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, claw sheaths rarely fossilise and are prone to deformation; past inferences are thus compromised. As the ungual phalanx within the claw is relatively resistant to deformation and more commonly preserved in the fossil record, geometry of this bone would 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 Kappa equal to 0.69. Our model predicts arboreal lifestyles for certain key taxa 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. 2/30

Quantifying shape and ecology in avian pedal claws: The relationship between the bony core and keratinous sheath

“…A strong phylogenetic signal was uncovered for claw shape whether it was derived using traditional or geometric morphometrics, demonstrating the importance of using phylogenetic comparative methods when examining claw shape. Recently, Cobb and Sellers ( [Preprint]) have argued that phylogenetic comparative methods cannot be used for comparative studies of birds, citing recent conflicting bird trees. However, there are multiple avian trees representing all known extant bird taxa, which largely agree (Jetz et al, ; Prum et al, ).…”

“…Ethier et al () warned workers not to use a single bird species in studies of claw development or morphology as a result of variable rates of claw growth due to fluctuating energy demands such as migration and breeding. Cobb and Sellers ( [Preprint]) found that the left and right claws of the same fossil taxon could be categorized in different ecological categories. Although this may also be related to taphonomy (Hedrick & Dodson, ; Hedrick, Schachner, Rivera, Dodson, & Pierce, ), no study has thus far looked at within‐individual variation in claw shape in extant or fossil birds.…”

“…Glen and Bennett () presented the first estimation of the keratinous sheath arc from the bony core arc using radiographs, but they did not present the strength of the correlation, only the conversion factor. Recently Cobb and Sellers ( [Preprint]) used radiographs to determine whether the bony ungual or keratinous sheath was a better predictor of avian ecology, but they did not test for the correlation between the two data types. We found that the log‐transformed bony core arc explains 79.2% of the variation in the log‐transformed keratinous sheath arc when incorporating phylogeny and body size, suggesting a relatively tight correlation (Figure a).…”

The diet of early birds based on modern and fossil evidence and a new framework for its reconstruction

“…It is, therefore, unhelpful to divide these studies by their subject, and so instead they are divided by their focus on angular or linear measurements. In addition to studies concerned directly with dietary cues (Csermely & Rossi, 2006; Einoder & Richardson, 2007; Fowler et al ., 2009, 2011; Csermely et al ., 2012; Mosto & Tambussi, 2014; M. Wang et al ., 2014 b ) and grasping ability (Kambic, 2008), we also include studies concerned purely with locomotion (Peters & Görgner, 1992; Feduccia, 1993; Clark et al ., 1998; Zhou, 1999; Hopson, 2001; Pike & Maitland, 2004; Glen & Bennett, 2007; Morschhauser et al ., 2009; Dececchi & Larsson, 2011; Tinius & Russell, 2017; Cobb & Sellers, 2020). Locomotion can provide insight into what resources would be available to individuals to feed on and how free the hind limbs were to manipulate food while feeding.…”

“…Goslow, 1972; Csermely & Gaibani, 1998; Ward, Weigl & Conroy, 2002; Sustaita et al ., 2013). Subsequent studies measuring curvature either measured the dorsal arc exclusively (Glen & Bennett, 2007; Csermely et al ., 2012; Mosto & Tambussi, 2014) or both the dorsal and ventral arcs (Fowler et al ., 2009, 2011; Birn‐Jeffery et al ., 2012; Tinius & Russell, 2017; Cobb & Sellers, 2020) [although note that Fowler et al . (2009, 2011) and Tinius & Russell (2017) use an alternative ventral arc proximal landmark (Fig.…”