Abstract
At least two lineages of Mesozoic birds are known to have possessed a distinct feather morphotype for which there is no neornithine (modern) equivalent. The early stepwise evolution of apparently modern feathers occurred within Maniraptora, basal to the avian transition, with asymmetrical pennaceous feathers suited for flight present in the most basal recognized avian, Archaeopteryx lithographica. The number of extinct primitive feather morphotypes recognized among non-avian dinosaurs continues to increase with new discoveries; some of these resemble feathers present in basal birds. As a result, feathers between phylogenetically widely separated taxa have been described as homologous. Here we examine the extinct feather morphotypes recognized within Aves and compare these structures with those found in non-avian dinosaurs. We conclude that the "rachis dominated" tail feathers of Confuciusornis sanctus and some enantiornithines are not equivalent to the "proximally ribbon-like" pennaceous feathers of the juvenile oviraptorosaur Similicaudipteryx yixianensis. Close morphological analysis of these unusual rectrices in basal birds supports the interpretation that they are modified pennaceous feathers. Because this feather morphotype is not seen in living birds, we build on current understanding of modern feather molecular morphogenesis to suggest a hypothetical molecular developmental model for the formation of the rachis dominated feathers of extinct basal birds.
Highlights
IntroductionScientific understanding of the evolution of modern feathers has increased exponentially over the past three decades thanks as much to the numerous feathered Jurassic and Cretaceous dinosaurs uncovered from northeastern China [1,2,3,4,5,6,7] as to the results of laboratory experiments on the molecular morphogenesis of feathers in living birds [8,9,10,11,12,13,14,15]
The rachis-dominated feathers of some basal birds have been suggested to represent (I) an intermediate morphology providing evidence that pennaceous feathers evolved from elongated scales [40,52], (II) a type of modified pennaceous feather [42], or (III) a completely new type of primitive feather [4]
These long rectricial feathers superficially resemble the filoplumes of modern birds in that they have a broad rachis relative to the “vaned” portion of the feather, modern filoplumes are typically shorter than the coverts they are associated with and have a very slender, hair-like rachis [48]
Summary
Scientific understanding of the evolution of modern feathers has increased exponentially over the past three decades thanks as much to the numerous feathered Jurassic and Cretaceous dinosaurs uncovered from northeastern China [1,2,3,4,5,6,7] as to the results of laboratory experiments on the molecular morphogenesis of feathers in living birds [8,9,10,11,12,13,14,15]. Some non-avian maniraptoran theropods clearly did possess feathers of essentially modern morphology, it appears most known taxa retain apparently primitive morphotypes, such as large single filaments [24], multiple filaments joined basally [20,25,26], distally branched filaments [18,26], and pennaceous feathers with proximally undifferentiated vanes [4], variably distributed throughout the clade [4,19]; integumentary structures lacking a rachis are here referred to informally as “proto-feathers.”. A close comparison of these feathers together with information from new enantiornithine specimens support previous interpretations that these rectrices, in basal birds, represent modified pennaceous feathers, and are distinct from the feather morphotype preserved in juvenile Similicaudipteryx yixianensis (STM4-1). We apply current knowledge on the cellular and molecular process of feather development to discuss the key morphogenetic process that may have produced this extinct morphology
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