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

Abstract

Claws are involved in numerous vital functions including locomotion and prey capture, and as a result, animals are expected to evolve claw morphologies that enable efficient performance of these biological roles whilst minimising stress/strain during functional performance. This is supported by past studies finding the geometry of pedal claws to correlate with mode of life for extant birds, squamates, and mammals, and this relationship has frequently been cited to infer lifestyles of Mesozoic theropods including Archaeopteryx and Microraptor. However, claw sheaths are comprised of soft tissues that rarely fossilise and are prone to breakage and deformation, and so many fossil claws are comprised solely of the internal ungual bone, and even fossilised claw sheaths from exceptionally preserved specimens tend to be broken and/or deformed. Past inferences based on intact extant claws are thus compromised by errors related to reconstruction and/or comparison of nonhomologous structures (claw sheaths and ungual bones). As the ungual phalanx within the claw is more commonly preserved intact in the fossil record, the geometry of this bone may provide a more useful metric for palaeontological analysis. In this study, ungual bones of the third pedal digit belonging to 95 species of extant birds representing 24 orders and 5 species of extant squamates have been radiographed using the portable dental imaging device the Nomad Pro Radiography Unit. Curvatures of the ventral and dorsal surface of this bone were measured by approximating the surface to a circular arc and taking degree of the arc using a custom-made software DinoLino.exe. A predictive model created using linear discriminant analysis (LDA) found a significant relationship between these measures on the ungual bone and four modes of life; ground-dwelling, perching, predatory, and climbing; with total weighted accuracy equal to 0.79 when tested on extant birds. Ranges and medians of the measured claw angles were relatively low for ground-dwelling taxa, relatively high for climbing taxa, and relatively intermediate for perching and predatory taxa; these results suggest ungual bones follow similar trends as keratinous sheaths with regard to ecomorphology. However, there is much overlap between ranges found for claw angle and also between behavioural categories. The predictive model predicts perching ecologies for Archaeopteryx, Balaur, Xiaotingia, Zhenyuanlong, and Microraptor, a predatory ecology for Confuciusornis, Anchiornis, Changyuraptor, and Talos, a ground-dwelling ecology for Borogovia, Eosinopteryx, and Halszkraptor, and a climbing ecology for Sapeornis. Some predictions were unexpected, but predictions were generally consistent with fossil evidence. Unfortunately, many fossil claws measured here possess morphologies that are intermediate relative to those of extant taxa and as a result many predictions by the model are based on low posterior probabilities. The results suggest ungual bone geometry is a useful metric that could shed light on debates in palaeontology including the theropod-bird transition, and the evolution of avian flight. However, further work could be useful in mitigating the confounding influences of scaling and phylogeny, investigating whether overlapping morphospaces may be distinguished, and improving strength of classifications.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	William Sellers (Supervisor) & Andrew Chamberlain (Supervisor)