"Common ancestor" in cladistics

[Manuel <mparrado@peoplepc.com>]

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.

Thanks in advance.

Manuel Parrado

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