The mysterious “twin dinosaurs”

Author: Julián Monge Nájera, Ecologist and Photographer 

70 million years ago, two similar-looking dinosaurs fed on the lush vegetation of southern Mongolia; anyone would have thought they were closely related; but in fact, they were not, they were the answer to a riddle: how can you be bulky enough to digest large amounts of vegetation, and, at the same time, escape from the deadly tyrannosaurid Tarbosaurus.

This is southern Mongolia now: 

But 70 million years ago, it looked more like this¹:

The Okavango Delta, in Africa, gives us an idea of Mongolia at the time, but the canopy-like forests were rich in araucarias rather than on the kind of trees that you find in Africa today. If you could visit Mongolia in the Cretaceous, you would also see these two similar-looking dinosaurs, Deinocheirus and Therizinosaurus:

Both were as heavy as elephants, but taller (over 5 m tall and around 10 m long), and fed mostly on vegetation; they were not unhappy about swallowing the myriad little animals that were attached to the foliage: this complemented their diet with protein and other valuable nutrients; in other words, both dinosaurs were herbivores with a touch of omnivory. 

When I first saw reconstructions of these two animals, I thought that they were closely related, just like two species of the same genus that coexist in place and time; at most, I imagined, they reduced competition by a phenomenon called niche-partitioning, with Therizinosaurus feeding on drier land and Deinocheirus closer to water (fish remains have been found associated with its fossils).

I was surprised, then, to learn that they are unrelated, doppelgangers rather than twins. Deinocheirusbelongs to the “ostrich dinosaurs”, herbivorous (or at most, omnivorous) dinosaurs, while Therizinosaurusevolved from carnivorous dinosaurs that secondarily became herbivores (or slightly omnivorous). We know that both fed mostly on vegetation because of their anatomy, adapted to the large digestive systems needed for processing plant tissue; this was one reason why they ended up looking so similar, with pot bellies and small heads. They probably were in an evolutionary race against predators like Tarbosaurus, and this led to them growing larger than elephants (their similar weight is explained by the lighter bones of dinosaurs).

Other convergent adaptations were their gigantic claws: 

We do not know for sure if these claws were hidden among feathers, as in the above reconstructions, based on authors who think that the hands were used only for sexual or warning display. I personally do not find this convincing, firstly, because these animals were too large to be covered with think plumage —they more probably were like ostriches and elephants, which have much exposed skin to prevent over-heating; secondly, because the anatomy of their arms show that they were spectacularly strong, and this means that they were used for something that required physical strength, maybe, as suggested by some paleontologists, to break open termite mounds, fight off predators and competitors, manipulate branches, extract roots from the ground, or several of these functions. Paleontologist Lev Alexandrovich Nesov went as far as imagining that Therizinosaurus hung from trees like sloths, but the adult dinosaurs were too heavy for this.

Let us take a closer look at each.

Deinocheirus² reached 11 m in length and its bones have been found with bite marks probably made by Tarbosaurus: 

It is related to Garudimimus, and this suggests that, just like cats, Deinocheirus became active for several short periods both day and night. 

On the other hand, Therizinosaurus³, just like pandas, were herbivores that carried the mark of their predatory past: they had a great sense of smell, and were agile, smart and social, possibly traveling in herds, which provided some protection against predators. They were related to Nanshiungosaurus, Segnosaurus, and Erlikosaurus, from Mongolia and China; and to Nothronychus from North America, all of which looked much like them. 

Therizinosaurus were so heavy that, atypically for their group, they had feet that looked like the feet of sauropods. We are lucky enough to have nest fossils: groups of mothers excavated subterranean nests where they laid compact circles of 8 spherical, rough eggs each. When the young dinosaurs hatched, there were no mothers around, but they were precocial and could look after themselves as soon as they emerged from the ground:

Still, these dinosaurs are mysterious, we only have a few fossils and the information that we ignore far surpasses what we have been able to extract from the impressions in stone of these long-gone animals that, 70 million years ago, breathed, thought and lived lives that we can hardly imagine.

*Edited by Katherine Bonilla y Carolina Seas.

Originally published  in Blog Biología Tropical: 24 September 2020

REFERENCES

¹ Chinzorig, T., et al. (2018). Ornithomimosaurs from the Nemegt Formation of Mongolia: manus morphological variation and diversity. Palaeogeography, Palaeoclimatology, Palaeoecology, 494, 91-100. DOI: 10.1016/j.palaeo.2017.10.031

² Lee, Y. N., et al. (2014). Resolving the long-standing enigmas of a giant ornithomimosaur Deinocheirus mirificus. Nature, 515(7526), 257-260. DOI: 10.1038/nature13874

³ Lautenschlager, S. (2014). Morphological and functional diversity in therizinosaur claws and the implications for theropod claw evolution. Proceedings of the Royal Society B: Biological Sciences, 281(1785), 20140497. DOI: 10.1098/rspb.2014.0497