The dinosaur clade Maniraptora includes the ancestors of birds, and most maniraptoran dinosaurs used their hands for grasping and in flight, but early maniraptorans (alvarezsaurs and therizinosaurs) had extraordinary claws of mysterious function. University of Bristol paleontologist Zichuan Qin and colleagues developed a comprehensive methodological framework to investigate what the functions of these most bizarre bony claws are and how they formed.
Life reconstruction of Paralitherizinosaurus japonicus. Image credit: Masato Hattori.
Early maniraptoran dinosaurs, like alvarezsaurs and therizinosaurs, occupied enigmatic ecological niches and had bizarre morphological characters.
Alvarezsaurs underwent miniaturization to become the smallest non-avian dinosaurs ever, but with short, strong arms and hands with a stout, rock-pick-like, single functional finger.
Some therizinosaurs evolved large body sizes, with elongate fingers with slender and sickle-like unguals, sometimes over 1 m long.
“Alvarezsaurs and therizinosaurs are definitely the strangest cousins among dinosaurs,” said University of Bristol’s Professor Michael Benton.
“Alvarezsaurs were the tiniest dinosaurs ever, the size of chickens, with stubby forelimbs and robust single claws, but their closest relative, the therizinosaurs, evolved in the exact opposite path.”
“Therizinosaurus is famous for its sickle-like claws, each as long as a samurai sword: Edward Scissor-hands on speed,” added Dr. Chun-Chi Liao, an expert on therizinosaurs from the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences.
“We all saw Therizinosaurus in ‘Jurassic World’ hitting deer and killing the giant predator Giganotosaurus.”
“However, this is unlikely. These long, narrow claws were too weak for combat.”
“Our engineering simulation shows that these claws could not withstand much stress.”
Phylogenetic comparison of von Mises stress plots of dinosaur manual unguals under three functional scenarios. Image credit: Qin et al., doi: 10.1038/s42003-023-04552-4.
For their study, the authors developed a new, computational approach in biomechanics to identify functions based on detailed comparison with living animals.
First, the claws were modeled in three dimensions from CT scans, then modeled for stress and strain using engineering methods, and finally matched to functions of pulling, piercing and digging by comparison with modern animals whose claw functions are known.
“Not all therizinosaur hand claws were so useless in combat, but most other related species could use their claws as powerful hooking tools when feeding on leaves from the trees,” Dr. Liao said.
“So, we conclude that the largest claws of any animal ever were actually useless in mechanical function, and so must have evolved under sexual selection to be used in display.”
“The adult Therizinosaurus I guess could wave the claws at a competitor and effectively say, ‘look at me, back off’ or wave them around in some way like a peacock can use its tail in display to attract females for mating.”
“Our previous work has shown that alvarezsaurs evolved to become the tiniest dinosaurs by the end of the Cretaceous, and these dinosaurian midgets were using their punchy little claws for digging into ant hills and termite mounds. They were ant-eaters,” Qin said.
“Our study shows that the early alvarezsaurs, like Haplocheirus from the Jurassic, had multifunctional hands, but they were not good at digging.”
“Their much smaller descendants had the efficient digging hands so they could feast on the Late Cretaceous termites.”
“Science and technology cannot bring dinosaurs back to life, but advanced computing and engineering techniques can show us how extinct animals lived,” said University of Bristol’s Professor Emily Rayfield.
“Especially for extinct animals like alvarezsaurs and therizinosaurs, they are so bizarre that we even can’t find any living animals like them.”
“Luckily, advanced technology can help us to simulate, on a computer, the functioning of extinct animals using fundamental engineering and biomechanical principles.”
“This study shows very well how selection for function can lead to the emergence of specific, sometime very bizarre, forms.”
The findings were published in the journal Communications Biology.
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Z. Qin et al. 2023. Functional space analyses reveal the function and evolution of the most bizarre theropod manual unguals. Commun Biol 6, 181; doi: 10.1038/s42003-023-04552-4
