Abstract
Birds are one of the most diverse clades of extant terrestrial vertebrates, a diversity that first arose during the Mesozoic as a multitude of lineages of pre-neornithine (stem) birds appeared but did not survive into the Cenozoic Era. Modern birds (Neornithes) inhabit an extensive array of ecologically distinct habitats and have specific and varied foraging strategies. Likewise, the morphological disparity among Mesozoic lineages appears to underscore a significant degree of ecological diversity, yet attempts to determine lineage-specific ecologies have mainly been limited to superficial narratives. In recent years, numerous studies have used various morphometric proxies to interpret the paleoecology of Mesozoic bird lineages, but largely without evaluating the interplay between ecological and phylogenetic signals. Moreover, most studies of this sort transform the original data into logarithms to control dimensionality, underestimating the biases induced upon such transformations. The goal of this study is to quantitatively address the ecomorphology of crown-group Neornithes using a dense sample of raw forelimb and hindlimb measurements, and to examine if such results can be used to infer the ecologies of Mesozoic bird lineages. To that end, scaling of limb measurements and ecological data from modern birds was assessed statistically using phylogenetic comparative methods, followed by the inclusion of fossil taxa. A strong relationship was recovered between humerus and hindlimb allometric scaling and phylogeny. Our results indicate that while some ecological classes of modern birds can be discriminated from each other, phylogenetic signature can overwhelm ecological signal in morphometric data, potentially limiting the inferences that can be made from ecomorphological studies. Furthermore, we found differential scaling of leg bones among Early Cretaceous enantiornithines and ornithuromorphs, a result hinting that habitat partitioning among different lineages could be a pervasive phenomenon in avian evolution.
Highlights
Modern birds comprise one of the most diverse clades of vertebrates alive today, yet members of this crown group, the Neornithes, are poorly represented in the fossil record before the Tertiary (Fountaine et al, 2005; Brocklehurst et al, 2012; Field et al, 2020)
The results indicated that limited curvature of the pedal claw was the ancestral state in theropods, and most Mesozoic birds displayed pedal claw geometry consistent with a primarily terrestrial lifestyle (Glen and Bennett 2007)
This study found that Mesozoic ornithurines shared morphospace with modern flapping and flap-gliding birds, while all other Mesozoic taxa occurred at either the fringes of or outside the morphospace defined by modern birds (Close and Rayfield 2012)
Summary
Modern birds comprise one of the most diverse clades of vertebrates alive today, yet members of this crown group, the Neornithes, are poorly represented in the fossil record before the Tertiary (Fountaine et al, 2005; Brocklehurst et al, 2012; Field et al, 2020). A wealth of pre-Tertiary fossils reveals the existence of numerous Mesozoic lineages outside of the crown clade Neornithes (Brusatte et al, 2015; Mayr 2016; Wang and Zhou 2017; Chiappe 2018; Chiappe and Bell 2020) These stem lineages represent an enormous diversity of forms, from the longlegged, cursorial Hollanda (Bell et al, 2010) to the long-winged, soaring Sapeornis (Serrano and Chiappe 2017), and from the small, flighted enantiornithines (O’Connor and Chiappe 2011; Liu et al, 2017) to the large, flightless hesperornithiforms (Bell and Chiappe 2016). Because the relationship between morphological traits and ecological characteristics is not always straightforward, quantitative analysis of comparative datasets for modern and fossil birds is a promising avenue of inquiry
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