Rachis morphology cannot accurately predict the mechanical performance of primary feathers in extant (and therefore fossil) feathered flyers

Abstract

It was previously suggested that the flight ability of feathered fossils could be hypothesized from the diameter of their feather rachises. Central to the idea is the unvalidated assumption that the strength of a primary flight feather (i.e. its material and structural properties) may be consistently calculated from the external diameter of the feather rachis, which is the only dimension that is likely to relate to structural properties available from fossils. Here, using three-point bending tests, the relationship between feather structural properties (maximum bending moment, Mmax and Young's modulus, Ebend) and external morphological parameters (primary feather rachis length, diameter and second moment of area at the calamus) in 180 primary feathers from four species of bird of differing flight style was investigated. Intraspecifically, both Ebend and Mmax were strongly correlated with morphology, decreasing and increasing, respectively, with all three morphological measures. Without accounting for species, however, external morphology was a poor predictor of rachis structural properties, meaning that precise determination of aerial performance in extinct, feathered species from external rachis dimensions alone is not possible. Even if it were possible to calculate the second moment of area of the rachis, our data suggest that feather strength could still not be reliably estimated.

Mid-Cretaceous amber inclusions reveal morphogenesis of extinct rachis-dominated feathers

“…42 from the same mid-Cretaceous deposit display consistent morphologies with those presented in this study, suggesting that this rachis construction was the dominate developmental pathway to produce RDFs in the mid-Cretaceous. Evaluating incongruences between inferred mechanical aspects of preserved RDFs from the entire Mesozoic in the light of the unexpected three-dimensional morphologies described here is outside of the scope of this study, but a greater understanding and consideration of the construction of RDFs in three dimensions at multiple scales will be critical in understanding their mechanical performance 41 . The excellent three-dimensional and micro-structural preservation in amber allows for unprecedented comparisons to modern feathers and can provide insight into extinct and extant feather development.…”

“…This hypothesis provides one alternative pathway (perhaps not the only one) of how the extinct RDFs could have developed. Clearly further studies are needed to test this hypothesis but the realization that certain extinct feathers are unlikely to have developed through identical developmental mechanisms that produce a modern feather cautions the use of modern developmental pathways and mechanical properties of feathers to infer aspects of the plumage of extinct, Mesozoic avians, a practice that has been common among researchers 39–41 . If the RDFs preserved as lithic fossils share the same construction and cross-sectional profile as RDFs in amber, as suggested from semi three-dimensional remains from China (Fig.…”

Mechanical and structural adaptations to migration in the flight feathers of a Palaearctic passerine

“…Our results additionally showed that observed variation in bending stiffness between robins was mostly explained by differences in the feather traits analyses (see Table 3; Table S1), although there is still some room for additional, less perceptible (De la Hera, Hedenström, et al, 2010;Lees et al, 2017), structural adjustments at microscopic level that would require additional research with the appropriate technology (e.g. tomography of the feather shaft; Weber et al, 2010).…”

Examining the accuracy of trackways for predicting gait selection and speed of locomotion

“…Other unknowns, not quantified in the current study, but likely to further cloud the inferences from tracks in isolation are the influence of sex differences on the kinematics of locomotion [16,[68][69][70] and ontogenetic influences [21] all of which cannot be quantified in extinct animals. Inferences into the biology of extinct forms commonly suffers from large errors [60,61,71] and rely on numerous assumptions when extrapolating from extant to extinct forms [65,72,73].…”