A new Jurassic theropod from China documents a transitional step in the macrostructure of feathers

Abstract

Genuine fossils with exquisitely preserved plumage from the Late Jurassic and Early Cretaceous of northeastern China have recently revealed that bird-like theropod dinosaurs had long pennaceous feathers along their hindlimbs and may have used their four wings to glide or fly. Thus, it has been postulated that early bird flight might initially have involved four wings (Xu et al. Nature 421:335-340, 2003; Hu et al. Nature 461:640-643, 2009; Han et al. Nat Commun 5:4382, 2014). Here, we describe Serikornis sungei gen. et sp. nov., a new feathered theropod from the Tiaojishan Fm (Late Jurassic) of Liaoning Province, China. Its skeletal morphology suggests a ground-dwelling ecology with no flying adaptations. Our phylogenetic analysis places Serikornis, together with other Late Jurassic paravians from China, as a basal paravians, outside the Eumaniraptora clade. The tail of Serikornis is covered proximally by filaments and distally by slender rectrices. Thin symmetrical remiges lacking barbules are attached along its forelimbs and elongate hindlimb feathers extend up to its toes, suggesting that hindlimb remiges evolved in ground-dwelling maniraptorans before being co-opted to an arboreal lifestyle or flight.

Additional information on the primitive contour and wing feathering of paravian dinosaurs

“…Another paravian, Serikornis , is preserved with dramatically long, dense contour plumage (Lefèvre et al . ), consistent with the ‘shaggy’ appearance of paravian plumage suggested here. We hypothesize that the ‘shaggy’ plumage of Anchiornis affected physiology by altering heat retention (relative to the combination of modern down and contour feathers), decreasing water repellence, and decreasing aerodynamic efficiency by increasing drag in comparison to modern feathers, which are known to function effectively in such roles (Lucas & Stettenheim ).…”

“…Although Lefèvre et al . () suggested that barbules were entirely lacking in Serikornis , this is unlikely and represents an extreme interpretation, as discussed above where the presence of iridescence in paravians is consistent with the likely presence of barbules (Maia et al . ; Li et al .…”

“…The contour feathers of the paravian Serikornis were described in a similar manner with two morphotypes present, one with ‘bundles of filaments that are joined together proximally and remain nearly parallel as they extend distally’ and the other with a ‘well‐defined rachis and transversely inserted barbs’ (Lefèvre et al . ). Some of the epidermal structures found associated with Yi , another taxon that has been placed within Pennaraptora (in a sister clade to Paraves), are also similar to the contour feathers observed here for Anchiornis (Xu et al .…”

“…A similar observation was made for the paravian Serikornis (Lefèvre et al . ). The wing feathers on the forelimb and hindlimb of this taxon appear to be open‐vaned.…”

“…Evidence cited by Lefèvre et al . () includes a lack of visible barbs under optical and electron microscopy as well as a lack of basal expansions on the preserved barbs that indicated barbule insertions. However, such expansions are noted to be lacking in taxa believed to have barbules.…”