Figure 1: a, Main slab of the holotype (STMN 26-3). b, Broken slab. The scale bar in b refers also to a. c, Close-up of skull and mandible. d, Interpretive drawing of skull and mandible. e, Close-up of dentition. Abbreviations: a, angular; aof, antorbital fossa; ca, caudal vertebrae; cv, cervical vertebrae; d, dentary; dv, dorsal vertebrae; emf, external mandibular fenestra; en, external naris; f, femur; h, humerus; isc, ischium; j, jugal; l, lacrimal; m, maxilla; n, nasal; pd, predentary; pf, prefrontal; pm, premaxilla; po, postorbital; pub, pubis; q, quadrate; qj, quadratojugal; scaco, scapulocoracoid; sa, surangular; tf, tibia and fibula. Click for larger image.
The really interesting feature is that this dinosaur has what appears to be feathers. The authors call them filaments, and the big question is whether how much like feathers they are. If in structure they are like feathers, then they might be homologous to them, and that then would have all sorts of implications about the evolution of feathers. If, on the other hand, these filaments are not structurally like feathers, then homology is doubtful, and the story told by this fossil change drastically.
Figure 2: a, Ventral to the cervical vertebral series. b, Dorsal to the dorsal vertebral series. c, Dorsal to the proximal–middle caudal vertebral series. d, Close-up of c. Arrows in c and d point to single filament exhibiting a clear, dark, midline 'stripe'. e, Schematic of long, central tail feather of Epidexipteryx (after ref. 12). f, Two types of integumentary filaments of Sinornithosaurus (after ref. 17). Click for larger image.
The question rests upon determining whether the filaments protrude from the epidermal or the dermal skin layer. If they are epidermal, then they a like (proto)feathers. If they are dermal (deeper in the skin), then they wouldn't function like feather, because they wouldn't protrude through the skin at all. the authors argue that the filaments are epidermal, and supports this by the clear, dark, midline 'stripe' in Figure 2, e.
One lesson from this is that it really, really takes an expert to interpret the evidence. Anyone else cannot looks at this fossil and determine whether these filaments are homologous or not to the feathers of other feathered dinosaurs.
The authors conclude thus.
When homology with 'protofeathers' is hypothesized, a possible scenario is that the elongate, singular, cylindrical filaments of Tianyulong represent elaborations of the initial stage in development models of the origin and evolution of feathers. The basal position of Heterodontosauridae within Ornithischia then suggests that such early-stage structures were present in the earliest dinosaurs (before or at the Saurischia–Ornithischia split) and inherited by basal members of each group. Later ornithischians and non-theropod saurischians for which skin impressions are known would therefore represent secondary losses of these structures at as-yet undetermined systematic positions. The structures in Tianyulong, as well as the bristle-like structures of the basal ceratopsian Psittacosaurus, which have frequently been perceived as nonhomologous with the filamentous structures of theropods, may truly be homologous, albeit derived in structure compared to those of theropods.
Zheng, X., You, H., Xu, X., & Dong, Z. (2009). An Early Cretaceous heterodontosaurid dinosaur with filamentous integumentary structures Nature, 458 (7236), 333-336 DOI: 10.1038/nature07856