Plumage

Engineering

Ecology

Aerospace engineering

Biology

Manus

Wing

Feather

Botany

Flight feather

Evolutionary biology

Paleontology

Larva

Theropoda

Anatomy

Myology

Cretaceous

Zoology

Moulting

Current Biology

The most basal avians Archaeopteryx and Jeholornis have elongate reptilian tails. However, all other birds (Pygostylia) have an abbreviated tail that ends in a fused element called the pygostyle. In extant birds, this is typically associated with a fleshy structure called the rectricial bulb that secures the tail feathers (rectrices) [1Baumel J.J. Functional morphology of the tail apparatus of the pigeon (Columba livia).Adv. Anat. Embryol. Cell Biol. 1987; 110: 1-115Crossref PubMed Google Scholar]. The bulbi rectricium muscle controls the spread of the rectrices during flight. This ability to manipulate tail shape greatly increases flight function [2Gatesy S.M. Dial K.P. From frond to fan: Archaeopteryx and the evolution of short-tailed birds.Evolution. 1996; 50: 2037-2048Crossref Google Scholar, 3Thomas A.L.R. Why do birds have tails? The tail as a drag reducing flap, and trim control.J. Theor. Biol. 1996; 183: 247-253Crossref Scopus (52) Google Scholar]. The Jehol avifauna preserves the earliest known pygostylians and a diversity of rectrices. However, no fossil directly elucidates this important skeletal transition. Differences in plumage and pygostyle morphology between clades of Early Cretaceous birds led to the hypothesis that rectricial bulbs co-evolved with the plough-shaped pygostyle of the Ornithuromorpha [4Clarke J.A. Zhou Z. Zhang F. Insight into the evolution of avian flight from a new clade of Early Cretaceous ornithurines from China and the morphology of Yixianornis grabaui.J. Anat. 2006; 208: 287-308Crossref PubMed Scopus (136) Google Scholar]. A newly discovered pengornithid, Chiappeavis magnapremaxillo gen. et sp. nov., preserves strong evidence that enantiornithines possessed aerodynamic rectricial fans. The consistent co-occurrence of short pygostyle morphology with clear aerodynamic tail fans in the Ornithuromorpha, the Sapeornithiformes, and now the Pengornithidae strongly supports inferences that these features co-evolved with the rectricial bulbs as a “rectricial complex.” Most parsimoniously, rectricial bulbs are plesiomorphic to Pygostylia and were lost in confuciusornithiforms and some enantiornithines, although morphological differences suggest three independent origins.

An Enantiornithine with a Fan-Shaped Tail, and the Evolution of the Rectricial Complex in Early Birds