Craniofacial development illuminates the evolution of nightbirds (Strisores).

Guillermo Navalón,Bhart-Anjan Bhullar,Jen A Bright,Jesús Marugán-Lobón,Sergio M Nebreda,Emily J Rayfield,Matteo Fabbri,Roger B J Benson

doi:10.1098/rspb.2021.0181

Abstract

Evolutionary variation in ontogeny played a central role in the origin of the avian skull. However, its influence in subsequent bird evolution is largely unexplored. We assess the links between ontogenetic and evolutionary variation of skull morphology in Strisores (nightbirds). Nightbirds span an exceptional range of ecologies, sizes, life-history traits and craniofacial morphologies constituting an ideal test for evo-devo hypotheses of avian craniofacial evolution. These morphologies include superficially ‘juvenile-like’ broad, flat skulls with short rostra and large orbits in swifts, nightjars and allied lineages, and the elongate, narrow rostra and globular skulls of hummingbirds. Here, we show that nightbird skulls undergo large ontogenetic shape changes that differ strongly from widespread avian patterns. While the superficially juvenile-like skull morphology of many adult nightbirds results from convergent evolution, rather than paedomorphosis, the divergent cranial morphology of hummingbirds originates from an evolutionary reversal to a more typical avian ontogenetic trajectory combined with accelerated ontogenetic shape change. Our findings underscore the evolutionary lability of cranial growth and development in birds, and the underappreciated role of this aspect of phenotypic variability in the macroevolutionary diversification of the amniote skull.

Highlights

Phenotypic macroevolution in vertebrates is generally studied through the lens of adult morphology
Skull evolution in hummingbirds (Trochillidae) is characterized by an abrupt and large early divergence towards positive values of PC1, characterized by a basal shift towards a longirostrine morphology accompanied by further changes in palatal and neurocranial morphology
The principal components analyses (PCA) plot of ontogenetic trajectories visually reveals all the earliest immature strisorean individuals share a common region of the morphospace, initially similar in shape among them and distinct from the non-strisorean taxa, from which they widely diverge through ontogeny, with hummingbird ontogenies progressing through shape space in a highly divergent direction from other strisoreans

Summary

Introduction

Phenotypic macroevolution in vertebrates is generally studied through the lens of adult morphology. The landmarks of the palate and occiput regions (landmarks 8, 9 and 10, electronic supplementary material, figure S1a) were only digitized in the better preserved and generally more ossified adult specimens, to explore the phylogenetic patterns of cranial shape evolution in greater detail (figure 2b).

Results

Conclusion