The robust ventral ramus is convex rostrally and differs from the flat form present in other enantiornithines.
The dorsal margin of the lacrimal is notably concave between the two dorsal rami, as in Pengornis but unlike other enantiornithines 15. As in Deinonychus, Archaeopteryx and some enantiornithines 15, 16, 17, 18, the T-shaped lacrimal has rostral and caudal rami that extend rostroventrally and caudodorsally, respectively (Fig. The premaxillary process of the nasal extends rostrally to the midpoint of the frontal process of the premaxilla, and together they form the dorsal margin of the external naris.
2e–g) as in the juvenile enantiornithine LP-4450-IEI from Spain 16, opposite the condition present in other enantiornithines such as Falcatakely and Longipteryx 6, 15. The nasals have well-developed maxillary processes (Supplementary Fig. The maxilla bears a lateral groove that contains a row of nutrient foramina, ending in a caudoventrally oriented margin that articulates with the jugal. 2c, d), and together they completely separate the external nares from the antorbital fenestra. The triradiate maxilla has an elongate dorsal process that contacts the nasal (Supplementary Fig. As in most other enantiornithines 15, the facial margin is dominated by the maxilla, rather than the premaxilla as in crown birds 8. As in Parabohaiornis 14, the maxillary process tapers into an elongate dorsal ramus that overlays a ventral notch and sits in a groove on the lateral surface of the maxilla. The premaxillae are partially fused rostrally, but are separated caudally along their frontal processes (Fig. The degree of bone fusion indicates that IVPP V12707 is more skeletally mature than other recently described juvenile enantiornithines 11, 12, 13. IVPP V12707 is interpreted as a late juvenile individual on the basis of its proportionately large skull and orbit, coarse surface texture in the proximal and distal regions of the major limb bones, incomplete fusion of the knee and ankle elements, the partially co-ossified pygostyle, and fused neurocentral sutures on all visible thoracic vertebrae. Here, we describe a new enantiornithine bird with an excellent preserved skull from the Early Cretaceous of northeastern China, which demonstrates that the mosaic assembly of the functional aspects of kinesis and complex avian skull osteology must have arisen among crownward lineages. Avian cranial kinesis, allowing the upper jaw to move independent of the braincase and lower jaw, impacted the radiation of the great diversity of skull shape and ecologies among crown birds 1, 9. However, uncertainties still remain for key cranial regions, particularly the palatal complex that is functionally integrated in cranial kinesis 9, 10. Those evolutionary hypotheses are supported by fossils of stem avialans that exhibit an incompletely known, though vast diversity of cranial morphologies 5, 6, 7, 8. While ongoing paleontological discoveries of stem avialans largely have bridged the morphological gap between crown taxa and their closest nonavialan theropod relatives 2, questions remain surrounding the evolutionary construction and function of the modern avian skull that was assembled heterogeneously and heterochroncially over time through the loss, thinning, and reorganization of various cranial elements and their connections present in the ancestral theropod skull 3, 4, 5. The evolutionary patterns and modes from their first global-scale diversification during the Mesozoic to the >10,000 species of living birds with their great diversity of ecologies, morphologies, and behaviors, are enduring research topics in evolutionary biology 1, 2.
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