A new Chinese specimen indicates that 'protofeathers' in the Early Cretaceous theropod dinosaur Sinosauropteryx are degraded collagen fibres

Alleged primitive feathers or protofeathers in the theropod dinosaur Sinosauropteryx have potentially profound implications concerning feather morphogenesis, evolution offlight, dinosaur physiology and perhaps even the origin of birds, yet their existence has never been adequately documented. We report on a new specimen of Sinosauropteryx which shows that the integumental structures proposed as protofeathers are the remains of structural fibres that provide toughness. The preservation in the proximal tail area reveals an architecture of closely associated bands offibres parallel to the tail's long axis, which originate from the skin. In adjacent more exposed areas, the fibres are short, fragmented and disorganized. Fibres preserved dorsal to the neck and back and in the distal part of the tail are the remains of a stiffening system of a frill, peripheral to the body and extending from the head to the tip of the tail. These findings are confirmed in the holotype Sinosauropteryx and NIGP 127587. The fibres show a striking similarity to the structure and levels of organization of dermal collagen. The proposal that these fibres are protofeathers is dismissed.     

Dorsal fin in the white shark, Carcharodon carcharias: a dynamic stabilizer for fast swimming

Transverse sections of the skin in the dorsal fin of the white shark, Carcharodon carcharias, tiger shark, Galeocerdo cuvier, and spotted raggedtooth shark, Carcharias taurus, show large numbers of dermal fiber bundles, which extend from the body into the fin. The bundles are tightly grouped together in staggered formation (not arranged in a straight line or in rows). This arrangement of dermal fibers gives tensile strength without impeding fiber movement. Tangential sections indicate that the fibers in all three species are strained and lie at angles in excess of 60 degrees . Of the three species investigated the dermal fibers in C. carcharias are the most densely concentrated and extend furthest distally along the dorsal fin. The overall results indicate that the dorsal fin of C. carcharias functions as a dynamic stabilizer and that the dermal fibers are crucial to this role. The fibers work like riggings that stabilize a ship's mast. During fast swimming, when the problems of yaw and roll are greatest, hydrostatic pressure within the shark increases and the fibers around the body, including in the dorsal fin, become taut, thereby stiffening the fin. During slow swimming and feeding the hydrostatic pressure is reduced, the fibers are slackened, and the muscles are able to exert greater bending forces on the fin via the radials and ceratotrichia. In C. carcharias there is a trade-off for greater stiffness of the dorsal fin against flexibility.     

Selective biodegradation of keratin matrix in feather rachis reveals classic bioengineering

Flight necessitates that the feather rachis is extremely tough and light. Yet, the crucial filamentous hierarchy of the rachis is unknown—study hindered by the tight chemical bonding between the filaments and matrix. We used novel microbial biodegradation to delineate the fibres of the rachidial cortex in situ. It revealed the thickest keratin filaments known to date (factor >10), approximately 6 µm thick, extending predominantly axially but with a small outer circumferential component. Near-periodic thickened nodes of the fibres are staggered with those in adjacent fibres in two- and three-dimensional planes, creating a fibre–matrix texture with high attributes for crack stopping and resistance to transverse cutting. Close association of the fibre layer with the underlying ‘spongy’ medulloid pith indicates the potential for higher buckling loads and greater elastic recoil. Strikingly, the fibres are similar in dimensions and form to the free filaments of the feather vane and plumulaceous and embryonic down, the syncitial barbules, but, identified for the first time in 140+ years of study in a new location—as a major structural component of the rachis. Early in feather evolution, syncitial barbules were consolidated in a robust central rachis, definitively characterizing the avian lineage of keratin.     

Do feathered dinosaurs exist? Testing the hypothesis on neontological and paleontological evidence

The origin of birds and avian flight from within the archosaurian radiation has been among the most contentious issues in paleobiology. Although there is general agreement that birds are related to theropod dinosaurs at some level, debate centers on whether birds are derived directly from highly derived theropods, the current dogma, or from an earlier common ancestor lacking suites of derived anatomical characters. Recent discoveries from the Early Cretaceous of China have highlighted the debate, with claims of the discovery of all stages of feather evolution and ancestral birds (theropod dinosaurs), although the deposits are at least 25 million years younger than those containing the earliest known bird Archaeopteryx. In the first part of the study we examine the fossil evidence relating to alleged feather progenitors, commonly referred to as protofeathers, in these putative ancestors of birds. Our findings show no evidence for the existence of protofeathers and consequently no evidence in support of the follicular theory of the morphogenesis of the feather. Rather, based on histological studies of the integument of modern reptiles, which show complex patterns of the collagen fibers of the dermis, we conclude that "protofeathers" are probably the remains of collagenous fiber "meshworks" that reinforced the dinosaur integument. These "meshworks" of the skin frequently formed aberrant patterns resembling feathers as a consequence of decomposition. Our findings also draw support from new paleontological evidence. We describe integumental structures, very similar to "protofeathers," preserved within the rib area of a Psittacosaurus specimen from Nanjing, China, an ornithopod dinosaur unconnected with the ancestry of birds. These integumental structures show a strong resemblance to the collagenous fiber systems in the dermis of many animals. We also report the presence of scales in the forearm of the theropod ornithomimid (bird mimic) dinosaur, Pelecanimimus, from Spain. In the second part of the study we examine evidence relating to the most critical character thought to link birds to derived theropods, a tridactyl hand composed of digits 1-2-3. We maintain the evidence supports interpretation of bird wing digit identity as 2,3,4, which appears different from that in theropod dinosaurs. The phylogenetic significance of Chinese microraptors is also discussed, with respect to bird origins and flight origins. We suggest that a possible solution to the disparate data is that Aves plus bird-like maniraptoran theropods (e.g., microraptors and others) may be a separate clade, distinctive from the main lineage of Theropoda, a remnant of the early avian radiation, exhibiting all stages of flight and flightlessness.     

A New Helical Crossed-Fibre Structure of β-Keratin in Flight Feathers and Its Biomechanical Implications

The feather aerofoil is unequalled in nature. It is comprised of a central rachis, serial paired branches or barbs, from which arise further branches, the barbules. Barbs and barbules arise from the significantly thinner lateral walls (the epicortex) of the rachis and barbs respectively, as opposed to the thicker dorsal and ventral walls (the cortex). We hypothesized a microstructural design of the epicortex that would resist the vertical or shearing stresses. The microstructures of the cortex and epicortex of the rachis and barbs were investigated in several bird species by microbe-assisted selective disassembly and conventional methods via scanning electron microscopy. We report, preeminent of the finds, a novel system of crossed fibres (ranging from ∼100–800 nm in diameter), oppositely oriented in alternate layers of the epicortex in the rachis and barbs. It represents the first cross-fibre microstructure, not only for the feather but in keratin per se. The cortex of the barbs is comprised of syncitial barbule cells, definitive structural units shown in the rachidial cortex in a related study. The structural connection between the cortex of the rachis and barbs appears uninterrupted. A new model on feather microstructure incorporating the findings here and in the related study is presented. The helical fibre system found in the integument of a diverse range of invertebrates and vertebrates has been implicated in profound functional strategies, perhaps none more so potentially than in the aerofoil microstructure of the feather here, which is central to one of the marvels of nature, bird flight.     

An enigmatic early mesozoic dinosaur trackway from Zimbabwe

A trackway from Zimbabwe of probably the smallest dinosaur footprints recorded in Africa, is described and tentatively assigned to the Early Jurassic. The footprints are possibly those of a theropod and show strong negative (outward) rotation of the pes and are associated with manus prints. The shape of the footprints, unusual negative rotation, posterior curvature of digit IV and curious positioning of the manus prints in relation to the pes are enigmatic but somewhat reminiscent of Atreipus. Although a number of propositions are considered the most likely is that the animal was an immature dinosaur using a quadrupedal gait. A second trackway of slightly larger footprints of a bipedal theropod dinosaur is also recorded along with other diminutive tracks that suggest an early dinosaur assemblage, possibly dating from near the Trias‐sic‐Jurassic boundary.     

First investigation of the collagen D-band ultrastructure in fossilized vertebrate integument

The ultrastructure of dermal fibres of a 200Myr thunniform ichthyosaur, Ichthyosaurus, specifically the 67nm axial repeat D-banding of the fibrils, which characterizes collagen, is presented for the first time by means of scanning electron microscopy (SEM) analysis. The fragment of material investigated is part of previously described fossilized skin comprising an architecture of layers of oppositely oriented fibre bundles. The wider implication, as indicated by the extraordinary quality of preservation, is the robustness of the collagen molecule at the ultrastructural level, which presumably contributed to its survival during the initial processes of decomposition prior to mineralization. Investigation of the elemental composition of the sample by SEM-energy dispersive X-ray spectroscopy indicates that calcite and phosphate played important roles in the rapid mineralization and fine replication of the collagen fibres and fibrils. The exceedingly small sample used in the investigation and high level of information achieved indicate the potential for minimal damage to prized museum specimens; for example, ultrastructural investigations by SEM may be used to help resolve highly contentious questions, for example, 'protofeathers' in the Chinese dinosaurs.     

The ichthyosaur integument: skin fibers, a means for a strong, flexible and smooth skin

The ichthyosaur skin is examined in order to further our understanding of the adaptation of these animals to the aquatic medium and their locomotory efficiency. Softtissue structures in two excellently preserved specimens of the ichthyosaur Stenopterygius quadricissus and in a partial skull of Ichthyosaurus provide unique data on the integument of advanced or tunniform ichthyosaurs. A system of fibers of three classes based on thickness and in different levels of the integument covered almost the entire surface of the body. The thickest fibers are located deepest in the skin and the thinnest outermost. The latter consist of at least two superimposed layers of fine fibers that extend in opposing directions to form a lattice or orthogonal meshwork. The angles of these fibers vary between 25 ° and 75 ° to the long axis of the animals, depending on their location in the body. The fibers of the two other size classes, lying deeper in the tissue, were observed in single layers. The thickest fibers extend in near parallel rows approximately 60 °-80 ° to the long axis of the animal in the area near the midpoint of the body and 90 ° in the post-dorsal fin region. The intermediate-sized fibers were apparently oriented at ca. 50 °-75 ° to the animal's long axis and were regularly spaced. Of considerable interest is their attachment dorsally to longitudinal fibers. This contrasts with the general condition of helically arranged fibers in fast-swimming marine vertebrates such as tuna and sharks, but compares with the condition in sirenians. Fibers were observed in the dorsal and caudal fins but not in the limbs. The fibers in ichthyosaurs are the thickest so far noted in marine vertebrates. The presence of a complex system of fibers, which includes an orthogonal meshwork of the finest of these, suggests that creasing of the skin would have been minimized, a condition highly important in reducing drag during the locomotion of marine animals.