Re: "Common ancestor" in cladistics

[Nick Pharris <npharris@umich.edu>]

Quoting Manuel <mparrado@peoplepc.com>:

> To help me get a feel for how paleontologists build phylogenic
> (phylogenetic?) relationships, can somebody tell me whether there are some
> genera or even species that have been identified as the so-called "common
> ancestor" of others?  Is there such a thing as an actual common ancestor or
> is that an abstract concept?  If there are, how are they identified to be
> such?  Some examples of actual dinosaurs would be great.

Well, you can't exactly call them abstract concepts, since the ancestors must
have actually existed at some point.

On the other hand, it is common in phylogenetic nomenclature to refer to a
common ancestor, without expecting that the actual ancestor will ever be found
(or recognized as the common ancestor even if it were found).

So, for instance, one definition of Dinosauria is "the common ancestor of
_Megalosaurus_ and _Iguanodon_ and all of its descendants".  Assuming
_Megalosaurus_ and _Iguanodon_ do not derive from two separate origins of life
(which I think we can all agree on), that ancestor must have existed at some
point.  But to the best of our knowledge no actual remains of that ancestor
have ever been discovered, and it is very likely that they never will be.

Nick Pharris
Department of Linguistics
University of Michigan