4.5 Article

Variations of Mesozoic feathers: Insights from the morphogenesis of extant feather rachises

Journal

EVOLUTION
Volume 74, Issue 9, Pages 2121-2133

Publisher

WILEY
DOI: 10.1111/evo.14051

Keywords

contour feather; Burmese amber; developmental constraint; evo-devo; Late Cretaceous

Funding

  1. Human Frontier Sciences Project [LT000728/2018]
  2. National Institutes of Health, USA [AR47364, AR60306]
  3. China Medical University (CMU)/ Hospital in Taiwan [5351285884]
  4. USC [5351285884]
  5. Integrative Stem Cell Center, CMU/ Hospital in Taiwan
  6. Taiwan Higher Education Sprout Project by the Ministry of Education in Taiwan
  7. Ministry of Science and Technology in Taiwan

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The rachises of extant feathers, composed of dense cortex and spongy internal medulla, are flexible and light, yet stiff enough to withstand the load required for flight, among other functions. Incomplete knowledge of early feathers prevents a full understanding of how cylindrical rachises have evolved. Bizarre feathers with unusually wide and flattened rachises, known as rachis-dominated feathers (RDFs), have been observed in fossil nonavian and avian theropods. Newly discovered RDFs embedded in early Late Cretaceous Burmese ambers (about 99 million year ago) suggest the unusually wide and flattened rachises mainly consist of a dorsal cortex, lacking a medulla and a ventral cortex. Coupled with findings on extant feather morphogenesis, known fossil RDFs were categorized into three morphotypes based on their rachidial configurations. For each morphotype, potential developmental scenarios were depicted by referring to the rachidial development in chickens, and relative stiffness of each morphotype was estimated through functional simulations. The results suggest rachises of RDFs are developmentally equivalent to a variety of immature stages of cylindrical rachises. Similar rachidial morphotypes documented in extant penguins suggest that the RDFs are not unique to Mesozoic theropods, although they are likely to have evolved independently in extant penguins.

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