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

Abstract

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.

Feather function and the evolution of birds

“…The potential for novelty or increased diversity in feather structures is demonstrated by feathers from early birds or relatives of early birds. Feathers evolved prior to birds (Ksepka, 2020), and morphological combinations of feather macro‐ and microstructure beyond those found in extant birds have been demonstrated by recent discoveries of novel Mesozoic feathers (Carroll, Chiappe & Bottjer, 2019; Wang et al ., 2020).…”

Description of natal down of the ostrich (Struthio camelus) and comparison with common quail (Coturnix coturnix): Developmental and evolutionary implications

“…Feather morphologies that do not fit easily into categories described from neognaths have also been reported in outgroups of Aves such as pterosaurs, ornithischians, and non‐avian theropods (Perrichot et al, 2008; Wang et al, 2020; Xu, 2020; Yang et al, 2019) but critically assessing the relationships among these feather types and their evolutionary implications requires a fuller understanding of extant variation in morphology and function. These unusual morphotypes include monofilamentous structures which resemble bristles or fur, branched structures that are both open‐ and closed‐vane, ribbon‐like structures, scale‐like structures with protruding filaments, brush‐like structures, branched structures without a standard rachis, and vaned feathers with an unusually wide rachis (Chen et al, 1998; Mayr et al, 2002; Perrichot et al, 2008; Qiang et al, 1998; Rauhut et al, 2012; Wang et al, 2020; Xing et al, 2020; Xu, 2020; Xu et al, 1999, 2000; Yang et al, 2019; Zhang et al, 2006; Zheng et al, 2009). Such morphotypes are sometimes described as dissimilar to those in extant birds, which raises questions about their diversity and the evolution of feathers.…”