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> _Euhelopus_, a camarasaurid;
> _Omeisaurus_, a shunosaurid related to _Datousaurus_;
> and _Mamenchisaurus_, between the Shunosauridae and the Diplodocidae.
> Have any of these been re-united into a larger Euhelopodidae?

All of the above, according to Paul Upchurch, as well as 
_Klamelisaurus_, and _Nurosaurus_ (?spelling).  
But, there's no good evidence that _Tienshanosaurus_ and 
_Rhoetosaurus_ are euhelopodids.  I don't know about _Chiayusaurus_.

Euhelopodids exhibit forked (or "skid-like") chevrons, like 
diplodocoids.  In my opinion, this evolved independently in both 
groups (to protect subvertebral blood vessels when the tail was 
dragged along the ground).  The situation is not known for 
_Euhelopus_, and I haven't seen a description yet for _Nuro_ (which, 
overall, looks very similar to _Euhelopus_). 

Of these, _Euhelopus_, _Nuro_, _Omei_, _Mamenchi_, and _Klameli_ 
comprise the "long-necked" euhelopodids.  All show a pronounced 
increase in both the number of cervical vertebrae and the total 
number of presacral vertebrae.   Diplodocids show an increase in the 
number of cervicals, but only a small (if any) increase in the total 
number of presacrals; extra cervicals were recruited from the dorsal 

_Shuno_ and _Omei_ also have a bony tail club, and other euhelopodids 
may have had them too. 

Euhelopodid skulls look a bit like the skull of _Camarasaurus_ 
(especially that of_Euhelopus_); but, as someone else pointed out, 
Upchurch found a number of persistently primitive features in the 
skulls of euhelopodids.