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RE: Looking to catch up on Sauropods.

Mike Lima wrote-

I just recently got a copy of "The Dinosauria" (I'm
just beginning to actually pick up and look for
literature that I should have gotten along time ago.
I'm new to this whole thing and much of the vocabulary
used in these posts goes way over my head).

You should have got the recently published second edition.

First off it has Diplodocidae as a family broken into
Diplodocinae, Dicraosaurinae, and Mamenchisaurinae
subfamilies. Now, I've also heard the diplodocid
family expressed the same way only with
"Rebbachisaurinae" replacing "Mamenchisaurinae" (which
I realize doesn't belong). However yet again, I've
also seen Rebbachisauridae and Dicraeosauridae as
families along with Diplodocidae under Diplodocoidea.
Which way is accepted today?

I've never seen Rebbachisauridae recently sunk as a subfamily of Diplodocidae. As far as I know, everyone keeps Dicraeosauridae and Diplodocidae as families as well (the diplodocoid clade they both belong to is Flagellicaudata, which does not include rebbachisaurids).

What families do Euhelopus and Omeisaurus belong?

Euhelopus is either a non-neosauropod eusauropod or a basal titanosaurian. It's been suggested Pukyongosaurus is a close relative, but Euhelopodidae is not recognized anymore. Classic 'euhelopodids' (e.g. Shunosaurus, Omeisaurus, Mamenchisaurus) are more primitive eusauropods than Euhelopus.

Omeisaurus is a non-neosauropod eusauropod. Mamenchisaurus (Wilson, 2002), Losillasaurus (Harris and Dodson, 2004), Cetiosauriscus (Rauhut et al., 2005) and Tehuelchisaurus (Upchurch et al., 2004) have been suggested as close relatives. However, it's also possible Tehuelchesaurus is a basal titanosauriform (Rauhut et al., 2005) and Mamenchisaurus is more closely related to neosauropods (Upchurch et al., 2004). Wilson suggested Omeisauridae as a family name, but if Mamenchisaurus is included, Mamenchisauridae may have priority.

I recall seeing in the archives an old post in which
Dr. Holtz? said that when the gaps in the
Nemegtosaurus and Quaesitosaurus skulls are filled
with Brachiosaurid parts it looks fine. Now I know
both of these genera are considered Titanosaurs
(though I'm not familiar with what a Titanosaur skull
looks like) are Nemegtosaurus and Quaesitosaurus now
considered to have more Brachiosaurid skulls than how
they are figured in "The Dinosauria?"

There's a great new paper on this topic-
Wilson, J.A. (2005). Redescription of the Mongolian sauropod _Nemegtosaurus mongoliensis_ Nowinski (Dinosauria: Saurischia) and comments on Late Cretaceous sauropod diversity. Journal of Systematic Palaeontology 3: 283-318.

Wilson's new reconstruction does look more brachiosaur-like than the Dinosauria's figures.

How are Diplodocoids defined? From the descriptions
I've heard on list of Nigersaurus (though I've never
seen any material) it seems like it lacks alot of the
characteristics that are normally associated with the
group (long neck, whip-like tail, skid-like chevrons,
bifid cervical neural spines)

You're apparently looking for a diagnosis (character list).
Wilson (2002) lists the following-
1. Posterolateral processes of premaxilla and lateral processes of maxilla without midline contact.
2. Premaxillary anterior margin without step.
3. Parietal excluded from margin of post-temporal fenestra.
4. Elongate basipterygoid processes.
5. Basipterygoid processes oriented anteriorly.
6. Rectangular-shaped dentary ramus.
7. Tooth rows restricted anterior to subnarial foramen.
8. Tooth crowns do not overlap.
9. Cylindrical tooth crowns.
10. More than four replacement teeth per alveolus.
11. Loss of triangular neural spines (reversal).
12. Anterior caudal neural spines broad.
13. Biconvex distalmost caudal centra.
14. Biconvex caudal centra elongate.

Lastly a couple questions about Saltasaurus and
Titanosaurs. "The Dinosauria" says that Saltasaurus
had two types of armor, osteoderms (which I can only
picture like those of a crocodile) and dermal plates.
What is a dermal plate? Are these two types of armor
illustrated well anywhere? Titanosaurus
madagascariensis had a scute found with it, is this
like a crocodile's scute?

The dermal plates are large (~10 cm) and keeled, while the ossicles are smaller (~6 mm) and irregularily shaped. Here's a dermal plate-

Mickey Mortimer