Re: "Common ancestor" in cladistics

["T. Michael Keesey" <mightyodinn@yahoo.com>]

--- Manuel <mparrado@peoplepc.com> wrote:

> I hope this is not an overdone question.
> 
> Recently, I have started reading very avidly about dinosaurs and paleontology
> in general.  I keep finding many authors (of books, websites, articles, etc.)
> referring to groups of dinosaurs as having "a common ancestor", "a more
> recent common ancestor than", "all descendants of the common ancestor of this
> and that", etc...
> 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.

There were such things, of course, but they are impossible to identify with
100% certainty. Henry Gee's _In Search of Deep Time_ covers this point well.
Without an unbroken chain of individual descendants, you can't prove ancestry,
and you'll never find an unbroken chain of descendants in the fossil record.

You may find something that exists at the right time and has no autapomorphies,
but it may simply be a close offshoot. Autapomorphies may be present but not
preserved (e.g. color pattern), or there may even be no autapomorphies
(depending on your species concept, of course).

The best we can do is find good models for ancestor. For example, _Eoraptor_ is
a pretty good model for the ancestral dinosaur. (But it can't be the ancestral
dinosaur, since other dinosaurs coexist with it, or even slightly predate it.)

=====
=====> T. Michael Keesey <http://dino.lm.com/contact>
=====> The Dinosauricon <http://dinosauricon.com>
=====> Instant Messenger <Ric Blayze>
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