Data from: Variations of mesozoic feathers: insights from the morphogenesis of extant feather rachises

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 non-avian and avian theropods. Newly
discovered RDFs embedded in early Late Cretaceous Burmese ambers (~99 Ma)
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, though they
are likely to have evolved independently in extant penguins.