Specimen preparation and fossil sampling

Extant specimens were prepared and dissected at the Texas Natural History Collections (TNHC) at the University of Texas at Austin. USNM-catalogued specimens were dissected and photographed at the Museum Support Center at the Smithsonian Institution, National Museum of Natural History (NMNH), Washington DC. All specimens were scanned at the University of Texas High-Resolution X-ray Computed Tomography Facility (UTCT). The protocols of iodine staining for soft-tissue contrast imaging were adapted from methods successfully developed for alligator and birds [18, 23, 29, 31, 32].

All specimens were fixed in 10% neutral buffered formalin solution (10% NBF) for approximately a week to two months based on the specimen size. Individual specimens were stained with either I₂KI (iodine-potassium iodine-10% NBF solution) or I₂E (iodine-ethanol solution). The concentration of the I₂KI solution was calculated by dividing the total weight of solutes (e.g., iodine and potassium iodine) by the volume of the solution. Alternatively, 1% I₂E (1g I₂ dissolved in 100 ml absolute ethanol [200 proof]) was used in staining the tongues of American alligator and Northern bobwhite (Alligator mississippians and Colinus virginianus) The concentration of staining solution, duration of staining and scanning parameters are detailed in the Tables ​Tables11 and ​and2.2. All smaller samples, including Ring-necked Pheasant (Phasianus colchicus), Chilean tinamou (Nothoprocta perdicaria), and Northern bobwhite (Colinus virginianus) were scanned using a microXCT 400 scanner (built by Zeiss, formerly Xradia, Inc.). The larger Dromaius novaehollandiae (Common Emu) were scanned using a BIR scanner (225 kV Feinfocus X-ray source and an Image Intensifier detector) in UTCT.

Fossil data were systematically assessed from the Shandong Tianyu Museum of Nature (n ~250) and Institute of Vertebrate Palaeontology and Palaeoanthropology (n ~50), and Beijing Museum of Natural History (n ~30). The best-preserved representative specimens are cited in the S1 Table. Reconstructions of extinct archosaur skulls and hyoids (Fig 8) were based on examination of fossil material in museum collections (Institute of Vertebrate Paleontology and Paleoanthropology, IVPP and Shandong Tianyu Museum of Nature, STM), Beijing Natural History Museum, as well as published resources (S1 Table). For all extinct archosaurs, the resting position of hyoid relative to the skull (i.e., the eye orbit) was assessed as similar to that preserved unless there was clear evidence of displacement post mortem.

Pterosaurs show convergent evolution of traits linked to tongue protrusion and mobility in birds (narrow midline element [achieved through fusion] and elongate, paired and rostrally positioned ceratobranchials). Within Dinosauria, hyoid elements are progressively more rostrally-placed in crownward bird-line species, a condition particularly evident in extant birds (compare orbital position). The ceratobranchial-basihyal contact is approximately even with the nasofrontal hinge (or zone of contact between the premaxillae and frontals) in birds, which is rostral to its position present in successive outgroups. A second ceratobranchial is absent in all archosaurs with the exception of one proposed example in Ankylosauria [10, 41]. However, unlike outgroup taxa, the position of ceratobranchial I in archosaurs is rostrally displaced relative to the center of the orbit. Skeletal hyoid elements are highlighted in different colors: red, ceratobranchial; blue, basihyal; yellow, paraglossal; green, epibranchials. References of reconstructions used are provided in S1 Table and the tree was adopted from ref. [52].

Institution abbreviations: BMMS, Bürgermeister Müller Museum Solnhofen; BMNH, Beijing Museum of Natural History; DNHM, Dalian Natural History Museum; ELDM, Erlianhaote Dinosaur Museum, Inner Mongolia; IVPP, Institute of Vertebrate Palaeontology and Palaeoanthropology; MNA, Museum of Northern Arizona; MACN, Meseo Argentino de Ciencias Naturales; NIGP, Nanjing Institute of Geology and Palaeontology; PKUP, Peking University Paleontological Collections; SAM, South African Museum; XHPM, Xinghai Museum of Prehistoric Life of Dalian; SDSM, South Dakota School of Mines and Technology; SMNK, Staatliches Museum Für Naturkunde Karlsruhe; STM, Shandong Tianyu Museum of Nature; TNHC, Texas Natural History Collection; UMNH, Natural History Museum of Utah; USNM, the National Museum of Natural History, Smithsonian Institution; UT-TMM, University of Texas at Austin, Vertebrate Paleontology Laboratory; ZDM, Zigong Dinosaur Museum, Zigong, China; ZLJ, Lufeng World Dinosaur Valley Park (see also S1 Table).