Functional space analyses reveal the function and evolution of the
most bizarre theropod manual unguals

Abstract

Maniraptoran dinosaurs include the ancestors of birds, and most used
their hands for grasping and in flight, but early-branching
maniraptorans had extraordinary claws of mysterious function.
Alvarezsauroids had short, strong arms and hands with a stout,
rock-pick-like, single functional finger. Therizinosaurians had
elongate fingers with slender and sickle-like unguals, sometimes over
one metre long. Here we develop a comprehensive methodological
framework to investigate what the functions of these most bizarre bony
claws are and how they formed. Our analysis includes finite element
analysis and a newly established functional-space analysis and also
involves shape and size effects in an assessment of function and
evolution. We find a distinct functional divergence among manual
unguals of early-branching maniraptorans, and we identify a complex
relationship between their structural strength, morphological
specialisations, and size changes. Our analysis reveals that efficient
digging capabilities only emerged in late-branching alvarezsauroid
forelimbs, rejecting the hypothesis of functional vestigial structures
like T. rex. Our results also support the statement that most
therizinosaurians were herbivores. However, the bizarre, huge
Therizinosaurus had sickle-like unguals of such length that no
mechanical function has been identified; we suggest they were
decorative and lengthened by peramorphic growth linked to increased
body size.

Open access:
https://www.nature.com/articles/s42003-023-04552-4