Definition of Dinosauria?

[Pachyrhinosaurus]

Most of us here have at least some interest in dinosaurs. As I was thinking, I noticed I didn't know the true definition of "dinosauria". I'm not thinking about the regular, look-it-up-in-a-dictionary definition, but the offical scientific one which uses coomon ancestors and the like. I think it was published in a paper, but as of now I am unsure of the paper or the definition. Does anybody know it? Thanks in advance.

[Patrx]

There are two that I have heard/read
"Triceratops, Neornithes, their most recent common ancestor, and all descendants" - Neornithes being birds.
or
"Triceratops horridus, Saltasaurus loricatus, and Passer domesticus, their most recent common ancestor, and all descendants" - Passer domesticus being the common house sparrow.

The latter definition is a little more inclusive, but otherwise, I'm not sure why there are two in common use.

[SBell]

There are two that I have heard/read
"Triceratops, Neornithes, their most recent common ancestor, and all descendants" - Neornithes being birds.
or
"Triceratops horridus, Saltasaurus loricatus, and Passer domesticus, their most recent common ancestor, and all descendants" - Passer domesticus being the common house sparrow.

The latter definition is a little more inclusive, but otherwise, I'm not sure why there are two in common use.

The latter would used under the assumption that the sauropods may not actually share a common ancestor with the theropods (including neornithines) but instead represent one of three radiations stemming from a common ancestor [side note, I have no opinion or expertise on this question] and so must be accounted for as a third branch of the clade (using the sauropod Saltasaurus) that would include birds and ornithischian dinosaurs (represented by Passer & Triceratops here).

Hennig would be mortified that the tree was being trifurcated at a single node, but sometimes nature does its own thing, regardless of people's theories.

[Gryphoceratops]

How dare nature not go along with human theories!   

[SBell]

How dare nature not go along with human theories!   

In pure Hennig cladistics, a node could only bifurcate--so to have a presumed ancestor lead to three different but certainly related lineages would require further resolution (in general, this is solved by the still-generally accepted saurischian-ornithischian dichotomy).  I'm not saying I necessarily agree with the second, extended dinosauria definition, but I understand why it exists.

[DinoToyForum]

A trichotomy (or polytomy) in a cladogram isn't to be taken literally - it is just a visual representation of a conflict in the data - lack of resolution. i.e. A cladogram isn't (necessarily) a family tree.

Hence, in a family tree or phylogenetic tree, it is bifurcations only, whereas a cladogram can be unresolved and, in every cladist's nightmare, could look like this one on the far right: *yuck* A cladogram is a representation of data, not of relationships.

[Dinoguy2]

There are two that I have heard/read
"Triceratops, Neornithes, their most recent common ancestor, and all descendants" - Neornithes being birds.
or
"Triceratops horridus, Saltasaurus loricatus, and Passer domesticus, their most recent common ancestor, and all descendants" - Passer domesticus being the common house sparrow.

The latter definition is a little more inclusive, but otherwise, I'm not sure why there are two in common use.

One that is coming into wider use is Iguanodon + Megalosaurus + Hylaeosaurus, as these are the original dinosaurs recognized by Owen. This is the one that will become official in the PhyloCode which mandates definitions should closely match original use.

Birds and sauropods were not originally considered to be dinosaurs, so it's a bad idea to include them in the definition. We shouldn't force birds and sauropods to be dinosaurs by definition, we should show them nesting among traditional dinosaurs using evidence.

[DinoToyForum]

"nesting", hah  Apart from that, I'm not sure I follow your reasoning. Surely we want a modern definition, not a historical one?

[SBell]

"nesting", hah  Apart from that, I'm not sure I follow your reasoning. Surely we want a modern definition, not a historical one?

Agreed--a cladogram, like any other method, is a hypothesis of ancestry and descent.  Our understanding of dinosaur relationships has certainly evolved past Owen's original decisions (for example, why wouldn't sauropods be included just because Owen didn't include them?).

[DinoToyForum]

"nesting", hah  Apart from that, I'm not sure I follow your reasoning. Surely we want a modern definition, not a historical one?

Agreed--a cladogram, like any other method, is a hypothesis of ancestry and descent. 

I conducted a cladistic analysis of biscuits once, which resulted in a cladogram. Is that cladogram a hypothesis of ancestry and descent for biscuits? 

Edit - by the way, I really did  (old old site) http://www.plesiosauria.com/dinobiscuits/biscuit.htm

[SBell]

"nesting", hah  Apart from that, I'm not sure I follow your reasoning. Surely we want a modern definition, not a historical one?

Agreed--a cladogram, like any other method, is a hypothesis of ancestry and descent. 

I conducted a cladistic analysis of biscuits once, which resulted in a cladogram. Is that cladogram a hypothesis of ancestry and descent for biscuits? 

Edit - by the way, I really did  (old old site) http://www.plesiosauria.com/dinobiscuits/biscuit.htm

True enough--I did one for nuts and bolts (hardware) in university.  I guess the use of cladograms has been extended from its original use of determining relationships (sort of like how taxonomy as a whole has grown beyond simple naming).

And it could be a biscuit ancestry test--the ancestral biscuit may have been a flat piece of breading, and over time the recipes evolved to add, recombine, and alter so that a basic sugar cookie could eventually become an oatmeal raisin  *yuck* (in this case, the recipes behind the biscuits are the ancestor-descendant, and the biscuits themselves are the expressions thereof).

[Pachyrhinosaurus]

"Triceratops, Neornithes, their most recent common ancestor, and all descendants"

Thank you. That's a great definition. I really like that neornithes are included. Who published this? It would be nice to quote him/her.

[SBell]

"Triceratops, Neornithes, their most recent common ancestor, and all descendants"

Thank you. That's a great definition. I really like that neornithes are included. Who published this? It would be nice to quote him/her.

I found the quote, verbatim, here:
http://www.amazon.ca/Dinosauria-Second-David-B-Weishampel/dp/0520242092/ref=sr_1_1?s=books&ie=UTF8&qid=1341934471&sr=1-1

The exact quote (thanks to Google Books) is near the bottom of this page (last paragraph, left column).

[Dinoguy2]

"nesting", hah  Apart from that, I'm not sure I follow your reasoning. Surely we want a modern definition, not a historical one?

Agreed--a cladogram, like any other method, is a hypothesis of ancestry and descent.  Our understanding of dinosaur relationships has certainly evolved past Owen's original decisions (for example, why wouldn't sauropods be included just because Owen didn't include them?).

I think you guys are misunderstanding. Sauropods aren't excluded just because they're not in the definition, just like tyrannosauroids are still included. Current research suggests they also descended from the common ancestor of megalosaurus and iguanodon, so they are also considered dinosaurs, along with birds. But if it later turned out we were wrong and sauropods are closer to, say, crocs, this would retain Dinosaurian to its original intended content, without including crocodiles as dinosaurs.

PhyloCode prefers historical definitions to prevent people enshrining their hypotheses in well known names. Including birds, which were only recently discovered to be dinosaurs, would be begging the question. Plus, if by some fluke band turned out to be right, dinosauria as we know it would cease to exist.