Evolution of dinosaur epidermal structures

Barrett, Evans and Campione (2015)
“find no compelling evidence for the appearance of protofeathers in the dinosaur common ancestor and scales are usually recovered as the plesiomorphic state, but results are sensitive to the outgroup condition in pterosaurs. Rare occurrences of ornithischian filamentous integument might represent independent acquisitions of novel epidermal structures that are not homologous with theropod feathers.”

Unfortunately
the Barrett team followed two false traditions with regard to pterosaurs, which gained their epidermal structures independent from dinos. The two clades are not related according to the large reptile tree which nests pterosaurs in a new clade of lepidosaurs.

Based on their false assumption of scaly pterosaurs
as an outgroup, their analysis recovered primitively scaled Dinosauria and Ornithischia. So we’re off to a bad start based on taxon exclusion and false inclusion. Scales have never been found on pterosaurs. Why didn’t they assume filamented pterosaurs? We have evidence for that. So there is a lack of logic here that would have changed their conclusion.

The actual outgroup
for dinosaurs is the Crocodylomorpha for which tiny back scales first appear on the lower back of tiny Scleromochlus and ultimately cover the entire dermal surface on large extinct and extant taxa. Tiny scales may have been present on basal dinos, but more likely they had naked skin, like birds without their feathers. Scales on bird feet are transformed feathers.

The Barrett team database
included 24 ornithischians, 6 sauropods and 40 theropods (including Mesozoic birds). All taxa were scored for the presence/absence of epidermal scales, unbranched filaments (protofeathers)/quills and more complex branched filaments (including feathers).

The Barrett team report,
“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.” Longtime readers know this is false based on a cladogram, the large reptile tree) that includes several hundred more taxa.

As noted above, scales are unknown in pterosaurs.
However, their known outgroup taxa, Longisquama, SharovipteryxCosesaurus and Macrocnemus all have scales. The former three also have ptero-hairs (pycnofibers) and are the only Triassic fenestrasaurs (including pterosaurs) known to have these epidermal structures.

Based on their appearance and location,
dinoaurian ‘quills’ appear to be hyper elongated primordia without branching.

The Barrett team concluded,
“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.” In Science “it seems most likely” is a very weak argument, further weakened by the fact that birds don’t have scales, except on their legs, and those are transformed feathers.

The Barrett team provided a cladogram
that depicted the extent to which scales, filaments and feathers were present. Notably they did not also include the extent of naked skin, which is a fourth possibility not covered by the text or graphic. The possibility exists that all dinosaur scales are transformed primordia (filaments) or transformed feathers. Dinosaur scales could also be novel epidermal structures that appear only on large dinosaurs just as croc scales are novel epidermal structures. Based on their appearance and location, dinoaurian ‘quills’ appear to be hyper elongated primordia.

Embryo birds
first develop primordial feathers in the middle of their backs, replaying phylogeny during ontogeny. With current data, that trait may go all the way back to basal archosaurs, like Scleromochlus.

Bottom line:
When you play with phylogenetic bracketing, you have to have a valid cladogram.

References
Barrett PM, Evans DC, Campione NE 2015. Evolution of dinosaur epidermal structures. Biol. Lett. 11: 20150229. online

 

 

 

 

 

 

 

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