Insight into the Evolution of the Avian Vocal Organ, or Syrinx, from Enhanced‐Contrast X‐ray Computed Tomography and Fossil Data

Abstract

From complex songs to simple honks, squawks, or caws, birds produce sounds using a unique vocal organ, the syrinx. Located close to the heart at the tracheobronchial junction, vocal folds or membranes attached to modified mineralized rings vibrate to produce sound. Not thought to commonly enter the fossil record, the few reported fossilized parts of the syrinx are geologically young (i.e., Pleistocene and Holocene). The only known older syrinx is an Eocene specimen, which was not described or figured. Data on the relationship between soft tissue structures and syringeal three‐dimensional geometry has also been limited. Our team of physiologists and paleontologists recently described the first remains of a fossil syrinx from the Mesozoic, which are preserved in three dimensions in a specimen from the Late Cretaceous of Antarctica. With both cranial and postcranial remains, the new specimen is the most complete to be recovered from a part of the radiation of living birds, Aves. Enhanced‐contrast x‐ray computed tomography (CT) of syrinx structure in twelve extant non‐passerine birds, as well as CT imaging of the Cretaceous and the Eocene syrinxes, informs both the reconstruction of ancestral states in birds and properties of the vocal organ in the extinct species. Fused rings in the Cretaceous fossil form a well‐mineralized pessulus, a derived neognath bird feature, proposed to anchor enlarged vocal folds, or labia. Left‐right bronchial asymmetry, only known in extant birds with two sound sources is similarly present.The new data show the fossilization potential of the avian vocal organ and beg the question why these remains have not been found in other dinosaurs. The lack of other Mesozoic tracheal and bronchial ring remains, and the poorly‐mineralized condition in archosaurian taxa without a syrinx, may indicate a complex syrinx was a late arising feature in the evolution of birds, well after the origin of flight and respiratory innovations. While a role for complexity in social structure and mating system has been extensively investigated with reference to increases in brain size in other vertebrates including humans, in the early evolution of birds these hypotheses have not been explored. We propose that first more complex visual signals and then vocal communication and song may have played important roles in the evolution of the larger extant avian brain.Support or Funding InformationGordon and Betty Moore Foundation Grant GBMF4498, US National Science Foundation (OPP ANT‐1141820, OPP 0927341 and EAR 1355292 to J.C.); Argentine Agencia Nacional de Promoción Científica y Técnica (PICT 2010‐066; F.E.N).