Geosciences & Sternberg Museum of Natural History

A new hesperornithiform bird specimen from Devon Island in the Canadian High Arctic is represented by three cervical vertebrae and is assigned to Canadaga arctica. The new specimen is only the second occurrence of C. arctica and corresponds in morphology and size to the type specimen from Bylot Island, also in the High Arctic. This new fossil adds to the record of North American hesperornithiforms, which had a well-documented Arctic presence. Body size comparisons of all North American specimens from the Campanian reveal that the largest known hesperornithiforms were from high latitudes, but otherwise no clear correlation between body size and latitude is apparent. The largest hesperornithiforms (Canadaga arctica and Heperornis regalis) are found at the highest latitudes, while the smallest forms (Baptornis advenus and Parahesperornis alexi) are found at the southern extent of the birds' range. Coniornis (a medium body-sized genus) is only found in the middle of the range. No size trends are discernable within the genus Hesperornis or within the species H. regalis. The presence of large hesperornithiforms at high latitudes may indicate that either strong seasonal distribution of resources contributed to larger body sizes at higher latitudes, or Campanian thermal gradients along the Western Interior Seaway were significant enough to affect body size for thermoregulatory reasons (sensu Bergmann's rule). The absence of body size trends within mid-latitude Hesperornis specimens suggests that the climatic gradient in the southern portion of the Seaway was not strong enough to force morphological evolutionary responses, or that character displacement, migration and/or other factors affected body size. Sample size and the inherent problems of an incomplete fossil record must also be considered.

The broad biogeographic distribution of Hesperornis fossils in Late Cretaceous Western Interior Seaway deposits has prompted questions about whether they endured polar winters or migrated between mid- and high latitudes. Here, we compare microstructures of hesperornithiform long bones from Kansas and the Arctic to investigate whether migration or Late Cretaceous polar climate affected bone growth. We also examine modern penguin bones to determine how migration and climate may influence bone growth in birds with known behaviours. Histological analysis of hesperornithiform samples reveals continuous bone deposition throughout the cortex, plus an outer circumferential layer in adults. No cyclic growth marks, zonation or differences in vasculature are apparent in the Hesperornis specimens. Comparatively, migratory Adélie and chinstrap penguin bones show no zonation or changes in microstructure, suggesting that migration is not necessarily recorded in avian bone microstructure. Non-migratory gentoos show evidence of rapid bone growth possibly associated with increased chick growth rates in high-latitude populations and large body size. The absence of histological evidence for migration in extinct Hesperornis and extant pygoscelid penguins may reflect that these birds reached skeletal maturity before migration or overwintering. This underscores the challenges of using bone microstructure to infer the effects of behaviour and climate on avian growth.

Here we describe a new hesperornithiform specimen from the Upper Cretaceous Kanguk Formation of Devon Island, Nunavut, Canada. This specimen (NUVF 286) is referred to cf. Hesperornis sp. based on size and shape of femora and teeth preserved with other skeletal elements. Previous osteohistologic analyses indicate a subadult ontogenetic stage at the time of death. This new cf. Hesperornis specimen includes the first teeth associated with a high-latitude hesperornithiform, allowing for comments on the trophic behavior of these birds. Paleoecological reconstructions based on current knowledge of polar assemblages and comparisons to modern birds suggest that high-latitude hesperornithiform birds may have had more varied diets than previously assumed. Résumé : Nous décrivons un nouveau spécimen d'hesperornithiforme de la Formation de Kanguk du Crétacé supérieur dans l'île Devon (Nunavut, Canada). Ce spécimen (NUVF 286) est référé a ` cf. Hesperornis sp. a ` la lumière de la taille et de la forme de fémurs et de dents préservés avec d'autres éléments du squelette. Des analyses ostéohistologiques antérieures indiquent un stade ontogénétique subadulte au moment de la mort. Ce nouveau spécimen de cf. Hesperornis comprend les premières dents associées a ` un hesperornithiforme de haute latitude, ce qui permet de formuler des commentaires sur le comportement trophique de ces oiseaux. Des reconstitutions paléoécologiques basées sur les connaissances actuelles sur des assemblages polaires et des com-paraisons a ` des oiseaux modernes donnent a ` penser que les oiseaux hesperornithiformes de haute latitude pourraient avoir présenté des régimes alimentaires plus variés que ce qui était présumé auparavant. [Traduit par la Rédaction]

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 characters1,2,3. Although its evolutionary importance continues to be affirmed3,4,5,6,7,8, 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.