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Re: FEATHERS DO NOT=ENDOTHERMY



On Monday, Oct. 14, Narren Naish wrote:

>Well...I hate to be the one to point it out, and don't think I'm
>necessarily saying that your point is _wrong_, but feathers may not be
>indicative of endothermy.  Chiappe, Chinsamy and Dodson looked at thin
>sections of _Patagopteryx_ and enantiornithine bone and concluded that
>these birds were not endothermic as are modern birds (I don't have the
>literature to hand, so I won't attempt to paraphrase their conclusions).
>Yet no-one (I think) would doubt that these forms are feathered - hey,
>we even have feather impressions for some of the enantiornithines.

        With regards to the thermal physiology of early birds, Chinsamy,
Chiappe & Dodson (1995) concluded that...

>The bone histology of these birds suggest they could have occupied an
>intermediate physiological condition between poikilothermic ectothermy
>and homeothermic endothermy, _P. deferrariisi_ being, however,
>apparently more advanced towards endothermy than the Enantiornithes.

        Having examined many, _many_ thin sections of bone microstructure
in dinosaurs, reptiles, mammals and birds, I am extremely dubious one can
conclude _anything_ about the metabolic rate of early birds from their
bone histology.  There is no direct experimental data documenting that
metabolic rate influences bone microstructure, and the that it
does is further weakened by the observation that some living and extinct
mammals exhibit bone structures indistinguishable from those of reptiles!
Some extant marsupials (kangaroos and wombats, for example) deposit bone
in a cyclical manner very similar to that seen in _Patagopteryx_ and some
reptiles.
        In addition, (Leahy, 1991) some extinct diprotodonts deposited
bone in a manner identical to that seen in modern crocodiles.  At the 1994
SVP meeting in Seattle, I presented a poster session which included thin
sections of a multituberculate femur with multiple deep set growth rings
and avascular lamellar bone identical to that seen in Enantiornithes.
        The thermal physiology of multis is unknown, though it is likely
they possessed some degree of tachymetabolic endothermy.  Marsupials are
fully endothermic, however, so in their case the presence of reptile-like
bone histologies strongly suggests that using bone histology as an
indicator of metabolic rate in fossil taxa could lead to significant
errors.
        Personally, I suspect that external insulation, in the form of
fur or feathers, is far more diagnostic of high resting metabolic rates,
since it is (a) absent from all living ectotherms, and (b) is functionally
linked to tachymetabolic endothermy.  Insulation reduces the rate of heat
transfer between an animal and its environment, thus allowing species with
high rates of endogenous heat production to maintain high stable body
temperatures, even in cold climates.  Insulation also functions to
reduce solar heat loads, thus reducing heat stress.
        By contrast, insulation would be a liability to ectotherms,
because it would reduce the rate of environmental heat gain, therefore
lengthening the basking time needed for attainment of preferred body
temperatures.  In addition, insulation by itself does not confer
significant thermal stability in the absence of a high metabolic rate,
as Cowles (1958) demonstrated many years ago by fitting lizards with
customized fur coats!  In my view, the presence of body feathers on
_Archaeopteryx_ represents direct, convincing evidence this protobird
already possessed a resting metabolism well above that of any living
reptile.
        Interestingly, the recent reports of the feathered theropod
from China (Browne, 1996), suggest the feather covering was down-like.
If true, this suggests an evolutionary scenario whereby feathers first
evolved as insulation in small, high metabolism theropods, and were
modified into flight structures later.  The hairy feathers present
in flightless birds may represent a secondary regression to the
theropod feather condition.

REFERENCES

        Browne, M. W. (1996).  Feathery fossil hints dinosaur-bird link.
The New York Times, 10/19, p. 1A, 10A.
        Bennett, A. F. (1987).  Structural and functional determinates of
metabolic rate.  American Zoologist, V. 28, pp. 699-708.
        Bennett, A. F. (1991).  The evolution of activity capacity.
Journal of Experimental Biology, V. 160, pp. 1-23.
        Brush, A. H. (1965).  Energetics, temperature regulation and
circulation in resting, active and defeathered California quail,
_Lophortyx californicus_.  Comparative Biochemistry and Physiology,
V. 15, pp. 399-421.
        Chinsamy, A., Chiappe, L. M. & Dodson, P. (1995).  Mesozoic
avian bone microstructure: physiological implications. Paleobiology,
V. 21 (4), pp. 561-574.
        Cowles, R. B. (1958).  Possible origin of dermal temperature
regulation.  Evolution, V. 12, pp. 347-357.
        Herremans, M., & Decuypere, E. (1986).  Heat production of
artificially defeathered dward cockerels as different ambient
temperatures.  Journal of Thermal Biology, V. 11 (2), pp. 127-130.
        Kellner, A.W.A., Maisey, J. G. & Campos, D.A. (1994).  Fossil
down feather from the lower Cretaceous of Brazil.  Palaeontology,
V. 37 (3), pp. 489-492.
        Leahy, G. D. (1991).  Lamellar-zonal bone in fossil mammals:
implications for dinosaur and therapsid paleophysiology.  Journal of
Vertebrate Paleontology, V. 11 (supplement to 3): p. 42A.
        Maloney, S. K., & Dawson, T. J. (1995).  The heat load from
solar radiation on a large, diurnally active bird, the emu (_Dromaius
novaehollandiae_).  Journal of Thermal Biology, V. 20 (5), pp. 381-387.
        Marsh, R. L., & Dawson, W. R. (1989).  Avian adjustments to cold.
pp. 206-253. _in_ Advances in Comparative and Environmental Physiology:
Animal Adaptation to Cold.  (Wang, L. C. H.) Ed. Springer-Verlag, New York.

        In addition, on the same date, Darren Naish also wrote:
>HYPSILOPHODONT SKIN
>
>With regard to dinosaur skin, I do recall reading a paper by Peter
>Galton where he mentioned an integumentary impression from a hypsilophodont
>(it was scaly).

        The reference in question is...

        Galton, P. M. (1974).  The ornithischian dinosaur _Hypislophodon_
from the Wealden of the Isle of Wight.  Bulletin of the British Museum of
Natural History, V. 25, pp. 1-152.

        Galton discussed the possibility of a "scaly" _Hypsilophodon_,
but concluded the available evidence for this was questionable.  The
smallest ornithischian dinosaur for which there is published evidence of
scaly skin is _Scelidosaurus_; however, there is an undescribed specimen
of the small ornithopod _Bugenasaura infernalis_ which appears to have
scale impressions on the head.  Whether the rest of the body was like this
is unknown.

Guy Leahy
n9435712@henson.cc.wwu.edu
Dept. of PEHR
Western Washington University
Bellingham, WA 98225-9067