[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Most dinosaurs were scaly




I concur with what Mickey has said. I would also like to raise the question of 
how confidant we can be that the all the scales of dinosaurs are true scales, 
and are not epidermal structures secondarily derived from feathers due to 
evo-devo mechanisms? 


At least some evo-devo studies suggest that the reticula scales on the plantar 
surface of avian feet are actually developmentally stunted feathers. To quote 
from Dhouailly (2009), "[R]eticula are not true cutaneous appendages, and 
appear to be feathers 
arrested in the initiation step of their morphogenesis: formation of a 
slight bump, without a placode." I believe similar suggestions were made by 
Sawyer and Knapp (2003).

At least some non-scale epidermal structures in modern avians morphologically 
mimic scales too. For instance, the facial wattles of Philepitta schlegeli 
(Schlegel's Asity) have an appearance to me that mimics scales: 
http://thefeaturedcreature.com/wp-content/uploads/2012/10/Screen%2Bshot%2B2011-06-05%2Bat%2B7.26.20%2BPM2.png

A similar thing can be said about the bare skin on the head and neck of 
Coragyps atratus (Black Vulture): 
http://en.wikipedia.org/wiki/File:Coragyps-atratus-002.jpg

If all we had were fossilized impressions of these bare skin structures, might 
we erroneously conclude they were true scales?

I also think inferring from osteoderms in thyreophorans and sauropods that 
their ancestors never had filamentous integumentary appendages (FIAs) might be 
misleading. Hair sprouts amongst the osteoderms of armadillos for instance: 
http://upload.wikimedia.org/wikipedia/commons/b/b4/Nine-banded_Armadillo.jpg Do 
fossil cingulates preserve hair impressions amongst their osteoderms? Seems 
unlikely.


The same might be true for scaled dinosaurs, even those with osteoderms. 
Additionally, feathers erupt from scutate scales in some birds on occasion. 
Sawyer & Knapps (2003) noted, "The growth of feathers from the distal ends of 
scutate scales occurs occasionally in normally scaled [chicken] breeds such as 
White Plymouth Rock and White Leghorn.
have been observed growing from the tips of the definitive scale ridges at 12 
days of incubation." So scales and feathers intermingling are not de facto 
impossible, although there appears to be significant developmental constraints. 
Maybe these constraints were more relaxed during the initial evolution of FIAs 
in dinosaurs.

-Zach

>________________________________
> From: Mickey Mortimer <mickey_mortimer111@msn.com>
>To: "dinosaur@usc.edu" <dinosaur@usc.edu> 
>Sent: Friday, December 27, 2013 11:48 PM
>Subject: RE: Most dinosaurs were scaly
> 
>
>Is it just me, or does this study seem to suffer from naively assuming body 
>covering doesn't covary with other factors we can test?  In other words, it's 
>obvious if you just plug in known skin coverings over a cladogram, scales will 
>be the ancestral state for Dinosauria.  No one's ever doubted that, Greg Paul 
>included.  The additional factor which has been discussed ever since his 1988 
>work is of course size.  What happens if you only plot small specimens on the 
>cladogram?  Ornithischians are all fuzzy/spiny, and sauropodomorphs are 
>unknown unless the titanosaur embryos count.  Even if the latter do count, 
>Sciurumimus makes theropods primitively fuzzy despite Juravenator, so that's 
>ambiguous basal Saurischia and fuzzy basal Dinosauria, which only gets more 
>support if pterosaurs are avemetatarsalians.  Maybe Barrett and Evans include 
>a caveat about this assumption, but since taking it into account nullifies 
>their entire conclusion, I don't think
 it could help.  This isn't even getting into the metabolic and growth evidence 
that 
>shows increasingly basal archosauromorphs weren't like living 
>'reptiles'.  You might as well determine ornithischians don't have cheeks 
>based only on their lack in birds, crocs and lepidosaurs.
>
>Mickey Mortimer
>
>
>----------------------------------------
>> Date: Fri, 27 Dec 2013 10:13:58 -0800
>> From: bcreisler@gmail.com
>> To: dinosaur@usc.edu
>> Subject: Most dinosaurs were scaly
>>
>> From: Ben Creisler
>> bcreisler@gmail.com
>>
>>
>> A news item and the original abstract from the 2013 SVP meeting:
>>
>> Nature news:
>>
>> http://www.nature.com/news/feathers-were-the-exception-rather-than-the-rule-for-dinosaurs-1.14379
>>
>>
>> The abstract from the SVP Meeting:
>>
>>
>> Poster Session III (Friday, November 1, 2013, 4:15 - 6:15 PM)
>> DINOSAUR INTEGUMENT: WHAT DO WE REALLY KNOW?
>> BARRETT, Paul, The Natural History Museum, London, United Kingdom; EVANS,
>> David, Univ of Toronto at Mississauga, Toronto, ON, Canada
>>
>> Osteoderms and scaly skin impressions are historically well known in 
>> non-avian
>> dinosaurs
>> these structures,
>> many dinosaur taxa possessed other integumentary features, including a
>> range of ‘quills,'
>> filaments, and feathers in non-avian theropods and ornithischians.
>> Feathers and their
>> homologs are commonly regarded as a synapomorphy of either coelurosaurian or
>> tetanuran theropods, but some authors have gone further, using the presence 
>> of
>> ornithischian feather-like structures to suggest that these structures
>> are plesiomorphic for
>> Dinosauria. This inference has wide-ranging implications for dinosaur
>> biology and
>> evolution.
>> However, to date, no studies have attempted to assess rigorously the
>> evolution of
>> dinosaur integumentary structures within a broad phylogenetic context.
>> We compiled a
>> complete database of all epidermal integumentary structures reported
>> in dinosaurs, by
>> major body region, in order to investigate the origin of feather
>> homol
nd the
>> evolution of integumentary structures in the clade. Scales are
>> definitively present in
>> virtually all major ornithischian clades. This, and the presence of
>> extensive armour in
>> thyreophorans suggests that genasaurian skins were primitively scaly. 
>> Similarly,
>> sauropodomorphs lack evidence for anything other than scales or
>> osteoderms. Fitch
>> optimization of integument types on dinosaur phylogenies shows that there is 
>> no
>> unequivocal support for inferring a deep origin of feather-like
>> structures, a result
>> supported by maximum likelihood ancestral state reconstructions for
>> these characters.
>> The structures in Tianyulong and Psittacosaurus are best regarded as
>> autapomorphic
>> integumentary modifications, and there is currently no strong evidence
>> that these features
>> are feather homologs. Further work on the chemical composition of
>> these structures, and
>> those in several non-coelurosaurian theropods, is needed. Although
>> ornithodirans exhibit
>> a range of integumentary novelties that may be related to the origin
>> of feathers, theropods
>> are currently the only dinosaurs that display unequivocal e
>> feathers and their
>> direct homologs.                           
>
>
>