getting a handle on Dinosaur taxonomy

[Eric Martichuski <herewiss13@hotmail.com>]

I'm having a hard time wrapping my brain around the relationships between 
dinosaurs, and I was hoping someone might be able to clarify matters in 
terms of modern species comparisons.

When we talk about two different hadrosaur species, would it be like 
comparing two antelope?  Or do you need to go further out to a comparison 
between all two-toed hoofed mammals.  Or is it just the variation between 
mule deer and white-tailed deer?

Are T. Rex and some other large contemporary theropod carnivore (I'm 
blanking on names) like Lions and Tigers?  Indian and African Lions?  Or 
more like Bears and Wolverines?

I realize the question covers a _lot_ of complicated ground, but I'm just 
trying to get a simple handle on the issue by analogy.  Most paleontology 
seems to deal with relationships by evolutionary descent (species x begat 
species y which begat species z), rather than looking at the separation 
between contemporaries.  You've got two ceratopsians.  Do I look at them 
like Moose and Elk or like red and grey Foxes?

I'm probably not helped by ignorance over where larger dinosaur grouping 
fall in the taxonomic order.  For example: is "Therepod" an order?  A class? 
 A family?

Thanks for any light you can shed on the subject,
Eric

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