or mating.  When it comes to feeding, these claws could have played a part in better manipulating branches and chutes towards the dinosaur's mouth to eat the leaves.  Despite the fact that Therizinosaurus could reach higher up with its neck than with its arms, they still could have been useful for bending plant limbs that started growing low and had edible foliage up high, towards, the dinosaur's mouth.  These claws also may have played a role in mate selection and display.  Who knows what kind of behavior Therizinosaurus may have exhibited when it wanted to impress a potential mate with those claws?  Birds today incorporate all sorts of body adaptations in mating displays and dances.  There is a strong chance many prehistoric dinosaurs were the same.  Since the fossil material from this dinosaur is also extremely limited right now, for all we know the extremely long claws may have been a display adaptation only present in mature males, for instance.  Only new discoveries made in the future could ever tell us that, however.

Quick sketch I did of a male Therizinosaurus using his claws in an attempt to woo a female. There is no fossil evidence that suggests there was a difference in claw size between the sexes.

Therizinosaurus would have had a relatively wide body, with a low center of gravity.  It's pubis bone, which usually is facing forward in most nonavian theropods, would have been angled backwards.  These are adaptations for a plant-eating lifestyle.  Since they likely evolved from meat-eating ancestors, therizinosauroids needed to change their internal anatomy, specifically their digestive tract, drastically in order to make the change to an herbivorous diet.  This requires a bigger gut, since plants are more work to digest, and therefore need more space to ferment inside.  For modern examples, look to large plant-eating animals today, like cows.  These animals have wide stomachs, with multiple chambers to slowly break down the plants that they eat.  Other herbivores, like horses, have enlarged cecums, the section between the small and large intestines, for the same purpose.  Think of Therizinosaurus' gut as a huge, living, fermentation chamber.

If Therizinosaurus did use its claws to help it forage, this is how it could have done it.  The arms couldn't reach above the head.

Therizinosaurus' feet were wide, and each had four toes touching the ground, for extra stability.  In most theropods, the first digit is greatly reduced and doesn't touch the ground, like a dewclaw.  This stability was important since Therizinosaurus was probably extremely heavy for a bipedal dinosaur. Other heavy-bodied dinosaurs could walk on all fours to better spread their weight.  In life Therizinosaurus was almost certainly feathered, since a beautifully preserved close relative, named Beipiaosaurus was discovered with the remains of a layer of shaggy feathers preserved still intact.  Since therizinosauroids obviously weren't flying, it is likely these feathers were more for display or temperature regulation.

That is all for this week!  As always feel free to comment below or on our facebook page!

References

Barsbold, R. (1976c). "New data on Therizinosaurus (Therizinosauridae, Theropoda) [in Russian]." In Devâtkin, E.V. and N.M. Ânovskaâ (eds.), Paleontologiâ i biostratigrafiâ Mongolii.Trudy, Sovmestnaâ Sovetsko−Mongol’skaâ paleontologičeskaâ kspediciâ, 3: 76–92.

Lautenschlager, S. "Morphological and Functional Diversity in Therizinosaur Claws and the Implications for Theropod Claw Evolution." Proceedings of the Royal Society B: Biological Sciences 281.1785 (2014): 20140497. Web.

Maleev, E.A. (1954). "New turtle−like reptile in Mongolia [in Russian]." Priroda, 1954(3): 106–108.

Perle, A., 1982, "On a new finding of the hindlimb of Therizinosaurus sp. from the Late Cretaceous of Mongolia", Problems on the Geology of Mongolia, 5: 94–98