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RE: Extinction Question and a Thank You!

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Dinosaur World
> What I am not clear on is why there are so many modern animals that have
> retained the same basic design as their Permian and Cretaceous counterparts?

Well, to be fair, at precisely the same level birds retain the "basic design" 
as their dinosaurian ancestors...

> If early Crocodiles, Alligators, Sharks, Insects etc. were subjected to
> global environmental changes that were long enough and strong enough to wipe
> out other species, why then do they still resemble their modern
> counterparts?

A) Short answer: they don't. The particular examples you give are part of a 
great paleo-myth perpetuated by countless books, movies,
and TV shows. There is considerable anatomical difference between a Hybodus and 
a Great White, for instance. Or a Goniopholis and a
Crocodylus. The problem here is that the general audience is simply not 
well-informed on the details of comparative anatomy to be
able to distinguish the difference. (Plus, many of the differences would be 
internal, and hence not lend themselves well to being
shown as paintings or TV appearances).

B) Longer answer: that being said, there certainly are some modern organisms 
who closely resemble their ancient ancestors. Blue
green bacteria are relatively unchanged over billions of years, at least 
anatomically. The living Neoceratodus lungfish is nearly
identical to the extinct Triassic Ceratodus.

But as much work has shown, there is a continuum between rapidly evolving 
organsims, stately evolving organisms, and very slowly
evolving organisms. If you have a continuous distribution of evolutionary 
rates, some lineages are going to be WAY out towards the
end of the distribution: these are the ones we call "living fossils". But it 
seems that there is no break in distribution between
"living fossils" and more rapidly evolving form.

> Wouldn?t a dramatic change in their environment demand that
> they would have to evolve to cope with it? In other words, shouldn?t there
> be a very recognizable difference between a pre- KT Crocodile and Post-KT
> Croc?

There are. You have to open up their snouts and look inside their mouths, 

(And, to be fair, there is probably greater differences between Early 
Cretaceous and latestst Cretaceous crocs than between a latest
Cretaceous croc and now).

> Could any of you give me your insight into this please?

Here's food for thought: there are no animals alive today that closely resemble 
a Permian land vertebrate. Every single living group
of land vertebrate is highly specialized with regards to their Permian 
ancestors and relatives. Hence, the similarities between then
(i.e., four-footedness) are simply superficial.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
        Mailing Address:
                Building 237, Room 1117
                College Park, MD  20742

Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796