Inferring lifestyle for Aves and Theropoda: A model based on curvatures of extant avian ungual bones

Abstract

Claws are involved in a number of behaviours including locomotion and prey capture, and as a result animals evolve claw morphologies that enable these functions. Past authors have found geometry of the keratinous sheath of the claw to correlate with mode of life for extant birds and squamates; this relationship has frequently been cited to infer lifestyles for Mesozoic theropods including Archaeopteryx. However, many fossil claws lack keratinous sheaths and thus cannot be analysed using current methods. As the ungual phalanx within the claw is more commonly preserved in the fossil record, geometry of this bone may provide a more useful metric for paleontological analysis. In this study, ungual bones of 108 birds and 5 squamates were imaged using X-ray techniques and a relationship was found between curvatures of the ungual bone within the claw of pedal digit III and four modes of life; ground-dwelling, perching, predatory, and scansorial; using linear discriminant analysis with weighted accuracy equal to 0.79. Our model predicts arboreal lifestyles for Archaeopteryx and Microraptor and a predatory ecology for Confuciusornis. These findings demonstrate the utility of our model in answering questions of palaeoecology, the theropod-bird transition, and the evolution of avian flight. Though the metric exhibits a strong correlation with lifestyle, morphospaces for PD-III curvatures overlap and so this metric should be considered alongside additional evidence.

Functional morphology of vertebrate claws investigated using functionally based categories and multiple morphological metrics

“…In predatory birds the cross‐sectional shape of the claw is roughly as wide as it is tall making it equally strong in all directions, a helpful feature for dealing with struggling prey items. Previous studies have found considerable overlap in the morphological characteristics of bird claws assigned to predatory and perching categories (Birn‐Jeffery et al, 2012; Cobb & Sellers, 2020; Pike & Maitland, 2004). It seems that this overlap is due to the shared biomechanical function (i.e., grasping) between these two groups, again highlighting the need to categorize specimens by function instead of ecology or life habit.…”

“…Although claws are composite structures where both the sheath and ungual likely contribute to overall function, measurements of ungual features (except for the flexor tubercle) do not factor into our analysis. However, recent studies suggest that a significant correlation exists between sheath and ungual morphology (Hedrick et al, 2019) and that either structure by itself serves well for inferring function (Cobb & Sellers, 2020). It is acknowledged that the measurements of these metrics are affected by physical wearing of the claw, mostly at the apex.…”

“…Quantitative approaches to claw functional morphology have been attempted by numerous researchers over the years, the vast majority of which are focused on claw curvature in birds. Some of these studies have been successful at distinguishing between different ecological categories (Cobb & Sellers, 2020; Glen & Bennett, 2007; Peters & Görgner, 1992; Richardson, 1942) while others have only been partially successful (Birn‐Jeffery et al, 2012; Pike & Maitland, 2004). In one study, the use of geometric morphometric methods showed promise for separating aquatic/terrestrial, arboreal, and predatory birds (McIntosh, 2017) whereas another study found that these methods were unable to significantly separate birds into predatory, ground‐dwelling, and generalist groups (Hedrick et al, 2019).…”

“…Despite the attention claw functional morphology has received, a survey of the results from these many studies demonstrates that the biomechanical and functional significance of many claw morphologies are still not explicitly clear (Peters & Görgner, 1992; Pike & Maitland, 2004; Sustaita et al, 2013; Tulli et al, 2009). It has been advocated by some that future research should focus on better morphological quantification and more accurate categorization of behavior and function in modern animals (Birn‐Jeffery et al, 2012; Cobb & Sellers, 2020; Hedrick et al, 2019; Higham, 2015; Pike & Maitland, 2004; Tinius & Russell, 2017). Thus far, no morphological study has utilized explicit categories based solely on function; animals have generally been assigned to categories based on either habitat affinity and/or food preference while the actual function of the claw is rarely considered.…”

A new darwinopteran pterosaur reveals arborealism and an opposed thumb

“…We are aware that several other analyses have assessed arboreality in extinct animals based on pedal claw morphology. 51,52 However, these datasets are mostly composed of avian measurements. As bipeds and quadrupeds are known to have very different biomechanical constraints, 53 studying pterosaurs using these datasets may lead to flawed interpretations.…”