DOI: 10.5061/dryad.zcrjdfnck
title: Data from: Morphology and distribution of scales, dermal ossifications, and other non-feather integumentary structures in non-avialan theropod dinosaurs

description: Modern birds are typified by the presence of feathers, complex
 evolutionary innovations that were already widespread in the group of
 theropod dinosaurs (Maniraptoriformes) that include crown Aves. Squamous
 or scaly reptilian-like skin is, however, considered the plesiomorphic
 condition for theropods and dinosaurs more broadly. Here, we review the
 morphology and distribution of non-feathered integumentary structures in
 non-avialan theropods covering squamous skin and naked skin as well as
 dermal ossifications. The integumentary record of non-averostran theropods
 is limited to tracks, which ubiquitously show a covering of tiny
 reticulate scales on the plantar surface of the pes. This is consistent
 also with younger averostran body fossils, which confirm an arthral
 arrangement of the digital pads. Among averostrans, squamous skin is
 confirmed in Ceratosauria (Carnotaurus), Allosauroidea (Allosaurus,
 Concavenator, Lourinhanosaurus), Compsognathidae (Juravenator), and
 Tyrannosauroidea (Santanaraptor, Albertosaurus, Daspletosaurus,
 Gorgosaurus, Tarbosaurus, Tyrannosaurus), whereas dermal ossifications
 consisting of sagittate and mosaic osteoderms are restricted to
 Ceratosaurus. Naked, non-scale bearing skin is found in the contentious
 tetanuran Sciurumimus, possibly ornithomimosaurians (Pelecanimimus) and
 tyrannosauroids (Santanaraptor), and also on the patagia of
 scansoriopterygids (Ambopteryx, Yi). Scales are surprisingly conservative
 among non-avialan theropods compared to some dinosaurian groups (e.g.,
 hadrosaurids); however, the limited preservation of tegument on most
 specimens hinders further interrogation. Scale patterns vary between
 and/or within body regions in Carnotaurus, Concavenator and Juravenator,
 and include polarised, snake-like ventral scales on the tail of the latter
 two genera. Unusual but more uniformly-distributed patterning also occurs
 in Tyrannosaurus, whereas feature scales are present only in Albertosaurus
 and Carnotaurus. Few theropods currently show compelling evidence for the
 cooccurrence of scales and feathers (e.g., Juravenator, Sinornithosaurus),
 although reticulate scales were probably retained on the mani and pedes of
 many theropods with a heavy plumage. Feathers and filamentous structures
 appear to have replaced widespread scaly integuments in maniraptorans.
 Theropod skin, and that of dinosaurs more broadly, remains a virtually
 untapped area of study and the appropriation of commonly-used techniques
 in other palaeontological fields to the study of skin holds great promise
 for future insights into the biology, taphonomy and relationships of these
 extinct animals.
description: To perform our
 review, we studied the non-feathered epidermal structures and dermal
 ossifications in 25 non-avialan theropod taxa bracketed phylogenetically
 between the early-diverging Theropoda <i>Tawa alae</i>
 (Nesbitt <i>et al.</i>, 2009) and the early-diverging avialan
 <i>Archaeopteryx lithographica</i> (Elzanowski, 2001;
 Christiansen & Bonde, 2004; Mayr, Pohl, & Peters, 2005;
 Foth, Tischlinger, & Rauhut, 2014). Only postcranial osteoderms
 were considered in this study. With the possible exception of
 <i>Tyrannosaurus</i> (the epipostorbital; see Carr, 2020), the
 cranial dermal ossifications reported in some theropods such as
 abelisaurids (Carrano & Sampson, 2008) are more cornified tissues,
 armour-like dermis and other types of dermal ornamentations than true
 osteoderms (Carr <i>et al.</i>, 2017; Delcourt, 2018).
 Specimens belonging to 22 taxa deposited in scientific collections of
 France, Germany, Portugal, Spain, Argentina, Brazil, USA, Canada, and
 China were examined first-hand and anatomical observations were assisted
 with the use of a digital camera and/or a digital microscope AM411T
 Dino-Lite Pro. The original integument was observed in the majority of
 theropod taxa, but high-resolution casts of skin were also used for
 specimens belonging to four non-maniraptoriform avetheropods (i.e.,
 <i>Allosaurus</i>, <i>Albertosaurus</i>,
 <i>Tarbosaurus</i> and <i>Tyrannosaurus</i>).
 Descriptions and illustrations in the literature were relied upon for
 three additional taxa (i.e., <i>Sciurumimus</i>,
 <i>Ornithomimus</i>, and <i>Epidendrosaurus</i>),
 including high-resolution photographs provided by two colleagues for one
 of them (i.e., <i>Sciurumimus</i>). Three-dimensional (3D)
 models of skin were generated for <i>Carnotaurus</i> (MACN-CH
 894) using photogrammetric data and the software Agisoft Photoscan 1.3.4,
 as well as for <i>Allosaurus</i> (UMNH VP C481) using a
 Creaform Go!SCAN 20 surface scanner at 0.2 mm resolution. The 3D models
 were exported, oriented, and scaled in Meshlab version 1.3.4BETA (Cignoni
 <i>et al.</i>, 2008) and deposited in MorphoMuseuM (https://morphomuseum.com/) where they are freely downloadable (see Hendrickx & Bell 2021). Laser-Stimulated Fluorescence (LSF) imaging was used to describe preserved integument using standard protocols (Kaye <i>et al.</i>, 2015; Wang <i>et al.</i>, 2017). Morphology of the non-avialan theropod skin was compared to that of other dinosaurs and living amniotes based on personal examination and high-quality photographs of the skin of ornithischian and sauropodomorph dinosaurs, living birds, crocodiles, squamates and turtles. To review the distribution and evolution of epidermal structures and dermal ossifications in dinosaurs, we mapped 71 tegument-based characters coded for 67 archosauriform taxa, among which 40 taxa were examined first hand (See Excel file "Dataset_theropod_integument_Aug_2021.xlsx"; Mesquite files "Mapping_Integument_characters_Aug_2021.nex" and "Mapping_Integument_characters_Aug_2021_no_DIP.nex"; TNT files "Mapping_Integument_characters_Aug_2021.tnt"and "Mapping_Integument_characters_Aug_2021_no_DIP.tnt"). Although characters on epidermal structures (i.e., smooth skin, scales, osteoderms and feathers) are incorporated in some data matrices, the majority of the integument-based characters we propose are new (See Excel file "Dataset_theropod_integument_Aug_2021.xlsx"). Taxa were bracketed phylogenetically between early-diverging archosauriforms (Campione <i>et al.</i> (2020) and references therein) and the early-diverging avialan <i>Archaeopteryx lithographica</i> (e.g., Foth, Tischlinger, & Rauhut, 2014). Non-ornithodiran archosauriforms were chosen as the outgroup over pterosaurs because the latter already show particularly derived integuments such as a non-scaly skin forming an airfoil and complex filamentous structures (Bakhurina & Unwin, 1995; Frey <i>et al.</i>, 2003; Kellner <i>et al.</i>, 2010; Barrett <i>et al.</i>, 2015; Yang <i>et al.</i>, 2019) whereas basalmost archosauriforms show the plesiomorphic condition of having body scales (Reisz & Sues, 2000; Campione <i>et al.</i>, 2020). Sauropodomorphs and ornithischians were also excluded as the outgroups because their phylogenetic distribution among dinosaurs is unsettled (see Baron, Norman, & Barrett, 2017; Langer <i>et al.</i>, 2017; Müller <i>et al.</i>, 2018; Baron, 2021) and their most-basal members preserving integument (monofilaments in the heterodontosaurid <i>Tianyulong</i>, polygonal basement scales in the sauropod <i>Mamenchisaurus</i>; (Ouyang & Ye, 2002; Zheng <i>et al.</i>, 2009) have very different epidermal structures. The distribution of integument-based characters was visualized on six topological trees representative of alternative phylogenetic hypotheses for non-avian theropod evolution (See Mesquite file "Mapping_Integument_characters_Aug_2021_no_DIP.nex"). These informal supertrees were built using Mesquite 3.2 (Maddison & Maddison, 2017) following the results obtained by Godefroit <i>et al.</i> (2014) and Boyd (2015) for ornithischians, Müller <i>et al.</i> (2018) for non-tetanuran saurischians, Rauhut <i>et al.</i> (2012) for non-coelurosaur tetanurans, Delcourt and Grillo (2018) for tyrannosauroids, and Pei <i>et al.</i> (2020) for maniraptoriforms (see Pittman <i>et al.</i>, 2020). Variations in the topology result from the differing placement of: 1) <i>Juravenator</i> as a non-avetheropod tetanuran (Tree1; Rauhut <i>et al.</i>, 2012; Foth <i>et al.</i>, 2020) or a compsognathid coelurosaur (Tree2; Pei <i>et al.</i>, 2020); 2) sauropodomorphs within the clade of Saurischia (Tree1; e.g., Langer <i>et al.</i>, 2017; Müller <i>et al.</i>, 2018) or as a sister-clade of Ornithoscelida (Tree3; i.e., Theropoda + Ornithischia; Baron, Norman, & Barrett, 2017); 3) anchiornithines as early-branching avialans (Tree1; Pei <i>et al.</i>, 2020) or troodontids (Tree4; Brusatte <i>et al.</i>, 2014); and 4) troodontids among Deinonychosauria (Tree1; Turner, Makovicky, & Norell, 2012; Pei <i>et al.</i>, 2020), as the sister clade of Dromaeosauridae + Averaptora (Tree5; Motta <i>et al.</i>, 2020), or as the sister clade of Avialae (Tree6; Avialae = Anchiornithinae + <i>Archaeopteryx</i>; Cau <i>et al.</i>, 2017; Foth & Rauhut, 2017). Our preferred topology (Tree1) follows a phylogenetic tree in which Sauropodomorpha and Theropoda form the clade Saurischia, <i>Juravenator</i> and <i>Sciurumimus</i> are classified as early-branching non-avetheropod Tetanurae (n.b., the integument of <i>Juravenator</i> is, however, described in the section on Compsognathidae), anchiornithines are placed as the earliest birds, and Deinonychosauria is resolved. Although included in the data matrix for comparative purposes, a large unnamed tyrannosauroid STM 1-5 (Xu <i>et al.</i>, 2010) and the contentious maniraptoran <i>Yixianosaurus longimanus</i> (Xu & Wang, 2003) were excluded in the analyses because their phylogenetic distribution among coelurosaurians are unsettled (Dececchi, Larsson, & Hone, 2012; Xu, Sullivan, & Wang, 2013; Lambertz, 2017). Conversely, because of its importance in our understanding of feather evolution (Xing <i>et al.</i>, 2016; Lambertz, 2017) and while its position among coelurosaurs is unsettled, the specimen DIP-V-15103 was included in the analysis following the topology of our preferred tree topology and classified as a basally branching non-neocoelurosaur coelurosaur (Tree0). Character distributions for integument-based features were visualized on each tree WinClada 1.00.08 (Nixon, 2002) based on the Nexus file created with Mesquite 3.6.1 (See Mesquite files "Mapping_Integument_characters_Aug_2021.nex" and "Mapping_Integument_characters_Aug_2021_no_DIP.nex"). An ancestral state reconstruction analysis was additionally performed in Mesquite v. 3.6.1 using the parsimony criterion with the integument based data matrix and the six aforementioned tree topologies (See Mesquite File "Ancestral_state_reconstruction_analysis_Aug_2021.nex"). The ParsimonyUnordered model was used as it is the software's standard parsimony mapping for categorical data. This calculates the most parsimonious ancestral states at the nodes of the tree assuming one step per state change (unordered or Fitch parsimony). In this analysis, the ichnotaxa <i>Grallator</i> and <i>Eubrontes</i> (e.g., Gatesy, 2001; Demathieu <i>et al.</i>, 2002; Milner <i>et al.</i>, 2006a) as well as the non-avialan coelurosaur DIP-V-15103 (Xing <i>et al.</i>, 2016), the oviraptorosaur <i>Ningyuansaurus</i> (Ji <i>et al.</i>, 2012) and the microraptorine IVPP V13476 (Xu & Li, 2016) were excluded as their phylogenetic affinities are insufficiently resolved.   References Bakhurina, N.N. & Unwin, D.M. (1995) A preliminary report on the evidence for ‘hair’ in <i>Sordes pilosus</i>, an Upper Jurassic pterosaur from Middle Asia. In <i>Sixth Symp. Mesozoic Terrestrial Ecosystems and Biota, Short Papers 1995</i> pp. 79–82. Beijing, China. Baron, M.G. (2021) Difficulties with the origin of dinosaurs: a short comment on the current debate. <i>Palaeovertebrata</i> <b>43</b>, 1–5. Baron, M.G., Norman, D.B. & Barrett, P.M. (2017) A new hypothesis of dinosaur relationships and early dinosaur evolution. <i>Nature</i> <b>543</b>, 501–506. Barrett, P.M., Evans, D.C. & Campione, N.E. (2015) Evolution of dinosaur epidermal structures. <i>Biology Letters</i> <b>11</b>, 20150229. Royal Society. Boyd, C.A. (2015) The systematic relationships and biogeographic history of ornithischian dinosaurs. <i>PeerJ</i> <b>3</b>, e1523. Brusatte, S.L., Lloyd, G.T., Wang, S.C. & Norell, M.A. (2014) Gradual assembly of avian body plan culminated in rapid rates of evolution across the dinosaur-bird transition. <i>Current Biology</i> <b>24</b>, 2386–2392. Campione, N.E., Barrett, P.M. & Evans, D.C. (2020) On the ancestry of feathers in Mesozoic dinosaurs. In <i>The Evolution of Feathers: From Their Origin to the Present</i> (eds C. Foth & O.W.M. Rauhut), pp. 213–243. Springer International Publishing, Cham. Carr, T.D. (2020) A high-resolution growth series of Tyrannosaurus rex obtained from multiple lines of evidence. <i>PeerJ</i> <b>8</b>, e9192. PeerJ Inc. Carr, T.D., Varricchio, D.J., Sedlmayr, J.C., Roberts, E.M. & Moore, J.R. (2017) A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system. <i>Scientific Reports</i> <b>7</b>, 1–11. Nature Publishing Group. Carrano, M.T. & Sampson, S.D. (2008) The phylogeny of Ceratosauria (Dinosauria: Theropoda). <i>Journal of Systematic Palaeontology</i> <b>6</b>, 183–236. Cau, A., Beyrand, V., Voeten, D.F.A.E., Fernandez, V., Tafforeau, P., Stein, K., Barsbold, R., Tsogtbaatar, K., Currie, P.J. & Godefroit, P. (2017) Synchrotron scanning reveals amphibious ecomorphology in a new clade of bird-like dinosaurs. <i>Nature</i> <b>552</b>, 395–399. Christiansen, P. & Bonde, N. (2004) Body plumage in <i>Archaeopteryx</i>: a review, and new evidence from the Berlin specimen. <i>Comptes Rendus Palevol</i> <b>3</b>, 99–118. Cignoni, P., Callieri, M., Corsini, M., Dellepiane, M., Ganovelli, F. & Ranzuglia, G. (2008) MeshLab: an open-source mesh processing tool. In <i>Eurographics Italian Chapter Conference</i> (eds V. Scarano, R.D. Chiara & U. Erra), pp. 129–136. The Eurographics Association, Salerno, Italy. Dececchi, T.A., Larsson, H.C.E. & Hone, D. (2012) <i>Yixianosaurus longimanus</i> (Theropoda: Dinosauria) and its bearing on the evolution of Maniraptora and ecology of the Jehol fauna. <i>Vertebrata PalAsiatica</i> <b>50</b>, 111–139. Delcourt, R. (2018) Ceratosaur palaeobiology: new insights on evolution and ecology of the southern rulers. <i>Scientific Reports</i> <b>8</b>, 1–12. Delcourt, R. & Grillo, O.N. (2018) Tyrannosauroids from the Southern Hemisphere: Implications for biogeography, evolution, and taxonomy. <i>Palaeogeography, Palaeoclimatology, Palaeoecology</i>. Demathieu, G., Gand, G., Sciau, J., Freytet, P. & Garric, J. (2002) Les traces de pas de dinosaures et autres archosaures du Lias inférieur des Grands Causses, Sud de la France. <i>Palaeovertebrata</i> <b>31</b>, 1–143. Elzanowski, A. (2001) A new genus and species for the largest specimen of <i>Archaeopteryx</i>. <i>Acta Palaeontologica Polonica</i> <b>46</b>, 519–532. Foth, C., Haug, C., Haug, J.T., Tischlinger, H. & Rauhut, O.W.M. (2020) Two of a feather: a comparison of the preserved integument in the juvenile theropod dinosaurs <i>Sciurumimus</i> and <i>Juravenator</i> from the Kimmeridgian Torleite Formation of southern Germany. In <i>The Evolution of Feathers: From Their Origin to the Present</i> (eds C. Foth & O.W.M. Rauhut), pp. 79–101. Springer International Publishing, Cham. Foth, C. & Rauhut, O.W.M. (2017) Re-evaluation of the Haarlem <i>Archaeopteryx</i> and the radiation of maniraptoran theropod dinosaurs. <i>BMC Evolutionary Biology</i> <b>17</b>, 236. Foth, C., Tischlinger, H. & Rauhut, O.W.M. (2014) New specimen of <i>Archaeopteryx</i> provides insights into the evolution of pennaceous feathers. <i>Nature</i> <b>511</b>, 79–82. Frey, E., Tischlinger, H., Buchy, M.-C. & Martill, D.M. (2003) New specimens of Pterosauria (Reptilia) with soft parts with implications for pterosaurian anatomy and locomotion. <i>Geological Society, London, Special Publications</i> <b>217</b>, 233–266. Gatesy, S.M. (2001) Skin impressions of Triassic theropods as records of foot movement. <i>Bulletin of the Museum of comparative Zoology</i> <b>156</b>, 137–149. Godefroit, P., Sinitsa, S.M., Dhouailly, D., Bolotsky, Y.L., Sizov, A.V., McNamara, M.E., Benton, M.J. & Spagna, P. (2014) A Jurassic ornithischian dinosaur from Siberia with both feathers and scales. <i>Science</i> <b>345</b>, 451–455. Goloboff, P.A. & Catalano, S.A. (2016) TNT version 1.5, including a full implementation of phylogenetic morphometrics. <i>Cladistics</i> <b>32</b>, 221–238. Hendrickx, C. & Bell, P.R. (2021) 3D model related to the publication: The scaly skin of the abelisaurid <i>Carnotaurus sastrei</i> (Theropoda: Ceratosauria) from the Upper Cretaceous of Patagonia. <i>MorphoMuseuM</i> <b>7</b>, 1–3. Hendrickx, C. & Carrano, M.T. (2016) Erratum on ‘An overview of non-avian theropod discoveries and classification’. <i>PalArch’s Journal of Vertebrate Palaeontology</i> <b>13</b>, 1–7. Hendrickx, C., Hartman, S.A. & Mateus, O. (2015) An overview of non-avian theropod discoveries and classification. <i>PalArch’s Journal of Vertebrate Palaeontology</i> <b>12</b>, 01–07. Ji, Q., Lü, J., Wei, X.F. & Wang, X.R. (2012) A new oviraptorosaur from the Yixian Formation of Jianchang, western Liaoning Province, China. <i>Geological Bulletin of China</i> <b>31</b>, 2102–2107. Kaye, T.G., Falk, A.R., Pittman, M., Sereno, P.C., Martin, L.D., Burnham, D.A., Gong, E., Xu, X. & Wang, Y. (2015) Laser-Stimulated Fluorescence in Paleontology. <i>PLOS ONE</i> <b>10</b>, e0125923. Public Library of Science. Kellner, A.W.A., Wang, X., Tischlinger, H., Campos, D. de A., Hone, D.W.E. & Meng, X. (2010) The soft tissue of <i>Jeholopterus</i> (Pterosauria, Anurognathidae, Batrachognathinae) and the structure of the pterosaur wing membrane. <i>Proceedings of the Royal Society of London B: Biological Sciences</i> <b>277</b>, 321–329. Lambertz, M. (2017) Phylogenetic placement, developmental trajectories and evolutionary implications of a feathered dinosaur tail in Mid-Cretaceous amber. <i>Current Biology</i> <b>27</b>, R215–R216. Langer, M.C., Ezcurra, M.D., Rauhut, O.W.M., Benton, M.J., Knoll, F., McPhee, B.W., Novas, F.E., Pol, D. & Brusatte, S.L. (2017) Untangling the dinosaur family tree. <i>Nature</i> <b>551</b>, nature24011. Maddison, W.P. & Maddison, D.R. (2017) Mesquite: a modular system for evolutionary analysis. Version 3.2. <i>Mesquite</i>. http://mesquiteproject.org [accessed 16 June 2017]. Mayr, G., Pohl, B. & Peters, D.S. (2005) A well-preserved <i>Archaeopteryx</i> specimen with theropod features. <i>Science</i> <b>310</b>, 1483–1486. Milner, A.R.C., Lockley, M.G. & Johnson, S. (2006) The story of the St. George Dinosaur Discovery Site at Johnson Farm: an important new Lower Jurassic dinosaur tracksite from the Moenave Formation of southwestern Utah. <i>The Triassic-Jurassic Terrestrial Transition. New Mexico Museum of Natural History and Science Bulletin</i> <b>37</b>, 329–345. Motta, M.J., Agnolín, F.L., Brissón Egli, F. & Novas, F.E. (2020) New theropod dinosaur from the Upper Cretaceous of Patagonia sheds light on the paravian radiation in Gondwana. <i>The Science of Nature</i> <b>107</b>, 24. Müller, R.T., Langer, M.C., Bronzati, M., Pacheco, C.P., Cabreira, S.F. & Dias-Da-Silva, S. (2018) Early evolution of sauropodomorphs: anatomy and phylogenetic relationships of a remarkably well-preserved dinosaur from the Upper Triassic of southern Brazil. <i>Zoological Journal of the Linnean Society</i> <b>184</b>, 1187–1248. Oxford Academic. Nesbitt, S.J., Smith, N.D., Irmis, R.B., Turner, A.H., Downs, A. & Norell, M.A. (2009) A complete skeleton of a Late Triassic saurischian and the early evolution of dinosaurs. <i>Science</i> <b>326</b>, 1530–1533. Nixon, K.C. (2002) WinClada, version 1.00.08. <i>Published by the author, Ithaca, New York</i>. Ouyang, H. & Ye, Y. (2002) <i>The first mamenchisaurian skeleton with complete skull, </i>Mamenchisaurus youngi. Sichuan Publishing House of Science and Technology, Chengdu, China. Pei, R., Pittman, M., Goloboff, P.A., Dececchi, T.A., Habib, M.B., Kaye, T.G., Larsson, H.C.E., Norell, M.A., Brusatte, S.L. & Xu, X. (2020) Potential for powered flight neared by most close avialan relatives, but few crossed its thresholds. <i>Current Biology</i> <b>30</b>, 4033-4046.e8. Pittman, M., O’Connor, J., Field, D.J., Turner, A.H., Ma, W., Makovicky, P.J. & Xu, X. (2020) Pennaraptoran systematics. In <i>Pennaraptoran Theropod Dinosaurs Past Progress and New Frontiers Bulletin of the American Museum of Natural History</i> (eds M. Pittman & X. Xu), pp. 7–36. American Museum of Natural History. Rauhut, O.W.M., Foth, C., Tischlinger, H. & Norell, M.A. (2012) Exceptionally preserved juvenile megalosauroid theropod dinosaur with filamentous integument from the Late Jurassic of Germany. <i>Proceedings of the National Academy of Sciences</i> <b>109</b>, 11746–11751. Reisz, R.R. & Sues, H.-D. (2000) The ‘feathers’ of <i>Longisquama</i>. <i>Nature</i> <b>408</b>, 428–428. Nature Publishing Group. Turner, A.H., Makovicky, P.J. & Norell, M. (2012) A review of dromaeosaurid systematics and paravian phylogeny. <i>Bulletin of the American Museum of Natural History</i> <b>371</b>, 1–206. Wang, X., Pittman, M., Zheng, X., Kaye, T.G., Falk, A.R., Hartman, S.A. & Xu, X. (2017) Basal paravian functional anatomy illuminated by high-detail body outline. <i>Nature Communications</i> <b>8</b>, 14576. Xing, L., McKellar, R.C., Xu, X., Li, G., Bai, M., Persons, W.S., Miyashita, T., Benton, M.J., Zhang, J., Wolfe, A.P., Yi, Q., Tseng, K., Ran, H. & Currie, P.J. (2016) A feathered dinosaur tail with primitive plumage trapped in Mid-Cretaceous amber. <i>Current Biology</i> <b>26</b>, 3352–3360. Xu, X. & Li, F. (2016) A new microraptorine specimen (Theropoda: Dromaeosauridae) with a brief comment on the evolution of compound bones in theropods. <i>Vertebrata PalAsiatica</i> <b>54</b>, 269–285. Xu, X., Sullivan, C. & Wang, S. (2013) The systematic position of the enigmatic theropod dinosaur <i>Yixianosaurus</i> longimanus. <i>Vertebrata PalAsiatica</i> <b>51</b>, 169–183. Xu, X. & Wang, X. (2003) A new maniraptoran dinosaur from the Early Cretaceous Yixian Formation of western Liaoning. <i>Vertebrata Pal Asiatica</i> <b>41</b>, 195–202. Xu, X., Zheng, X. & You, H. (2010) Exceptional dinosaur fossils show ontogenetic development of early feathers. <i>Nature</i> <b>464</b>, 1338–1341. Yang, Z., Jiang, B., McNamara, M.E., Kearns, S.L., Pittman, M., Kaye, T.G., Orr, P.J., Xu, X. & Benton, M.J. (2019) Pterosaur integumentary structures with complex feather-like branching. <i>Nature Ecology & Evolution</i> <b>3</b>, 24–30. Nature Publishing Group. Zheng, X.-T., You, H.-L., Xu, X. & Dong, Z.-M. (2009) An Early Cretaceous heterodontosaurid dinosaur with filamentous integumentary structures. <i>Nature</i> <b>458</b>, 333–336.
description: Each file is self-explanatory.