The close contact between the skin and surrounding amber, paired with the mummified external appearance of the skin where it has shriveled across the surface of the vertebrae, suggest one of two scenarios. Either the tail bearer was dead and partially desiccated before encapsulation, or else it rapidly dried due to resin interactions. Early-stage drying is further supported by the limited amount of cloudy amber surrounding the tail ( Figures 1 C and S2 ), which is a preservational feature related to decay products or moisture interacting with resin []. However, drying and resin impregnation were not sufficient to preserve cellular detail in the soft tissues. Based on the clays observed where bone breaches the amber surface, skeletal material was likely exposed on the surface after resin polymerization. The bone has been partially dissolved and infilled with clay from the surrounding matrix [], much like insect body cavities in this deposit ( Figure S2 A). Presence of Fewithin the carbonized remains suggests that organic components were trapped early and remained undisturbed by subsequent events. Further taphonomic constraints are difficult to infer. It is unclear whether the lack of melanosomes within the keratin sheets of the surrounding feathers ( Figures 2 B and S3 ) might provide additional taphonomic information or whether their absence results from weakly pigmented feathers or the small sample area available for SEM analyses. Artificial maturation experiments [] have shown the breakdown of modern melanosomes at a range of temperatures, but this work was conducted at temperatures that would also degrade amber. The taphonomic pathway that led to the preservation of DIP-V-15103 is not entirely clear, but it suggests promise for more detailed examinations of organics or pigmentation in vertebrate inclusions.

The tail within DIP-V-15103 is visible to the naked eye as an elongate and gently curved structure (length = 36.73 mm). A dense covering of feathers protrudes from the tail, obscuring underlying details, so Synchrotron Radiation (SR) X-ray phase-contrast μCT scanning was employed to examine concealed osteological and soft tissue features ( Figure 1 ). Soft tissues—presumably muscles, ligaments, and skin—are visible sporadically through the plumage, clinging to the bones in a manner suggestive of the desiccation common to other vertebrate remains in amber []. These tissues have largely been reduced to a carbon film, retaining only traces of their original chemical composition. Based on analyses further described in the Supplemental Information , SR μ-XFI shows that iron is present in the carbonized soft tissues and as a series of fine linear features corresponding to exposed plumage ( Figure 2 ). Copper is slightly more abundant in amber containing plumage, but this signal is cryptic and not a clear indicator for preserved pigments. Elements such as Ca, Sc, Zn, Ti, Ge, and Mn appear to be associated with clay minerals filling voids in the amber. We derived the valence state of iron in the sample qualitatively by comparison to the standard XAS of Fe foil, Fe, Fe, and FeO. Our calculations indicate that more than 80% of iron in the sample is ferrous (Fe). Similar measurements have been made on vessels preserved within Tyrannosaurus and Brachylophosaurus bones and have been interpreted as indicating the presence of goethite and biogenic iron oxides produced from hemoglobin decomposition []. The presence of large quantities of Fein DIP-V-15103 suggests that some primary iron from hemoglobin or ferritin remains trapped within the inclusion. SEM analyses show that pyrite (FeS) is also present, but not as a significant contributor to the distribution of iron within the specimen ( Figure S3 ).

(A) Elemental maps and region of interest (ROI) image for exposed soft tissue preservation in DIP-V-15103; black carbon film surrounds clay minerals infilling void between vertebrae or partially replacing them; milky amber related to decay surrounds vertebrae and plumage (ROI prior to clay flake removal is better visible in Figure S3 H).

Arrowheads in (A) and (D) mark rachis of feather featured in Figure 4 A. Asterisks in (A) and (C) indicate carbonized film (soft tissue) exposure. Arrows in (B) and (E)–(G) indicate shared landmark, plus bubbles exaggerating rachis dimensions; brackets in (G) and (H) delineate two vertebrae with clear transverse expansion and curvature of tail at articulation. Abbreviations for feather rachises: d, dorsal; dl, dorsalmost lateral; vl, ventralmost lateral; v, ventral. Scale bars, 5 mm in (A), (B), (D), and (F) and 2 mm in (C), (E), (G), and (H). See also Figure S2

DIP-V-15103 is interpreted as a non-avialan coelurosaur tail: its vertebral profiles and estimated length rule out avebrevicaudan birds, oviraptorosaurs, and scansoriopterygians—lineages generally characterized by a short caudal series with subequal centra [], with the exception of Epidendrosaurus. The branched feathers have a weak pennaceous arrangement of barbs consistent with non-avialan coelurosaurs, particularly paravians. Although the feathers are somewhat pennaceous, none of the observed osteological features preclude a compsognathid [] affinity. The presence of pennaceous feathers in pairs down the length of the tail may point toward a source within Pennaraptora [], placing a lower limit on the specimen’s phylogenetic position. However, the distribution and shape of the feathers only strongly supports placement crownward of basal coelurosaurs, such as tyrannosaurids and compsognathids. In terms of an upper limit, the specimen can be confidently excluded from Pygostylia; in addition, it can likely be excluded from the long-tailed birds, based on pronounced ventral grooves on the vertebral centra. Additional taxonomic assessment details are provided in the Supplemental Information

SR X-ray μCT scanning of DIP-V-15103 ( Figure 1 ) revealed that soft tissues have a density insufficiently different from the partially replaced skeletal elements to permit X-ray imaging and virtual dissection of osteology alone. Consequently, many diagnostic and comparative osteological details remain obscured. However, two vertebrae are clearly delineated ventrally ( Figures 1 F–1H). Extrapolating lengths of these vertebrae, the preserved tail section contains at least eight full vertebrae and part of a ninth. The vertebrae are elongate, with anteroposterior lengths double the maximum diameter of the tail ( Table S1 ). Vertebral proportions and tail flexion preclude membership within the Pygostylia [as in]. Even with the skin adpressed to the bony surface, no features other than the grooved ventral sulci of two centra are clearly visible. This lack of topography suggests that the vertebrae lack prominent neural arches, transverse processes, or hemal arches. Therefore, the preserved segment is only a small mid to distal portion of what was likely a relatively long tail, with the total caudal vertebral count not reasonably less than 15, and likely greater than 25. Based on specimen size, it also seems likely that the tail belonged to a juvenile.

Plumage

29 O’Connor J.K.

Sun C.

Xu X.

Wang X.

Zhou Z. A new species of Jeholornis with complete caudal integument. 9 Xu X.

Zhou Z.

Dudley R.

Mackem S.

Chuong C.M.

Erickson G.M.

Varricchio D.J. An integrative approach to understanding bird origins. Both SR X-ray μCT reconstruction and standard light microscopy confirm feather attachments throughout the preserved tail length ( Figure 1 ). A bilaterally paired series of posterodorsally oriented feathers extends from the dorsal midline ( Figures 1 D and 1E). Another row of feathers is present at mid-height on each side of the tail, with feathers extending posterolaterally at roughly 45° to its long axis ( Figures 1 D–1G). These follicle pairs appear evenly spaced along the length of the tail. Where the outlines of two vertebral centra are visible, follicles are located at the mid-lengths of centra and at intervertebral joints. Ventral plumage is sparse, consisting of fine feathers that follow the long axis of the tail closely ( Figures 1 B, 1G, and 1H). Overall, the plumage forms laterally directed keels on either side of the vertebral column, providing a unique opportunity to observe feather counts and orientations within the contour-like caudal plumage of a coelurosaur. DIP-V-15103 does not show the splaying of large pennaceous rectrices observed alongside the posteriormost caudals of long-tailed birds []. Either splaying was absent in this individual or it was only present caudally, beyond the preserved region. Nevertheless, the arrangement of feathers into lateral keels in DIP-V-15103 is similar to the paravian tail fan or frond []. Such arrangements, composed of different feather types, can occur not just at the distal tip but also along the entire length of the tail. Amber preservation suggests that the tail fans and fronds preserved in paravians are not merely a taphonomic artifact of compression.

8 Xu X.

Zheng X.

You H. Exceptional dinosaur fossils show ontogenetic development of early feathers. 14 Xing L.

McKellar R.C.

Wang M.

Bai M.

O’Connor J.K.

Benton M.J.

Zhang J.

Wang Y.

Tseng K.

Lockley M.G.

et al. Mummified precocial bird wings in mid-Cretaceous Burmese amber. If DIP-V-15103 indeed represents a juvenile coelurosaur tail, the feathers most likely characterize adult plumage; however, there is some room for uncertainty. Basal taxa within Pennaraptora, such as Similicaudipteryx, are thought to have undergone dramatic molts that affected the tail region [], while some basal members of Pygostylia have precocial juveniles with adult-like plumage []. The pennaceous feathers and barbules of DIP-V-15103 suggest an adult-like plumage, in which feathers would not have been replaced by different morphotypes in subsequent molts. Alternatively, the feather bearer may not have conformed to the molt patterns found in modern birds.

Figure 3 Photomicrographs of DIP-V-15103 Plumage Show full caption (A) Pale ventral feather in transmitted light (arrow indicates rachis apex). (B) Dark-field image of (A), highlighting structure and visible color. (C) Dark dorsal feather in transmitted light, apex toward bottom of image. (D) Base of ventral feather (arrow) with weakly developed rachis. (E) Pigment distribution and microstructure of barbules in (C), with white lines pointing to pigmented regions of barbules. (F–H) Barbule structure variation and pigmentation, among barbs, and ‘rachis’ with rachidial barbules (near arrows); images from apical, mid-feather, and basal positions respectively. Scale bars, 1 mm in (A), 0.5 mm in (B)–(E), and 0.25 mm in (F)–(H). See also Figure S4 Figure 4 DIP-V-15103 Structural Overview and Feather Evolutionary-Developmental Model Fit Show full caption (A and B) Overview of largest and most planar feather on tail (dorsal series, anterior end), with matching interpretive diagram of barbs and barbules. Barbules are omitted on upper side and on one barb section (near black arrow) to show rachidial barbules and structure; white arrow indicates follicle. 5 Prum R.O. Development and evolutionary origin of feathers. 12 McKellar R.C.

Chatterton B.D.E.

Wolfe A.P.

Currie P.J. A diverse assemblage of Late Cretaceous dinosaur and bird feathers from Canadian amber. (C) Evolutionary-developmental model and placement of new amber specimen. Brown denotes calamus, blue denotes barb ramus, red denotes barbule, and purple denotes rachis [as in]. Scale bars, 1 mm in (A) and (B). The feathers of DIP-V-15103 are similar to each other in morphology, regardless of position on the tail ( Figures 3 and S4 ). All preserved feathers have a weakly defined rachis that is nearly indistinguishable from the barb rami apically and that is slightly thickened basally ( Figure 3 ). Both rachises and barbs are sub-cylindrical in cross-section. Although the rachis thickens basally, the maximum diameter near the follicle is approximately three times that of an adjacent barb ramus ( Figures 3 and S4 ). Feathers near the anterior end of the dorsal series have the greatest basal expansion observed among the plumage, with rachis widths approaching 60 μm ( Figures 3 4 A, and 4 B). Rachises among these feathers become as narrow as 18 μm in apical positions, while barb rami have widths ranging from 15 to 23 μm. Within individual feathers, barbs are positioned alternately along the rachis, approaching an opposite arrangement basally, with wide spacing between and a weak planar arrangement ( Figure 4 ). Flexion within the amber indicates that barb rami were flexible, and the rachis itself was somewhat flexible. The open, flexible structure of these feathers is more analogous to modern ornamental feathers than to flight feathers, showing structural similarities to the distal components of contour feathers in certain Anseriformes ( Figures 3 and S5 ). The paired feather arrangement is similar to rectrices in modern birds, suggesting that tracts had become established in basal tail plumage before pygostyle development, with tail plumage becoming more specialized over time. If the entire tail bore plumage similar to that trapped in DIP-V-15103, the feather bearer would likely have been incapable of flight.

4, The feathers of DIP-V-15103 display exquisitely preserved barbules. Strikingly, the simple barbules branch not only within individual barbs but also unmodified from the rachis ( Figures 3 S4 G, and S4H). In this regard, the feathers are comparable to the contours of many modern birds, which also possess some barbules that originate from the rachis (rachidial barbules), although usually from the proximal barb base and in reduced form. In DIP-V-15103, barbules branch in an evenly spaced, paired, and nearly symmetrical manner. This pattern remains consistent in both proximal and distal barbules, from proximal to distal barbs, and along the rachis. Barbules are consistently blade shaped, with pigmentation outlining five basal cells followed by a poorly differentiated pennulum lacking discernible nodes or nodal protrusions ( Figures 3 E–3H). Close spacing between barbules, combined with the orientation of their flattened surfaces (parallel to the feather’s long axis), yields open-vaned feathers that are largely pennaceous.

5 Prum R.O. Development and evolutionary origin of feathers. 5 Prum R.O. Development and evolutionary origin of feathers. 19 Alibardi L. Cells of embryonic and regenerating germinal layers within barb ridges: implication for the development, evolution and diversification of feathers. 30 Alibardi L.

Sawyer R.H. Cell structure of developing downfeathers in the zebrafinch with emphasis on barb ridge morphogenesis. 6 Xu X.

Zhou Z.

Prum R.O. Branched integumental structures in Sinornithosaurus and the origin of feathers. 8 Xu X.

Zheng X.

You H. Exceptional dinosaur fossils show ontogenetic development of early feathers. 31 Xu X.

Tang Z.L.

Wang X.L. A therizinosauroid dinosaur with integumentary structures from China. The weakly developed rachis and contiguous barbule branching in DIP-V-15103 represents a novel combination among theropods. Within the evolutionary developmental model of feathers [], DIP-V-15103 appears to be intermediate between stages IIIa (rachis with naked barbs) and IIIb (barbs with barbules, lacking a rachis), but it does not exactly fit stage IIIa+b (rachis with barbs bearing barbules) ( Figure 4 C). In DIP-V-15103, barbs exhibit an alternating arrangement along a poorly defined rachis, with nearly dichotomous branching apically, and barbules continue along the surface of the rachis and barbs. The weakly developed rachis appears to have formed through fusion of individual barbs that already possessed barbules (stage IIIb) instead of fusion of naked barbs (stage IIIa) []. The barb branching pattern continues largely uninterrupted toward the follicle, as do the pervasive, undifferentiated barbules. Unless the condition observed in DIP-V-15103 represents a secondary reduction of the rachis, the evolutionary pathway for feathers in this coelurosaur may have been through stage IIIb (barbs with barbules), not stage IIIa (fusion of naked barbs). Cytological observations of barbule development along the barb vane ridge support the evolutionary coupling of barbs and barbules []. Feather morphology of DIP-V-15103 contrasts with the reduced rachis and long, naked, filamentous barbs in the branched caudal plumage of the dromaeosaurid Sinornithosaurus [] and the therizinosauroid Beipiaosaurus []. This suggests either a greater diversity of tail plumage in coelurosaurians than previously suspected or a simplified form of more-derived pennaceous feathers in DIP-V-15103.

32 Alibardi L. Cell structure of developing barbs and barbules in downfeathers of the chick: Central role of barb ridge morphogenesis for the evolution of feathers. 33 Alibardi L. Cell interactions in barb ridges of developing chick downfeather and the origin of feather branching. 5 Prum R.O. Development and evolutionary origin of feathers. 4 O’Connor J.K.

Chiappe L.M.

Chuong C.M.

Bottjer D.J.

You H. Homology and potential cellular and molecular mechanisms for the development of unique feather morphologies in early birds. 5 Prum R.O. Development and evolutionary origin of feathers. 10 Chen C.-F.

Foley J.

Tang P.C.

Li A.

Jiang T.X.

Wu P.

Widelitz R.B.

Chuong C.M. Development, regeneration, and evolution of feathers. The unusual barbule configuration in DIP-V-15103 suggests that barbules were primitively distributed evenly throughout the length of the feather and only later became restricted to the barbs and proximal rachis and oriented so that their edges face the feather surfaces, as in modern avians. In modern birds, barbule cells originate in the subperiderm and merge into a syncytium on either side of the barb vane ridge []. The symmetrical arrangement of barbules along the barbs in DIP-V-15103 implies symmetry of barbule cells across the barb vane ridge. The contiguous barbule branching along the rachis probably occurs along the barb vane ridge leading to the apicalmost barb. In the lineage leading to birds, the barbules became spatially restricted to the barbs and the proximal portion of the rachis, presumably to accommodate increasing barb number and density related to rigid pennaceous feathers (stage IIIa+b and/or stage IV) []. Alternatively, the barbule pattern in DIP-V-15103 may represent a highly derived and potentially experimental character state unrelated to the avian lineage. Whichever the case, DIP-V-15103 suggests that non-avialan theropods had a greater variety of feather forms than predicted from developmental phenotypes in modern feathers [].

34 Thomas D.B.

Nascimbene P.C.

Dove C.J.

Grimaldi D.A.

James H.F. Seeking carotenoid pigments in amber-preserved fossil feathers. Traces of pigmentation exist within the entombed plumage. Discrete bands corresponding to basal cells within each barbule are visible due to loosely confined pigments ( Figures 3 C–3H). Pigmentation is more pronounced within apical portions of each barbule and in the barb rami and rachis of dorsal feathers ( Figures 1 C and S4 H). Coloration varies little within individual feathers, but dorsal plumage is significantly darker than ventral plumage. Preserved coloration suggests a chestnut brown dorsal surface, contrasting against pale or almost white ventral plumage ( Figures 1 A–1C and S4 A–S4D); however, taphonomic impacts on visible colors are unclear. A small section of the pale ventral plumage was available for SEM observations. No melanosomes were observed, suggesting that ventral plumage was either unpigmented or pigmented through alternative means, such as carotenoids []. Keratin sheets are visible within the feather layer, displaying the distinctive, porous, laminar structure also observed in modern avian barbules under SEM ( Figures S2 A and S2B).