Abstract
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.
Highlights
IntroductionThe 33◦ N (Arkansas) [5] to 79◦ N (Ellesmere Island) [6] palaeolatitudinal distribution of Hesperornis fossils, coupled with an abundance of juvenile Hesperornis remains in the Arctic (and the absence of juveniles from lower latitudes) has led to the hypothesis that Hesperornis migrated along the Western Interior Seaway (WIS) [2,6,7,8], perhaps to breed in higher latitude environments
Hesperornithiforms were a group of flightless seabirds that inhabited Late Cretaceous marine environments throughout the Northern Hemisphere
The 33◦ N (Arkansas) [5] to 79◦ N (Ellesmere Island) [6] palaeolatitudinal distribution of Hesperornis fossils, coupled with an abundance of juvenile Hesperornis remains in the Arctic has led to the hypothesis that Hesperornis migrated along the Western Interior Seaway (WIS) [2,6,7,8], perhaps to breed in higher latitude environments
Summary
The 33◦ N (Arkansas) [5] to 79◦ N (Ellesmere Island) [6] palaeolatitudinal distribution of Hesperornis fossils, coupled with an abundance of juvenile Hesperornis remains in the Arctic (and the absence of juveniles from lower latitudes) has led to the hypothesis that Hesperornis migrated along the WIS [2,6,7,8], perhaps to breed in higher latitude environments These birds were highly adapted for foot-propelled diving [9,10,11], and given the secondary loss of flight, could have migrated by swimming north and south through the Seaway. Patterns of Hesperornis bone histology are compared with those of extant high-latitude penguins
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