The Definition and Description of a "Transitional"

I really don't like talk of transitionals as discrete entities. Every organism that has ever existed (including us) is a transitional form. Except of course, those lineages that became extinct without producing descendents...But a very good transitional anyway is Archaeopteryx, or perhaps Ambulocetus...

Ok...It would be good to define a transitional - perhaps with respect to character states? A transitional could be defined as an organism that has a synapomorphy of an ancestral taxa as a plesiomorphy, and a plesiomorphy of a descendent taxa as a synapomorphy. How does that sound?Easier description: Transitional exhibits as an ancestral character state what is a derived character state in the the 'older' form, and exhibits as a derived character state was is an ancestral character in the later form.I think we may be onto something here...

Sorry, I'm using too much jargon...Here's maybe a simpler definition. We have Lineage A and Lineage B. Lineage A/B is postulated to be a 'transitional form'. A 'unique/derived' character in Lineage A can be noted to be an ancestral, primitive character in Lineage A/B, and an ancestral, primitive character in lineage B is a 'unique/derived' character in Lineage A/B.

It's not so much their existence, but rather their definition as 'ancestral' or 'derived' that is more important me thinks.

Sure - I'll try my best.You said in a previous post

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"We have to separated higher taxa (today). For example, reptiles and mammals. (A and B).
A transitional (A/B -- a reptimammal) would have a specific characteristic that we now associate only with A (reptiles) AND some other specific characteristic that we now associate only with B (mammals)."

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It's not so much whether the transitional possesses characters that 'we would associate only with ...', although in most cases we would. The reason that this isn't our exclusive caveat is that we may discuss a character that is present in BOTH A and B.We need more to focus on whether our characters are derived, ie, evolved within that lineage, or whether they are ancestral, ie, inherited by common descent from an ancestor.An example, perhaps, using Archaeopteryx and the theropods/bird relationships.Dromaeosaurids, such as Velociraptor possess a unique, derived character (synapomorphy), and that is axial rotation of the wrist. In Archaeopteryx, this is present also, but as an inherited, ancestral characteristic (plesiomorphy). Archaeopteryxalso possesses feathers. Now, let us for argument's sake say that feathers first appeared in Archaeopteryx (although they didn't, but it just complicates things). Here, they would be a unique, derived characterstic - synapomorphy. Then we examine modern birds. Feathers and rotatable wrists are present as ancestral characters - plesiomorphies.So here we see it is not really pertinent to talk about 'what we now associate only with'. In evolution we don't really do that. What we do is look at which characters are derived (i.e appear in that lineage). Monophyletic clades are defined only be derived characters of the basal group. So, a transitional form can be defined as a lineage that possesses a synapomorphy of a basal lineage as a plesiomorphy, and a plesiomorphy of a descendent lineage as a synapomorphy. Note that this doesn't preclude the descendent lineage also possessing the synapomorphy of the ancestral lineage as well.

Your Admin Avatar is totally hot. Who is it?

As a matter of fact I'm not a girl. Do you have a problem with that?

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But don't human heads vary, sometimes greatly, in size from one to another?

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No, actually, they don't - certainly not with respect to the cranio-dental morphological features displayed by earlier hominid remains (such as Australopithecus afarensis or the members of the Paranthropus clade)