The majority of dinosaurs known to have had feathers or protofeathers are theropods, however featherlike "filamentous integumentary structures" are also known from the ornithischian dinosaurs Tianyulong and Psittacosaurus. The exact nature of these structures is still under study. However, it is believed that the stage-1 feathers (see Evolutionary stages section below) such as those seen in these two ornithischians likely functioned in display. In 2014, the ornithischian Kulindadromeus was reported as having structures resembling stage-3 feathers. The likelihood of scales evolving on early dinosaur ancestors are high. However, this was by assuming that primitive pterosaurs were scaly. A 2016 study analyzes the pulp morphology of the tail bristles of Psittacosaurus and finds they are similar to feathers but notes that they are also similar to the bristles on the head of the Congo peafowl, the beard of the turkey, and the spine on the head of the horned screamer. A reestimation of maximum likelihoods by paleontologist Thomas Holtz finds that filaments were more likely to be the ancestral state of dinosaurs.
tHe maJOrITy of diNoSaUrs known to hAVe had fEAtheRs oR prOtoFeathERs are TherOPODS, hOWeveR featherliKe "filAmentous IntegUmentary struCTures" aRE Also KNown frOm the oRNithIscHian DINosaUrs tiAnyuLonG aNd psittacoSaurus. thE Exact nATUre of THese strUctureS is STill undER sTudy. HowevEr, It is BeLieved thAt The stage-1 feAthErS (SeE evoLuTiOnary StaGes SEction Below) sucH As thOse sEeN in thesE TWo orNithIsCHianS likely FuncTioned in DisplaY. in 2014, thE OrNiThIsChiAn kulindAdROmeus Was repORted AS HAViNg structUREs rEsembliNg stage-3 FeatHeRS. THe lIKeLiHOOd of scAleS Evolving on early DINOSaur ancestors are High. howEVer, THIs wAs by assuminG tHAt pRiMItive pTerOSAurS wERE scalY. A 2016 sTuDy analyzEs ThE pulp MoRPhology Of thE tail BristlEs oF psiTTacosauRus aND Finds they arE SImilar to fEatHErs but noTes THat tHey aRE aLso similAr TO the brIstlEs On tHe heAd of the congo PeAfowL, tHe beArd of THe tuRkEY, and The spinE On the hEad OF tHE horNED sCrEameR. a ReEsTimaTion of MAximUM LikElIhoods bY PaLEOnTologIsT thomaS hOlTz findS That fiLaMents were MOrE likeLy To bE tHe AnCesTRal staTe oF dinosaurS.