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Okay, I'm still in need of some taxonomic re-education.  Although I know 
dinosauria isn't a formal taxon, I'm finding some puzzling new 
definitions and restrictions of the term.

For instance, I've been running across statements such as this:

>I've seen often stated, I think even in this mailing list, that there
>were no aquatic dinos, yet some exist even today (penguins).

and also:

>The ref on sci.anthropology.paleo may be a systematic mistake by the
>anthropologists (in that they may think ichthyosaurs, plesiosaurs, etc. are
>dinosaurs, which they are most emphatically not!).
>Except for pengiuns, auks, hesperornithiforms, and other aquatic birds,
>is no strong evidence for dinosaurs specialized for marine habits.

Traditionally, my understanding was that "dinosaurs" were extinct 
Archosaurian Diapsids, which would include Thecodonts, Pterosaurs, 
Saurischians, and Ornithischians, (and Placodonts?), excluding only the 
Crocodylia.  But am I to understand that birds, or more accurately, 
animals in the class Aves, are also members of dinosauria?  If dinosaurs 
are Archosaurs, and Archosaurs belong to the Diapsids, and Diapsids 
belong to the class Reptilia, how can this be?  I'm aware of the Bakker 
revolution (revelation?  resolution?  recognition?  retribution?), that 
dinosaurs were physiologically more similar to birds than to modern 
reptiles, but I didn't know that this similarity and common ancestry 
warranted a re-definition of "dinosauria".

I know of course that Archaeopterygiformes and other groups are teetering 
on the line between what has been typically classified as Avian and 
typically classified as Reptilian, however I was under the impression 
that there were certain anatomical distinctions that could easily exclude 
them from the group called "dinosauria".  (Feathers withstanding.)

Also, I've always found it rather frustrating that creatures such as 
Elasmosaurus and Dimetrodon are not also called "dinosaurs".  I know some 
of them don't have the double temporal opening, but there is a group of 
prehistoric reptile with which we are all familiar that include the 
Archosaurians, the Pelycosaurians, the Ichthyosaurians 
(Ichthyopterygians?), the Plesiosaurians, and others.  Why CAN'T the term 
"dinosaur" apply to these?  Or at least, why can't there be a term that 
would refer to this group, which would be limited to prehistoric/extinct 
vertebrates that are not mammals, aves, crocodiles, or amphibians, rather 
than always having to say, "Dinosaurs and other prehistoric reptiles."

BTW:  The Webster's Dictionary defines the word "dinosaur" as "1) any of 
a group (Dinosauria) of extinct chiefly terrestrial carnivorous or 
herbivorous reptiles, 2) any of various large extinct reptiles."  These 
definitions are much more ambiguous than what is being represented by 
professionals.  The words "extinct" and "reptiles" appear to leave 
penguins out in the cold.  "Chiefly" terrestrial seems to exclude 
Plesiosaurs, Ichtyosaurs and Nothosaurs, but the second definition is not 
so prejudice.  And weren't some dinosaurs omnivorous and many quite small?

Let me present my question this way:  If all of you were asked to write a 
definition of "dinosaur" for the latest edition of the dictionary, what 
would it currently be?

[ To cut off some of what I expect to be many redundant replies to
  this message, let me answer it with what I expect would be the most
  common responses:  Dinosauria includes the most recent common
  ancestor of _Iguanodon_ and _Megalosaurus_ and all of that animal's
  descendents.  Alternatively, you might use _Tricerotops_ and birds
  in place of _Iguanodon_ and _Megalosaurus_.  -- MR ]

--Christopher Sirmons Haviland (incertae sedis)