To refresh - I had asserted, too dogmatically, that taxonomic
classification is arbitrary and he replied with reference to the
shape of the "jaw."
I replied - The shape of the dental arcade is entirely determined by
the relative sizes of the incisors, canines and post-canine (cheek
teeth) teeth. It simply is not true to claim that Australopithecines
had a U shaped, or parabolic, arcade (as do modern humans). Some
did, most did not. Many of them had cheek teeth larger than modern
gorillas, that is, huge. Others had incisors larger than modern
chimpanzees, that is, huge. All of them had canines shorter than modern
male chimps or gorillas but still usually much larger than modern
humans with regard to the amount of space they take up in the arcade.
Large canines, large incisors and small cheek teeth typically
produce the rectangular shape common to modern pongids. (The V shape
that Glenn referred to is produced by large cheek teeth, small
canines and tiny incisors.) Each of these tooth types has its own
function and, so, is related to the habitual diet. Chimps consume a
lot of fruit; gorillas a lot of tough herbivorous stuff. The canines
of the males are used primarily for social display.
Paleoanthropologists claim the existence of at least three different
species of Australopithecines none of which are assigned solely on
the basis of shape of dental arcade.
My comment with regard to arbitrariness becomes relevant here. If
one has a fossil (fragment) of an upper jaw (maxilla) one cannot, in
good conscience or, I claim, in good scientific practice, name a
species and proceed to describe it (with apologies to Cuvier). But
this is exactly what was done in the case of Ramapithecus. This
taxon has now been eliminated on the strength of opinion. If one has
a fossil of a leg or, better, a pelvis it is easy to recognize
bipedalism. This attribute, by the way, appears in the fossil record
without ANY precursors. But this is sufficient only to recognize a
stage, not a species. Hence the arbitrariness.
The criterion for accepting a species is reproductive isolation ("in
nature"). At the risk of pointing out the painfully obvious, this
information is never preserved in the fossils. I assert that the use
of biological taxonomic labels for fossils is totally inappropriate.
Better would be something like "Joe" or "Lucy" or "stage 1". The use
of taxonomic labels for fossils (empirically) implies the acceptance of a specific
theory of evolution - the neoDarwinian at the moment - and all the
metaphysics that attend it.
I apologize for getting "into" this. It is, however, important to realize that
taxonomy cannot be done on an attribute by attribute basis - this
form has a jaw shaped like that one so they are related. It simply
does not work that way.