Morphometric comparison of the Hesperornithiformes and modern diving birds

Abstract The Cretaceous Hesperornithiformes are an extinct group of aquatic birds long recognized to have practiced foot-propelled diving. This specialization is seen today in a number of modern avian families (loons, grebes, cormorants, and some ducks) that have convergently derived a diving lifestyle. Historically, hesperornithiforms have usually been compared to modern loons and grebes as analogues. However, these comparisons are based on qualitative observations of skeletal morphology and have never been tested using morphometric data. Studying the morphological details underlying this functional convergence provides an opportunity to better understand the lifestyle of the extinct Hesperornithiformes, particularly in regard to adaptations for foot-propelled diving and ecological niches these birds may have occupied. This study presents a morphometric analysis of the hindlimb of hesperornithiforms and modern foot-propelled diving birds. Measurements were collected from the tarsometatarsus, tibiotarsus, and femur of eight genera of modern birds and six genera of hesperornithiforms. In order to explore variation within the data, a number of different combinations of measurements (for example, tarsometatarsus and tibiotarsus, or tibiotarsus and femur) were subjected to principal components analyses (PCA). The primary reason for investigating the data in different subsets was to increase taxonomic inclusion among the fossil specimens, where missing data was the largest impediment to the study. Results of this study show that hesperornithiforms rarely shared morphospace with loons or grebes, but more often did share morphospace with cormorants and diving ducks. Therefore, loons and grebes may not serve as the most appropriate analogue for the Hesperornithiformes. Use of these sorts of analyses, in conjunction with detailed morphological work, may enhance our understanding of the evolution of complex ecological strategies and niche partitioning among birds and other organisms.