Complete Ichthyornis skull illuminates mosaic assembly of the avian head

Abstract

The skull of living birds is greatly modified from the condition found in their dinosaurian antecedents. Bird skulls have an enlarged, toothless premaxillary beak and an intricate kinetic system that includes a mobile palate and jaw suspensorium. The expanded avian neurocranium protects an enlarged brain and is flanked by reduced jaw adductor muscles. However, the order of appearance of these features and the nature of their earliest manifestations remain unknown. The Late Cretaceous toothed bird Ichthyornis dispar sits in a pivotal phylogenetic position outside living groups: it is close to the extant avian radiation but retains numerous ancestral characters. Although its evolutionary importance continues to be affirmed, no substantial new cranial material of I. dispar has been described beyond incomplete remains recovered in the 1870s. Jurassic and Cretaceous Lagerstätten have yielded important avialan fossils, but their skulls are typically crushed and distorted . Here we report four three-dimensionally preserved specimens of I. dispar-including an unusually complete skull-as well as two previously overlooked elements from the Yale Peabody Museum holotype, YPM 1450. We used these specimens to generate a nearly complete three-dimensional reconstruction of the I. dispar skull using high-resolution computed tomography. Our study reveals that I. dispar had a transitional beak-small, lacking a palatal shelf and restricted to the tips of the jaws-coupled with a kinetic system similar to that of living birds. The feeding apparatus of extant birds therefore evolved earlier than previously thought and its components were functionally and developmentally coordinated. The brain was relatively modern, but the temporal region was unexpectedly dinosaurian: it retained a large adductor chamber bounded dorsally by substantial bony remnants of the ancestral reptilian upper temporal fenestra. This combination of features documents that important attributes of the avian brain and palate evolved before the reduction of jaw musculature and the full transformation of the beak.

Evolutionary and ontogenetic changes of the anatomical organization and modularity in the skull of archosaurs

“…This is likely explained by the similar modular division (as observed in Figs. 5B and 5D; Table S4), presence of anatomical features characteristic of modern birds, such as the loss of the postorbital bones, the fusion of the left and right premaxilla to form the beak, a bicondylar quadrate that form a joint with the braincase, and the arrangement of the rostrum, jugal, and quadratojugal required for a functional cranial kinetic system (Jollie, 1957;Bock, 1964;Clarke, 2004;Bhullar et al 2016;Field et al 2018). Within the neognaths, the skull of Geospiza fortis is more complex and more homogenous than either juvenile or adult Gallus gallus.…”

IsOculudentavisa bird or even archosaur?

“…This is also the condition among Mesozoic birds, including Archaeopteryx [5, 10, 11], Sapeornis [12], enantiornithines [6, 8] and ornithuromorphs [8]. In Ichthyornis , maxilla is elongate and extends further caudally beneath the jugal [13], which means the ventral margin of the orbit is still mostly composed by the jugal, different from Oculudentavis . In addition, we need to note that the skull of Jeholornis was incorrectly reconstructed with a maxilla extending most of the orbit, and a shortened jugal [1], which certainly lead to a strong similarity between the skull of Oculudentavis and Jeholornis .…”

Evolutionary insights from an ancient bird

“…The restoration, mounting and illustration of the specimens were, shall we say, somewhat overenthusiastic. The specimens could be convincingly described only after the mounts had been disassembled and prepared afresh more than a century later 1,4 .…”

“…Whatever the answers to these questions turn out to be, Field and colleagues' beautifully rendered 3D scans and reconstructions of this iconic fossil avian, along with their comparisons of these structures with those of earlier and later birds, provide an important resource to aid our understanding of early bird evolution. ■ 1 report the reconstruction of the skull of an extinct species. Their reconstruction fills in some structures missing from previously available fossils, thereby illuminating the transition between the loss of ancient dinosaur features and the evolution of characteristics found in present-day birds.…”