Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!samsung!munnari.oz.au!bruce!dbrmelb!davidp
From: davidp@dbrmelb.dbrhi.oz (David Paterson)
Newsgroups: sci.bio
Subject: Re: Vertebrate growth & evolution
Message-ID: <655@dbrmelb.dbrhi.oz>
Date: 26 Nov 89 22:08:19 GMT
References: <MZO6H0u00Ws9A6TERS@andrew.cmu.edu>
Organization: CSIRO, Div. Building Constr. and Eng'ing, Melb., Australia
Lines: 52

>          Are you saying that Dinosaurs stopped growing when they
> reached maturity?  I've never heard that before, but 
> if there is evidence to support that, I would like to hear about it.

Yes, so would I.

Let me put it this way. The closest living relatives of the dinosaurs
are the birds and many species of birds stop growing when they reach
maturity. So, in the absence of any other evidence, there is a better
that 50% chance that dinosaurs stopped growing when they reached maturity.

It is widely believed that the second closest living relatives of the
dinosaurs are the snakes and lizards (see, for instance, the back 
cover of every second issue of New Scientist). But, a few years ago,
a study of molecular evolution revealed a closer link between the
birds and mammals than between these and reptiles.

There are other peices of evidence supporting this result.
Mammals, birds and dinosaurs are warm blooded but reptiles are not.
Mammals, birds and dinosaurs have an upright stance but reptiles do
not. Pterosaurs are thought to be closely related to birds and dinosaurs
but pterosaurs and mammals have hair. Mammals and birds stop growing
when they reach maturity but reptiles do not. Mammals and birds have
toenails but reptiles do not.

So, if mammals are the second closest living relatives of the dinosaurs
then there is an excellent chance that the dinosaurs stopped growing
when they reached maturity. If it is true that ALL mammals and birds
stop growing when they reach maturity then the chance that dinosaurs
stop growing when they reach maturity approaches certainty. However,
this takes me a long way beyond my present knowledge.

Some readers will be temped to write in and say that the second closest
living relatives of the dinosaurs MUST be the snakes and lizards because
snakes and lizards, birds and dinosaurs share the same skull type which
is different from that of mammals. But it is possible to arrange an
evolutionary sequence: fish skull to tortoise skull to Ichthyosaur skull
to lizard and bird skull to mammal skull. And mammal skull may have
evolved from bird skull AFTER the evolutionary split between birds and
lizards.
 
If I'm wrong, please write in and tell me.

There is a further possibility. Some Pterosaurs have hair; feathers are
structurally more similar to hairs than scales; birds and mammals both
have toenails. These are pieces of evidence that suggest that some
dinosaurs were covered in hair. These would probably be small dinosaurs;
some large dinosaurs are known to have been naked like some large mammals
and some large lizards. Keep a lookout for hairy dinosaurs.

				    David Paterson
				    CSIRO, Highett, Australia.


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