Juvenile specimens of Xiphactinus audax are rarely preserved in the fossil record and are typically represented by disarticulated cranial elements. Here we report the smallest specimen of Xiphactinus to date, consisting of the left and right dentaries and the right preoperculum. Based on the size of the new specimen and the rarity of small ichthyodectids in the Niobrara Chalk, it is interpreted as a juvenile. We suggest that the early life of Xiphactinus was spent in shallow margins of seaways for protection and to utilize rich food resources, and they were rare in open marine water. Today, juvenile predatory large-bodied fish are rare in open-water sediments, but shallow, nearshore deposits are lacking from the Western Interior Seaway, providing a complicating bias. The teeth of this juvenile X. audax specimen indicate that the diet did not notably change during growth and that even the small specimens would have been piscivorous predators. This is typical of large-bodied predatory fish such as tarpon that utilize estuaries and other shallow environs to mitigate juvenile mortality and do not change their diets much throughout ontogeny. This use of nearshore environments could explain why our Xiphactinus is so small and why such small specimens are rare.

The uppermost Pennsylvanian Pony Creek Shale Lagerstatte (Pony Creek Shale Member, Wood Siding Formation, Wabaunsee Group) near Maple Hill, Wabaunsee County, Kansas, contains exceptionally preserved horseshoe crab (Xiphosura) fossils. Associated with these fossils is an ichnofauna that includes trace fossils assumed to be produced by xiphosurids, as well as coprolites and a few burrows. Of the trace fossils recovered from the Pony Creek Shale Member, 26 specimens represent the ichnogeneraArborichnus,Crescentichnus, Kouphichnium,andSelenichnites, all of which are commonly associated with xiphosurids. The other trace fossils consist of the ichnogenusRhizocorallium, several unidentified burrows, and coprolites, some preserving animal remains. The xiphosuran locomotion traces from the Pony Creek Shale Member are primarily crawling trails. These trace fossils are distinguished by back and forth scratch imprints, as opposed to the typical xiphosurid tracks that show independent foot marks in the substrate. Other trace fossils include resting traces and feeding traces. Previous paleoenvironmental interpretations and the trace and body fossil record indicate a marginal-marine depositional system for the Pony Creek Shale.

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 characters1–3. Although its evolutionary importance continues to be affirmed3–8, 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 distorted9. 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. High-resolution computed tomography of three-dimensionally preserved specimens of Ichthyornis dispar clarifies the mosaic evolution of the avian head, revealing a kinetic feeding apparatus reminiscent of modern birds, a transitional beak and a dinosaurian temporal region.