Mid-Cretaceous amber inclusions reveal morphogenesis of extinct rachis-dominated feathers

Abstract

We describe three-dimensionally preserved feathers in mid-Cretaceous Burmese amber that share macro-morphological similarities (e.g., proportionally wide rachis with a “medial stripe”) with lithic, two-dimensionally preserved rachis-dominated feathers, first recognized in the Jehol Biota. These feathers in amber reveal a unique ventrally concave and dorsoventrally thin rachis, and a dorsal groove (sometimes pigmented) that we identify as the “medial stripe” visible in many rachis-dominated rectrices of Mesozoic birds. The distally pennaceous portion of these feathers shows differentiated proximal and distal barbules, the latter with hooklets forming interlocking barbs. Micro-CT scans and transverse sections demonstrate the absence of histodifferentiated cortex and medullary pith of the rachis and barb rami. The highly differentiated barbules combined with the lack of obvious histodifferentiation of the barb rami or rachis suggests that these feathers could have been formed without the full suite and developmental interplay of intermediate filament alpha keratins and corneous beta-proteins that is employed in the cornification process of modern feathers. This study thus highlights how the development of these feathers might have differed from that of their modern counterparts, namely in the morphogenesis of the ventral components of the rachis and barb rami. We suggest that the concave ventral surface of the rachis of these Cretaceous feathers is not homologous with the ventral groove of modern rachises. Our study of these Burmese feathers also confirms previous claims, based on two-dimensional fossils, that they correspond to an extinct morphotype and it cautions about the common practice of extrapolating developmental aspects (and mechanical attributes) of modern feathers to those of stem birds (and their dinosaurian outgroups) because the latter need not to have developed through identical pathways.

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

“…1J, S1C, and S1D), producing three feather morphotypes that are unknown in extant feathers. A dorsal groove corresponding to the midline ridge reported in a previous work (Carroll et al 2019) is absent in any of the morphotype I specimens presented in this article. To understand the developmental etiology of the absent medulla and ventral cortex, we further subdivided morphotype I RDFs into two subcategories based on whether a midline cortical ridge is present (morphotype IA; Fig.…”

“…The detailed structures of these distinct rachises have traditionally been obscured by the carbonized nature of the compressed fossils (O'Connor et al 2012). Fortunately, recent studies of isolated feathers embedded in early Late Cretaceous Burmese ambers (about 98.8 ± 0.6 million years ago, Cenomanian [Shi et al 2012]) have revealed that the rachises of RDFs are not cylindrical (Xing et al 2018;Carroll et al 2019). Instead, these rachises consist of only the dorsal cortex, lacking the medulla and ventral cortex, and therefore the rachises appear as if they are ventrally opened (Xing et al 2018;Carroll et al 2019).…”

“…Fortunately, recent studies of isolated feathers embedded in early Late Cretaceous Burmese ambers (about 98.8 ± 0.6 million years ago, Cenomanian [Shi et al 2012]) have revealed that the rachises of RDFs are not cylindrical (Xing et al 2018;Carroll et al 2019). Instead, these rachises consist of only the dorsal cortex, lacking the medulla and ventral cortex, and therefore the rachises appear as if they are ventrally opened (Xing et al 2018;Carroll et al 2019). This unique structure raises the questions of how a rachis without a medulla and a ventral cortex could be formed, and whether this configuration represents a distinct phenotype that evolved independently of the common cylindrical rachidial phenotype (Carroll et al 2019).…”

“…Instead, these rachises consist of only the dorsal cortex, lacking the medulla and ventral cortex, and therefore the rachises appear as if they are ventrally opened (Xing et al 2018;Carroll et al 2019). This unique structure raises the questions of how a rachis without a medulla and a ventral cortex could be formed, and whether this configuration represents a distinct phenotype that evolved independently of the common cylindrical rachidial phenotype (Carroll et al 2019).…”

Cretaceous amniote integuments recorded through a taphonomic process unique to resins

“…Other semi-fluid preservative materials, like asphalt (highly viscous liquid or a semi-solid form of petroleum) or tar seeps, do not pull off vertebrate vestiture while allowing the survival of the individual 47 . Paired ornamental feathers of enantiornithine tails in the manner of their distal parts in relative live position (each pair) from the Burmese amber record have been reported 48,49 . Xing et al 48 commented that the paired feathers, which they found abundantly, could easily be removed, maybe as a sacrificial gesture in defensive behaviour.…”