Evolution of dinosaur epidermal structures

[HD-man]

Remember Barrett/Evans' abstract from SVP 2013 (See page 82: http://vertpaleo.org/PDFS/0d/0d20d609-f7e6-4bb3-a0c4-765fcffde49b.pdf )? The paper has since been published ( http://rsbl.royalsocietypublishing.org/content/11/6/20150229 ). There aren't many papers I can read most/all of the way through, so it's especially exciting that I can read this 1. I'm surprised someone else hasn't already created a thread for it given that it covers a lot of stuff discussed on this forum. I bolded my favorite parts of the discussion below.

Quoting Barrett et al. (See "4. Discussion"):

Current data do not provide definitive support for the suggestion that protofeathers were synapomorphic for dinosaurs. Instead, it seems most likely that scaly skin, unadorned by feathers or their precursors, was primitive for Dinosauria and retained in the majority of ornithischians, all sauropodomorphs and some early-diverging theropods (filaments are thus far unknown in ceratosaurians, abelisaurids and allosauroids: electronic supplementary material, S1). Additional examples of protofeathers would be required from early dinosaur lineages or non-dinosaurian dinosauromorphs to optimize this feature to the base of Dinosauria. In particular, the ancestral condition in pterosaurs is pivotal in this regard, but currently unknown. Filaments/feathers were probably present in the coelurosaur common ancestor and might have characterized all theropods, though support for the latter proposal is equivocal (electronic supplementary material, S1 and S2). There is some evidence across the dinosaur tree for homoplastic loss of these structures (e.g. some tyrannosaurids possess scales, but lack evidence for other epidermal structures; electronic supplementary material, S1), so integument evolution cannot be regarded as a progression to more complex epidermal structures.
Caution is warranted as taphonomy controls the types of integumentary structures preserved in different taxa. Feather-like structures are most frequently preserved in low-energy environments and may have been lost in coarse-grained, high-energy settings, though rare feather-like structures are known from the latter [3]. No dinosaur skin impressions (except footprints) are known from the Late Triassic and they are rare from the Early–Middle Jurassic (electronic supplementary material, S1) periods when dinosaur-bearing lacustrine/lagoonal deposits are scarce. Hence, potential taphonomic windows for early dinosaur filaments/feathers are not available currently: new localities are needed to resolve whether early dinosaurs were scaled or feathered. Conversely, the exceptional preservation of coelurosaurs in Late Jurassic and Cretaceous lagerstätte might not be indicative of skin structures in earlier relatives.
Previous authors noted that ornithischian quills/protofeathers were morphologically distinct from those in theropods [6,19]. Our analyses support suggestions that these features should not be regarded a priori as homologous with theropod epidermal structures: ornithischian quills/protofeathers plausibly represent epidermal structures that evolved independently, and may be indicative of a more general ornithodiran tendency to experiment with epidermal features. The latter possibility is suggested by: possession of feathers and protofeathers in theropods [2–5]; presence of filamentous coverings in at least some pterosaurs [18]; and development of elaborate midline scale frills in hadrosaurs and sauropods (electronic supplementary material, S1). As archosaur scales, claws and feathers are composed of β-keratins [20,21], it is possible that the elaboration of all complex ornithodiran epidermal structures was underpinned by the same developmental and regulatory mechanisms ([22]: which, for unknown reasons, were not expressed in the majority of non-coelurosaurian dinosaurs). Molecular phylogenies of β-keratin families indicate that those found in feathers are the latest diverging among archosaurs and may not have appeared prior to the evolution of crown birds, whereas scale and 'feather-like' β-keratins diverged earlier [21]. This suggests a scenario in which scales and feather-like structures may have appeared (and diversified) via numerous independent acquisitions in Ornithodira, with true feathers appearing only in birds and their proximate theropod outgroups.
The identification of branched feathers in theropods is uncontroversial and supported by morphology, developmental models and known β-keratin composition in some taxa [23]. Phylogenetic congruence and developmental data also support the homology of branched feathers and theropod filamentous protofeathers [23]. However, gross morphological differences are not sufficient to establish homologies between true feathers and 'protofeathers' in other ornithodiran lineages as these structures potentially represent modified epidermal scales (like those of Iguana, Kolekanos or Longisquama [6,24]) or other tissues (e.g. degraded collagen fibres [25]). Additional homology tests are required, such as identification of β-keratin antibody reactivity, detailed comparisons between collagenous and epidermal structures, and identification of microstructural differences between elongate scales and protofeathers [5,22,26]. However, as reptilian scale and feather β-keratins are compositionally similar [20,21], it may be difficult to distinguish elongate scales from genuine protofeathers on the basis of biogeochemistry and gross morphology.
Current data indicate that feathers and their filamentous homologues are probably theropod synapomorphies but fail to support the hypothesis that protofeathers are plesiomorphic for Dinosauria. The origins of 'protofeathers' and 'quills' outside Theropoda are ambiguous: interpretations of their homology should be correspondingly cautious, especially in those ornithodirans phylogenetically distant from birds.

[Halichoeres]

Thanks for posting, seems like a sound argument.

[Derek.McManus]

 :)Thank you for posting, very interesting!

[Tyto_Theropod]

Very handy - must remember this thread next time I'm drawing a n awkwardly-placed Theropod!

[HD-man]

Thanks for posting, seems like a sound argument.

:)Thank you for posting, very interesting!

Anytime. Glad I could help.

Very handy - must remember this thread next time I'm drawing a n awkwardly-placed Theropod!

What do you mean by "awkwardly-placed"? Just wondering.

[Tyto_Theropod]

Thanks for posting, seems like a sound argument.

:)Thank you for posting, very interesting!

Anytime. Glad I could help.

Very handy - must remember this thread next time I'm drawing a n awkwardly-placed Theropod!

What do you mean by "awkwardly-placed"? Just wondering.

As in, phylogenetically...odd.