The patterns and modes of the evolution of disparity in Mesozoic birds

Abstract

The origin of birds from non-avian theropod dinosaurs is one of the greatest transitions in evolution. Shortly after diverging from other theropods in the Late Jurassic, Mesozoic birds diversified into two major clades—the Enantiornithes and Ornithuromorpha—acquiring many features previously considered unique to the crown group along the way. Here, we present a comparative phylogenetic study of the patterns and modes of Mesozoic bird skeletal morphology and limb proportions. Our results show that the major Mesozoic avian groups are distinctive in discrete character space, but constrained in a morphospace defined by limb proportions. The Enantiornithines, despite being the most speciose group of Mesozoic birds, are much less morphologically disparate than their sister clade, the Ornithuromorpha—the clade that gave rise to living birds, showing disparity and diversity were decoupled in avian history. This relatively low disparity suggests that diversification of enantiornithines was characterized in exhausting fine morphologies, whereas ornithuromorphs continuously explored a broader array of morphologies and ecological opportunities. We suggest this clade-specific evolutionary versatility contributed to their sole survival of the end-Cretaceous mass extinction.

An Early Cretaceous enantiornithine bird with a pintail

“…The presence of metatarsal V is another plesiomorphy in ornithothoracines only present in pengornithids and Vorona (Figure S2). 3,7,14 Metatarsal V is short, measuring 1/4 the length of metatarsal IV; the distal half is tapered and weakly deflected cranially. The proximal phalanx of the hallux is the longest, but that of digit II is the most robust, as in other pengornithids.…”

“…35 The Enantiornithes and Ornithuromorpha, as the two dominant groups of Mesozoic birds with their almost worldwide distributions, provide a rare opportunity to study the 65 Ma of parallel diversification of these two sister clades within the context of phylogeny. 3,4,40 Paleontologists traditionally focus on their skeletal morphologies that exhibit clade-specific evolutionary versatility. 3 However, differences in plumage between enantiornithines and ornithuromorphs are emerging in light of the recent exceptional discoveries.…”

“…3,4,40 Paleontologists traditionally focus on their skeletal morphologies that exhibit clade-specific evolutionary versatility. 3 However, differences in plumage between enantiornithines and ornithuromorphs are emerging in light of the recent exceptional discoveries. 6,14,18,20,26 Specifically, an investigation of approximately 450 Early Cretaceous enantiornithines reveals a ratio close to 1:1 between specimens that have and do not have the central paired elongate rectrices (Table S3), speaking to the wide distribution of this sexual signaling trait across the Enantiornithes.…”

“…In order to investigate the relationship of IVPP V27883 with respect to other Mesozoic birds, we performed a comprehensive phylogenetic analysis using the latest version of the dataset from the Mesozoic Avian Phylogeny (MAP) project, which is actively being maintained by the Institute of Vertebrate Paleontology and Paleoanthropology. 3 The dataset was modified with the addition of the IVPP V27883 and minor revisions of character coding for a few taxa (Methods S2). The revised dataset consists of 280 characters and 81 taxa that encompass nearly all well-recognized Mesozoic birds (n = 78).…”

“…The strict consensus tree is consistent with most recent studies in the systematic relationships of major clades (Figure S4). 3,6,7,46 . Following previous study, 47 the Ornithuromorpha refers to the first ancestor of Neornithes that is not an ancestor of Enantiornithes, and all of its descendants.…”