An evolutionary timeline
Within the avian stem lineage, the transition from a larynx-based sound source to a tracheobronchial syrinx occurred within Dinosauria, at or before the origin of Aves about 66-68 million years ago. The earliest fossilized record of syringeal remains is from a single specimen of Vegavis iaai from the same epoch.
(...) An intact specimen from the late Cretaceous, however, highlights the fossilization potential of the ancestral structure and may indicate that the syrinx is a late-arising feature in avian evolution.
There is uncertainty about the relationship between the larynx and syrinx during this morphological shift, but there are two predominant evolutionary possibilities: regimes unrelated to sound production could have led to the loss in vocal function of the larynx. A new structure, the syrinx, then arose after selection for acoustic function. Conversely, the larynx could have retained some vocal capabilities, though at a diminished capacity. The syrinx then evolved to supplement sound production, which would have been followed by the loss of the larynx as a sound source. The former scenario would have led to a “silent” period in the evolution of avian ancestors. The current fossil record does not provide definitive evidence for whether the function of the larynx was lost before the syrinx was gained.
The fossil record does, however, provide clues for the evolutionary timeline of some syringeal elements. For example, increased mineralization at the tracheobronchial juncture is likely a late-arising feature in avian evolution. Despite new discoveries of preserved avian tracheobronchial rings from the Cenozoic, these structures have not been recovered from Mesozoic archosaurs. This might be a product of weak mineralization in the bronchi and trachea of Mesozoic archosaurs, a condition which would inhibit preservation potential. Thus, a shift towards a mineralized structure may have been preceded by many key avian adaptations, including respiratory shifts, increases in metabolic rates, and feather ornamentation.
Evolutionary causation
The archosaurian shift from the larynx to syrinx must have conferred a selective advantage for crown birds, but the causes for this shift remain unknown. To complicate matters, the syrinx falls into an unusual category of functional evolution: arising from ancestors with a larynx-based sound source, the syrinx contains significant functional overlap with the structure it replaced. In fact, there is no evidence that an original, simplified syrinx could produce calls with a larger frequency range or longer or louder calls that an alligator-like larynx, which would have potentially increased fitness.
Aucun commentaire:
Enregistrer un commentaire