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Four articles, one journal (long)

Four interesting articles, all in the same issue of American Zoologist.   I
have included the abstracts to pique your curiosity, but if you want to
scratch that itch, get thee to a library!   



Full source     American Zoologist, 2000, Vol 40, Iss 4, pp 513-529
Authors PFA Maderson, L Alibardi
Title   The development of the sauropsid integument: A contribution to the
problem of the origin and evolution of feathers

Abstract        Developmental anatomical data are insufficient to discuss
plausible intermediates between an ancestral, scaled, reptilian skin and
appendage-bearing, avian skin. We also review adult tissue replacement and
ubiquitous mechanisms underlying skin morphogenesis. Combining developmental
data sensu late with consideration of necessary biological roles permits
evaluation of major form/function trends in skin evolution, New data on
feathers reveal retention of the sauropsid synapomorphy of vertical
alteration of alpha- and beta -keratogenesis. By identifying roles that were
obligatorily maintained throughout evolution, we demonstrate constraints on
hypothetical skin morphologies in preavian taxa. We analyze feather origins
as a problem of emergence of complex form via modulations of morphogenesis.
While existing data do not permit presentation of sequential, hypothetical,
intermediates culminating in a plumage, the analysis: (1) implies that a
protofeather and its follicle are most easily derived from isolated,
flattened, elongate, reptilian scales; (2) explains diversification of
feather morphs from a contour-like ''basic'' feather and the similarity
between feather and hair follicles; and thus (3) reveals several
developmental constraints on structures proposed as antecedent to avian
feathers, whether hypothetical constructs or palaeontological
interpretations. Although these conclusions do not depend on any previous
scenario, they are consistent with Regal's (1975) model and the limited,
fossil evidence, especially that of the ''basal archosaur'' Longisquama.

Full source     American Zoologist, 2000, Vol 40, Iss 4, pp 553-574
Authors DG Homberger, KN deSilva
Title   Functional microanatomy of the feather-bearing integument:
Implications for the evolution of birds and avian flight

Abstract        A selective regime favoring a streamlining of body contours
and surfaces is proposed as having been instrumental in driving the
morphological and functional transformations of an unfeathered reptilian
integument into a feather-bearing avian one, This hypothesis is consistent
with a new, structurally and functionally coherent analysis of the
microanatomy of the avian feather-bearing integument as a complex,
integrated organ system that includes an intricate, hydraulic
skeleto-muscular apparatus of the feathers, a dermo-subcutaneous muscle
system of the integument, and a subcutaneous hydraulic skeletal system
formed by fat bodies. Key elements of the evidence supporting the new
hypothesis are (1) the presence of depressor feather muscles that are not
needed as antagonists for the erector feather muscles, but can counteract
external forces, such as air currents; (2) the fact that the highly
intricate feather-bearing integument represents a machinery to move feathers
or to stabilize them against external forces; (3) the crucial role of the
coat of feathers in streamlining the body contours and surfaces of birds;
(4) the aerodynamic role of feathers as pressure and turbulence sensors and
as controllable temporary turbulators; and (5) the critical role that a
streamlined body plays in avian flight and is likely to have played in the
evolutionary transformations from ecologically and locomotorily versatile
quadrupedal reptiles to volant bipedal birds without passing through
parachuting or gliding stages. These transformations are likely to have
occurred more than once. The ancestral birds were probably small, arboreal,
hopping, and using flap-bounding, or intermittent bounding, flight.

Full source     American Zoologist, 2000, Vol 40, Iss 4, pp 676-686
Authors SF Tarsitano, AP Russell, F Horne, C Plummer, K Millerchip
Title   On the evolution of feathers from an aerodynamic and constructional
view point

Abstract        The evolution of birds and feathers are examined in terms of
the aerodynamic constraints imposed by the arboreal and cursorial models of
flight evolution, The cursorial origin of flight is associated with the
putative coelurosaurian ancestry of birds. As presently known, coelurosaurs
have a center of mass located in the pelvic region and an elongated pubis
that is ventrally or anteriorly directed. Both of these characteristics make
it difficult to postulate an origin of flight that would involve a gliding
phase because the abdomen cannot be flattened into an aerodynamic shape.
Moreover, the cursorial model must counteract gravity using the hindlimb
and, thus, selection for the power requirement for lift-off would not fetus
on the forelimb, Therefore, if the hypothesis proposing a coelurosaurian
ancestry of birds is to remain viable, it must be via an as yet undiscovered
taxon that is compatible with the morphological and aerodynamic constraints
imposed by flight evolution. The arboreal model, currently centers around
non-dinosaurian taxa and is more parsimonious in that early archosaurs have
short pubes that do not preclude an aerodynamic body profile. Moreover, the
arboreal proavis uses gravity to create the airflow over the body surfaces
and is, thus, energy efficient. Consideration of the initial aerodynamic
roles of feathers and feather design are consistent with a precursory
gliding phase. Whether avian ancestry lies among coelurosaur theropods or
earlier archosaurs, we must remain mindful of the complex aerodynamic
dictates of gliding and powered flight and avoid formalistic approaches that
co-opt sister taxa, with their known body form, as functional ancestors.

Full source     American Zoologist, 2000, Vol 40, Iss 4, pp 687-694
Authors LD Martin, SA Czerkas
Title   The fossil record of feather evolution in the Mesozoic

Abstract        The oldest known feathers from the Late Jurassic are already
modern in form and microscopic detail. Because these oldest examples are
assignable to an extinct branch (Sauriurae) of the basal avian dichotomy,
their features must have been established at a significantly earlier date.
The skin of a wide variety of dinosaurs is now known and is unlikely to
represent a predecessor to a feather bearing integument, Examples of
feathered dinosaurs result from erroneous identification of internal
structures as part of the skin covering, and from the confusion of
flightless birds from the Early Cretaceous of China with dinosaurs.