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Re: Thermogenic muscle hypothesis for origin of birds



Yeah Newman presented an earlier version of this hypothesis in 2011. The
"mechanical
environment of embryonic development" is a new point, we'll have to see
what it means.

Below are some of my critiques of the hypothesis from 2011, issues I'll be
looking for in the full paper. I must say that, from the new abstract, the
paper looks really interesting, and it sounds like he may have addressed
the issues I raised, probably just through his own work and that of his
co-authors:

In his new paper Stuart A. Newman
offers an interesting new character in bird evolution, the loss of the
mitochondrial gene UCP1. Newman¹s insights could be incorporated with other
data from diapsid paleontology to produce a strong hypothesis of bird
origins.
...

Newman¹s work, while thought - provoking, must start by
substantiating its anatomical claims. Newman provides no references or
measurements to quantify his statement that birds and other dinosaurs have
³massive skeletal muscles² compared to other tetrapods. Do, say, chimney
swifts, really have proportionally larger thigh muscles than basal mammals
like
kangaroos? A distribution of muscle masses relative to body size in various
groups should be presented. If this putative hyperplasia is substantiated,
then
we can move to the next steps.
 

Newman suggests that disproportionately large avian muscle
mass may have prevailed in ancestral populations due to its beneficial
function
in endothermy, and that it may therefore be independent of flight
adaptations.
That latter hypothesis is testable. By comparing anatomical measurements
one
can assess the muscle masses of flying birds against flightless ones, to
see if
and how the muscle masses of pectoral and pelvic locomotor modules vary.
If the
pelvic plus pectoral musculature mass remains relatively constant for body
size, then Newman¹s hypothesis may be supported. If the loss of pectoral
muscle
mass in flightless birds is not offset by increased pelvic mass, then it
may be
weakened.
 

Newman must clarify his hypothesis. He states that the loss
of UCP1 led to a crisis that the ³reptilian ancestors of birds² only
survived
by developing hyperplastic muscles. Newman mentions that lizards also lack
UCP1, yet no such crisis seems to have confronted them, and no hyperplasia
seems to have resulted. Newman probably means that the lineage that gave
rise
to birds only retained or reversed to endothermy by resorting to
hyperplasia,
but this is not clear. Instead of a crisis, it seems more likely that the
broad
archosaurian radiations experimented with several mechanisms for
maintaining
endothermy after the loss of UCP1, with huge body masses, feathers, and
pterosaur fur being just three examples, and muscular hyperplasia being
the new
one. I suggest that Newman formulate the hypothesis to specifically state
that
UCP1 may have been lost earlier, but that avian muscle hyperplasia arose
first
in or near the small - bodied ancestor of the Paraves, and test the
hypothesis
as such. Turner et al. (2007) used regression analysis to determine that
the
last common ancestor of all Paravians had a body mass as low as 600 grams,
a
point where endothermy requires active rather than passive mechanisms, and
this
is, in my mind, the most likely point where the avian lineage could have
acquired its muscle hyperplasia. Similar hyperplasia may have arisen
independently in small, ancestral, pterosaurs.
 

Newman¹s paper can be much improved by taking into
consideration what is already known of the phylogenetic pattern and
geological
history of amniotes. He implies that UCP1 was lost in the last common
ancestor
of lizards and birds, which was the ancestral saurian, and that this
created a
crisis that dinosaurs responded to by muscular adaptations. Yet there may
be
something like 30 million years between the first saurian (Late Permian)
and
the first dinosaur (lowest Late Triassic) ­ and the intervening time saw
several radiations of marine diapsids, lepidosaurs, basal archosaurs,
crocodyliforms, and even pterosaurs from this lineage. Moreover, the
interval
between the first saurian and the first bird is far longer, possibly 110
million years. 
 

Newman must specify the node or nodes at which he
hypothesizes that UCP1 was lost. If there was a crisis that birds
responded to
then it must have happened in an immediate ancestor to birds ­ perhaps the
ancestral Paravian in the Middle Jurassic. If not, then the loss of UCP1
may
have happened in the ancestral saurian and was tolerated just fine by a
wide
radiation of highly successful diapsids for the last 260 million years.
 
At a minimum Newman could check the alligator genome, which
has been sequenced, for UCP1. If crocodilians retain it then it is
parsimonious
to conclude that lizards and birds underwent independent losses of the
gene. In
that case it was likely lost in the immediate ancestors of birds. Otherwise
UCP1 was more probably lost in the ancestral saurian and tolerated in a
diversity of ways by its descendants.
 

Newman suggests that birds survived the end of the
Cretaceous because they were endothermic while other dinosaurs were
poikilothermic. In this he does not reconcile his views with what is known
about the pattern of survival of vertebrate groups at the KT horizon.
Enantiornithine birds and at least two lineages of non ­ avian dinosaurs
that
likely had the same hyperplastic muscular proportions as birds went
extinct at
the KT transition. Moreover, as should be obvious, many groups such as
choristoderes, crocodilians, turtles, and frogs that were poikilotherms
survived. 
 

The loss of UCP1 is a character that may be of future
importance in theories of bird origins. It may help offer pleiotropic
explanations for the origin and development of some avian features. But
UCP1
cannot be informative about the origin of birds if it is viewed outside of
its
phylogenetic context.




Jason Brougham
Senior Principal Preparator
American Museum of Natural History
jaseb@amnh.org
(212) 496 3544





On 4/4/13 3:07 PM, "don ohmes" <d_ohmes@yahoo.com> wrote:

>
>
>My initial impression is that this is a streeeeeeeee-tch...
>
>To put it mildly.
>
>And ignores possible theropod/avian relationships.
>
>That said, I have only read the abstract...
>
>What prompted me to post a comment onlist, however - the abstract seems
>to propose that changes in egg shape/size select for altered morphology
>in the subsequent adult - and this is a new idea to me, and one that
>implies potential for abrupt changes... as they say.
>
>Is it indeed a novel idea? Or did I just miss it sometime in the last
>century?  
>
>------------------------------
>On Thu, Apr 4, 2013 11:58 AM EDT Ben Creisler wrote:
>
>>From: Ben Creisler
>>bcreisler@gmail.com
>>
>>
>>A new online paper:
>>
>>
>>Stuart A. Newman, Nadezhda V. Mezentseva & Alexander V. Badyaev (2013)
>>Gene loss, thermogenesis, and the origin of birds.
>>Annals of the New York Academy of Sciences (advance online publication)
>>DOI: 10.1111/nyas.12090
>>http://onlinelibrary.wiley.com/doi/10.1111/nyas.12090/abstract
>>
>>Compared to related taxa, birds have exceptionally enlarged and
>>diversified skeletal muscles, features that are closely associated
>>with skeletal diversification and are commonly explained by a
>>diversity of avian ecological niches and locomotion types. The
>>thermogenic muscle hypothesis (TMH) for the origin of birds proposes
>>that such muscle hyperplasia and the associated skeletal innovations
>>are instead the consequence of the avian clade originating from an
>>ancestral population that underwent several successive episodes of
>>loss of genes associated with thermogenesis, myogenesis, and
>>skeletogenesis. Direct bird ancestors met this challenge with a
>>combination of behavioral strategies (e.g., brooding of nestlings) and
>>acquisition of a variety of adaptations for enhanced nonshivering
>>thermogenesis in skeletal muscle. The latter include specific
>>biochemical alterations promoting muscle heat generation and dramatic
>>expansion of thigh and breast muscle mass. The TMH proposes that such
>>muscle hyperplasia facilitated bipedality, freeing upper limbs for new
>>functions (e.g., flight, swimming), and, by altering the mechanical
>>environment of embryonic development, generated skeletal novelties,
>>sometimes abruptly, that became distinctive features of the avian body
>>plan.
>