Functional constraints on the number and shape of flight feathers

Abstract

As a fundamental ecological aspect of most organisms, locomotor function significantly constrains morphology. At the same time, the evolution of novel locomotor abilities has produced dramatic morphological transformations, initiating some of the most significant diversifications in life history. Despite significant new fossil evidence, it remains unclear whether volant locomotion had a single or multiple origins in pennaraptoran dinosaurs and the volant abilities of individual taxa are controversial. The evolution of powered flight in modern birds involved exaptation of feathered surfaces extending off the limbs and tail yet most studies concerning flight potential in pennaraptorans do not account for the structure and morphology of the wing feathers themselves. Analysis of the number and shape of remex and rectrix feathers across a large dataset of extant birds indicates that the number of remiges and rectrices and the degree of primary vane asymmetry strongly correlate with locomotor ability revealing important functional constraints. Among these traits, phenotypic flexibility varies reflected by the different rates at which morphological changes evolve, such that some traits reflect the ancestral condition, whereas others reflect current locomotor function. While Mesozoic birds and Microraptor have remex morphologies consistent with extant volant birds, that of anchiornithines deviate significantly providing strong evidence this clade was not volant. The results of these analyses support a single origin of dinosaurian flight and indicate the early stages of feathered wing evolution are not sampled by the currently available fossil record.

Earliest evidence of avian primary feather moult

“…The sequential moult observed in the two specimens described here indicates that these birds maintained a relatively high functionality of the flight feathers even during the moulting process [10]. This moult strategy characterizes modern birds that are highly dependent on flight and is also present in the slightly younger (120 Ma Jiufotang Formation) four-winged and probably volant dromaeosaurid Microraptor [13,[21][22][23]26]. In these specimens, flight ability is also supported by the strong asymmetry of the flight feathers (STM7-21 = 3.67 and STM7-22 = 2.91; the degree of vane asymmetry was calculated as the ratio between the trailing and leading vane edges [26]).…”

“…This moult strategy characterizes modern birds that are highly dependent on flight and is also present in the slightly younger (120 Ma Jiufotang Formation) four-winged and probably volant dromaeosaurid Microraptor [13,[21][22][23]26]. In these specimens, flight ability is also supported by the strong asymmetry of the flight feathers (STM7-21 = 3.67 and STM7-22 = 2.91; the degree of vane asymmetry was calculated as the ratio between the trailing and leading vane edges [26]). However, other aspects of the wing anatomy call into question the aerodynamic abilities of confuciusornithiforms, such as their narrow rachises and very elongate primaries [29], but see Foth et al [6].…”

“…In addition, assuming that our identification is correct and both specimens are in active moult, there is the possibility of short, growing primary feathers that are not visible. Previous studies utilizing a sample of nine well-preserved specimens indicate that Confuciusornithiformes had nine or 10 functional primary feathers [25,26]. In order to simplify the description below, we refer to these two specimens as having 10 functional primaries, numbered from the innermost primary (P1) toward the wingtip.…”

“…Similarly, it is possible that a sequential moult was the ancestral condition in Pennaraptora, or that this moult strategy co-evolved with flight, evolving independently with each appearance of volant ability. Determining between these hypotheses will be difficult; the extreme plasticity of moult patterns documented in modern birds means that, if extinct pennaraptorans were similar to their extant counter parts, the sequential moult would be the first flight-related characteristic to be lost after volant abilities themselves in secondarily flightless taxa [13,26]. Documentation of active flight feather moult in extinct pennaraptoran dinosaurs is very rare [34].…”

“…However, this is consistent with the fact that moult is only documented in volant taxa, and further indicates these taxa were dependent on flight for survival. It is currently unknown whether volant abilities evolved once at the base of Pennaraptora [26], or multiple times independently [30][31][32][33]. Similarly, it is possible that a sequential moult was the ancestral condition in Pennaraptora, or that this moult strategy co-evolved with flight, evolving independently with each appearance of volant ability.…”