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RE: Are dinosaurs really reptiles?



Birds nested in Dinosauria, which are nested within Reptilia, which are
nested within Amniota, which are nested in Amphibia, which are nested
within Osteichthyes, which makes us hairy fishes! ;-)

-----Original Message-----
From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu] On Behalf
Of Roberto Takata
Sent: Friday, April 25, 2008 4:18 AM
To: trex_kid@hotmail.com
Cc: dinosaur@usc.edu
Subject: Re: Are dinosaurs really reptiles?

Well, it is an issue more or less well settled down.

In a phylogenetic systematics (a classification system that takes into
account only derived - evolutionarily novel - characteristics) there
is no such thing as class, order or so on. Just monophyletic groups (a
group that is formed by an ancestral form and all of its descendants).
And a monophyletic group could and must be nested within a broader
(more inclusive) monophyletic group: just in the same way that Primate
is within Mammalia and Mammalia within Synapsida and Synapsida within
Amniota and so on. (Only the group that include all life forms will
not be included in any other group.)

Most researches agree that Reptilia is a group that include
crocodiles, turtles, lizzards, snakes, tuataras their most recent
common ancestor and all its descendants. So, birds and dinosaurs are
included within Reptilia - they share a exclusive ancestor with
crocodyles. But mammals and their extinct relatives are not.

If we exclude dinosaurs from Reptilia, it won't be a monophyletic group
anymore.

Alternatively we could expand Reptilia to include Mammalia and their
relatives - but in doing so it will be the same as Amniota.

(Robert T Bakker once argued to elect a class to dinosaurs - including
birds - but he was not using the phylogenetic way of thought.)

[]s,

Roberto Takata

[]s,

Roberto Takata

On Fri, Apr 25, 2008 at 6:57 AM, Brandon Pilcher <trex_kid@hotmail.com>
wrote:
"Traditionally, dinosaurs have been considered reptiles, and it's true
that many share some physical characteristics with reptiles (scaly
skin). However, I know that there are paleontologists who think
dinosaurs are distinct enough from traditional reptiles to justify
getting their own taxonomic class. I am inclined towards this view.
The very fact that birds, traditionally considered to be non-reptiles,
are the closest living relatives of dinosaurs, not any extant
reptiles, convinces me to consider the Dinosauria as separate from
Reptilia. There's no good reason to classify a group of animals inside
one class when those animals are more closely related to members of
another class (birds) than to those within the class they've been
assigned."