Meeting Abstract
Cranial kinesis in birds is a remarkable anatomical specialization involving the development of novel joints resulting in a complex biomechanical system that allows the bill to move independently of the braincase. The early evolutionary history of the specialized musculoskeletal structures necessary for a fully kinetic skull, however, is poorly understood. Whereas nearly all Mesozoic bird skull fossils are crushed flat, fossil marine toothed bird Hesperornis regalis, from Late Cretaceous Kansas, is the closest fossil taxon to crown Aves with known from nearly complete, undistorted skull material, making it crucial for understanding the emergence of kinesis in birds. The three-dimensionally preserved skull material can be integrated with osteological correlates to musculature and mobile joints from living species to reconstruct the cranial kinetic system of Hesperornis. We μCT scanned and digitally prepared Hesperornis skull elements to construct a model for comparative and functional studies. We used contrast-stained μCT scans of palaeognathous and neognathous birds, and a crocodylian, to precisely identify osteological correlates to the cranial kinetic system and cranial musculature. With these comparative data, we developed a reconstruction cranial muscles in Hesperornis. The palate, rostrum, and some correlates to musculature compare favorably with palaeognaths, lacking mobile interpterygoid joints or a nasofrontal hinge, and kinesis relying on elastic deformation of the rostral bones. We also identify a suite of osteological correlates to jaw adductor muscles on the braincase resembling those in disparate clades of extant pursuit-diving birds, showing a mosaic of deeply plesiomorphic characters occurring in concert with remarkable convergence and specialization.
