Abstract
Just as skeletal characteristics provide clues regarding behavior of extinct vertebrates, phylogenetically-informed evaluation of endocranial morphology facilitates comparisons among extinct taxa and extant taxa with known behavioral characteristics. Previous research has established that endocranial morphology varies across Aves; however, variation of those systems among closely related species remains largely unexplored. The Charadriiformes (shorebirds and allies) are an ecologically diverse clade with a comparatively rich fossil record, and therefore, are well suited for investigating interspecies variation, and potential links between endocranial morphology, phylogeny, ecology and other life history attributes. Endocranial endocasts were rendered from high resolution X-ray computed tomography data for 17 charadriiforms (15 extant and two flightless extinct species). Evaluation of endocranial character state changes on a phylogeny for Charadriiformes resulted in identification of characters that vary in taxa with distinct feeding and locomotor ecologies. In comparison with all other charadriiforms, stem and crown clade wing-propelled diving Pan-Alcidae displayed compressed semicircular canals, and indistinct occipital sinuses and cerebellar fissures. Flightless wing-propelled divers have relatively smaller brains for their body mass and smaller optic lobes than volant pan-alcids. Observed differences between volant and flightless wing-propelled sister taxa are striking given that flightless pan-alcids continue to rely on the flight stroke for underwater propulsion. Additionally, the brain of the Black Skimmer Rynchops niger, a taxon with a unique feeding ecology that involves continuous forward aerial motion and touch-based prey detection used both at day and night, is discovered to be unlike that of any other sampled charadriiform in having an extremely large wulst as well as a small optic lobe and distinct occipital sinus. Notably, the differences between the Black Skimmer and other charadriiforms are more pronounced than between wing-propelled divers and other charadriiforms. Finally, aspects of endosseous labyrinth morphology are remarkably similar between divers and non-divers, and may deserve further evaluation.
Highlights
Variation in brain and inner ear morphology across Aves has been documented [1,2,3,4], variation within major avian sub-clades has received comparatively little attention
Work we address the following four broad questions: (1) How does the within-clade endocranial morphological variation among charadriiforms compare to the morphological differences documented for other avian clades; and, is charadriiform endocranial morphology relatively conserved despite ecological and ethological differences? are there endocranial apomorphies of Charadriiformes relative to outgroup taxa that might be used to identify fossil charadriiform taxa? (2) Are there endocranial apomorphies that characterize clades within Charadriiformes? For example, are there differences in the endocranial morphology of wing-propelled diving Pan-Alcidae relative to volant charadriiform taxa given that extensive osteological modifications in panalcids have been previously documented [28,31,34]? (3) Is the relative volume of the charadriiform endocranial cavity correlated with variables such as endosseous labyrinth volume, body mass, phylogeny, flightlessness, developmental strategies and dive depth
Many of the differences in charadriiform endocranial morphology appear to correlate with phylogeny and derived ecologies
Summary
Variation in brain and inner ear morphology across Aves has been documented [1,2,3,4], variation within major avian sub-clades has received comparatively little attention. Rather than assessing broad-scale morphological trends among taxa, studies with dense taxonomic sampling have often focused on particular endocranial features such as the olfactory lobe, the wulst (i.e., sagittal eminence), or relative size of different brain regions [10,11,12,13,14]. Recent studies of moa (Dinornithiformes), Haast’s Eagle Harpagornis moorei (Accipitridae) and penguins (Pan-Sphenisciformes) [15,16,17] provide some of the first examples of endocranial studies that sampled multiple closely related species and examined in detail, the morphological differences among the endocranial anatomy of those taxa. The behavior, cognitive ability, and potential adaptability of extant birds has been linked with the relative size and gross morphology of different regions of the brain (e.g., [20,21,22,23]).
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