A discovery of the sound-producing vocal organ known as the syrinx in a bird fossil from the end of the 'age of dinosaurs' highlights the anatomical basis for myriad aspects of avian social and behavioural evolution. See Letter p.502
The origin of modern birds and their tremendous diversification continue to inspire many studies aimed at elucidating the functional, ecological and evolutionary context of the group's success. The past three decades of palaeontological research have deepened our understanding of birds as close relatives of the predatory theropod dinosaurs, with a wealth of anatomical evidence supporting this idea. Remarkable non-avian dinosaur fossils exist with several hallmark 'avian' features, including a diversity of feather types1 and a complex lung air-sac system2, both of which have a broad distribution among non-avian theropod dinosaurs such as Velociraptor. A study on page 502 by Clarke et al.3 reports the identification of an avian-specific feature — the sound-producing syrinx — in a Late Cretaceous (69 million to 66 million years ago) fossil of a bird recovered from the Antarctic Peninsula.
Modern birds are a tremendously diverse group, numbering about 10,000 living species4. Thanks to an impressive range of anatomical and behavioural specializations, birds have spread to almost every environment on the planet. Although considerable ink has been spilled regarding the roles of avian feathers, wings, feeding systems and body-size differences as drivers of this diversification, the fossil record has produced only limited information regarding the evolution of avian social and behavioural systems. The evolution of the syrinx, that one avian-specific feature responsible for the vast array of sounds produced by birds, is sorely understudied.
The work of Clarke and colleagues represents one of only a handful of examples from the entire avian fossil record in which the mineralized cartilage rings of the syrinx, which support the sound-producing soft-tissue membranes, have been identified. So far, the sound-producing system for non-avian dinosaurs has been thought to be the muscular tube in the throat known as the larynx. Such inferences rest on generalized comparisons with other (non-avian) ancestral groups known as tetrapods, and there is a relative dearth of studies that even consider the origin of the syrinx.
Clarke et al. assigned the fossil to the bird species Vegavis iaai, which belongs to the group that includes ducks, geese and swans. The specimen was recovered from Antarctic rocks in layers just below and older than the layer corresponding to the Cretaceous–Palaeogene extinction event some 66 million years ago, which wiped out the non-avian dinosaurs and many other organisms around the globe. The current research builds on the earlier landmark study5 establishing the species V. iaai as the only unambiguously identified modern bird fossil known from the entirety of the Mesozoic Era (252 million to 66 million years ago). Now V. iaai is revealing clues about the evolution of the avian vocalization apparatus, and about vocal communication in the group more generally.
The researchers scanned the fossil specimen using a high-resolution X-ray imaging technique termed microcomputed tomography, which enabled them to construct a three-dimensional representation of the syrinx, a structure about 1 centimetre in diameter. To improve the anatomical interpretations and constrain the range of sounds that the bird might have produced, the authors used another type of tomography that provides information on both hard and soft tissues to assemble an additional data set for samples from 12 groups of present-day birds. This approach of contextualizing fossil data with information derived from extant animals, in which both hard and soft tissues can be linked to functional and performance parameters, represents the current standard in palaeontology. Both hard and soft tissues are necessary for sound production, with the cartilaginous rings providing support for the vibrating soft-tissue membranes.
The authors defined the basic shape and size of the fossil syrinx and its associated bronchial (lung) and tracheal (windpipe) segments, noting several characteristics, such as the fusion of cartilage rings and asymmetry between the left and right sides of the syrinx, that are useful for making comparisons with structural data from the present-day birds (Fig. 1). The researchers refrained from over-interpreting the fossil, particularly with regard to inferring the presence of the intricate muscular and membranous tissues known to be crucial for complex vocalization in living birds. Instead, the authors note general features that are similar to those in present-day ducks and geese, an interpretation that is consistent with other skeletal features affiliating Vegavis with this branch of the bird family. This structural-reconstruction analysis suggests that V. iaai had the ability to produce basic honks and other simple sounds typical of ducks and geese.
Despite a truly amazing fossil record containing evidence that has expanded the distribution of certain avian features such as feathers among non-avian theropod dinosaurs, fossilized components of the vocal apparatus have yet to be discovered in non-avian theropods. Taking the fossil record at face value, Clarke and colleagues suggest that the syrinx therefore represents a feature that evolved relatively late in the bird lineage compared with feathers and flight. In other words, the syrinx might be a defining avian feature that played a vital part in the tremendous diversification of the group. But as we repeatedly find in the world of palaeontology, inferences derived from the fossil record are ephemeral entities lasting only until the next spectacular fossil is recovered.
Clarke and colleagues' study also highlights the potential for other researchers to take a much closer look at both new and previously recovered fossils for evidence of the syrinx. I suspect that there are additional examples of this seemingly avian-specific structure already in museum collections around the globe. Moreover, studies such as that by Clarke et al. will certainly inform fossil examination, and, perhaps most importantly, how scientists frame questions about bird evolution. For example, comparative biologists have had no reason to consider the possibility that something like the syrinx would be discoverable in a fossil — now that scientists can characterize its precise size and shape, an entire field of enquiry about how the syrinx functions in living birds, and its role in social evolution, is ripe for the picking.
As any real 'birder' enthusiast will be quick to inform the casual naturalist, a bird's appearance is only one aspect of its biology, with chirps, honks and other more complex vocalizations offering the best insight into the vast array of behaviours and social interactions exhibited by the plentiful species of extant birds. Clarke and colleagues have uncovered one key piece of that puzzle in a small bird fossil from Antarctica, foreshadowing the soundscape of things yet to come during avian diversification.Footnote 1
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