Data from: Fossil evidence of the avian vocal organ from the Mesozoic

From complex songs to simple honks, birds produce sounds using a unique
vocal organ called the syrinx1, 2. Located close to the heart at the
tracheobronchial junction, vocal folds or membranes attached to modified
mineralized rings vibrate to produce sound1, 2, 3, 4, 5, 6, 7. Syringeal
components were not thought to commonly enter the fossil record6, and the
few reported fossilized parts of the syrinx are geologically young8, 9,
10, 11 (from the Pleistocene and Holocene (approximately 2.5 million years
ago to the present)). The only known older syrinx is an Eocene specimen
that was not described or illustrated12. Data on the relationship between
soft tissue structures and syringeal three-dimensional geometry are also
exceptionally limited5. Here we describe the first remains, to our
knowledge, of a fossil syrinx from the Mesozoic Era, which are preserved
in three dimensions in a specimen from the Late Cretaceous (approximately
66 to 69 million years ago) of Antarctica. With both cranial and
postcranial remains, the new Vegavis iaai 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 Vegavis
and Eocene syrinxes, informs both the reconstruction of ancestral states
in birds and properties of the vocal organ in the extinct species. Fused
rings in Vegavis form a well-mineralized pessulus, a derived neognath bird
feature, proposed to anchor enlarged vocal folds or labia5. Left-right
bronchial asymmetry, as seen in Vegavis, is only known in extant birds
with two sets of vocal fold sound sources. 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 tracheobronchial remains, and the poorly mineralized condition in
archosaurian taxa without a syrinx, may indicate that a complex syrinx was
a late arising feature in the evolution of birds, well after the origin of
flight and respiratory innovations.