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Juvenile characteristics (was) MORE FEATHERS & NEKKID MAMMALS

Jeff Poling wrote:

>    1)  Hoatzins are born with two fully functional digits that are later
> absorbed into the typical avian carpusmetacarpus (or whatever the hand is
> called).  I doubt this sort of thing is unique, where juveniles have
> characters different from adults.
  Forgive me for diverging for a moment here, but isn't it frequently
found that embryos -during their developement- go through phases which
display traits which are primitive for that species, such as gills and
tails in human embryos-which are later re-absorbed into the system?
  Does this type of throw-back-ness have a technical name?
  Is development (and loss) of such characteristics AFTER birth/hatching
the same sort of thing?  Human babies are frequently born with hair over
every surface of the body which is lost shortly after birth (unless you
have a wierd genetic trait which means you grow up looking like Jojo the
dog-faced boy).
  Would it be sense to say in that the Hoatzin and Marsh hen chicks
(which also have juvenile fingers), that fingers are thus primitive for
these types and that they have the same sort of delayed absorbtions as
Jojo does?  Or are these traits found to be some wierd genetically
triggered thing which was re-evolved specifically for each type?

-Betty Cunningham