This generation of paleontologists grew up on the skeletons of Gregory Paul but his art is getting harder and harder to find online even without purchasing copyrights. I understand a man has to eat and in that pursuit I have always tried to ask permission before, or been willing to take down art after it is posted if asked to do so. I can't even get in touch with Mr. Paul most of the time and therefore I have to turn to the equally brilliant and accurate work of Scott Hartman, who I can ask for permission from and if I have ever forgotten to do so I apologize to him and everyone that that affects. Regardless, Mr. Hartman's skeletals are for the generation coming up, what Paul's were for the current generation and that is due in part to the fact that I find them all over the internet in papers, on his sites, and embedded in dinosaur stories and art collections made independently. The fact that he has produced a very diagnostic specimen of Mamenchisaurus will help in viewing other artist's illustrations of the long necked sauropod. This particular Mamenchisaurus appears in Mr. Hartman's blog on an update concerning tail rigidity and flexibility and, as you can see, the tail of Mamenchisaurus does not possess the bony rods that have come to be associated with rigid balancing tails and therefore must have some side to side flexibility, which we can discuss with the illustrations shortly.
This illustration is an exact copy, though the positioning is obviously contrary and then somewhat rotated, of Scott Hartman's skeletal. The brachiosaurid nature of the skull presented in the actual skeleton and in the above skeletal drawing is well represented here, remember that for the next illustrations. The tail here is visibly flexing and has a little of the whip-like structure that we will talk about in a moment also. This is an attribute given to diplodocids, if you remember that old entry, but has been transferred to many sauropods with longer tails that taper drastically like Mamenchisaurus. The illustration is a good fleshing out of Hartman's skeletal, and it uses the brachiosaur-like skull of M. youngi, but not every illustration has done that in the past. One thing about the tail which this and all of the other illustrations are "missing" is the supposed tail club found with M. hochuanensis. The purpose or the and the extent of use of the tail club are as yet unknown.
I think the most iconic of Mamenchisaurus images belongs to Mark Hallett. Hallett clearly did not use M. youngi as a model as we can tell by looking at the skulls of the juvenile and the adult. The fact is that not all of the skulls have been fully found or pieced together for every species of Mamenchisaurus. This is problematic for a number of reasons and one of those would certainly be that older illustrations, like this masterpiece, may have been partly inspired but what was assumed to be a very diplodocid or apatosaur like body and the related assumption that skull formation would then be very Diplodocus or Apatosaurus like in appearance.
Finding the majority of the skull in M. youngi may have been an amazing stroke of luck, but it does also make fantastic illustrations like this make the head look rather strange if the M. youngi brachiosaurid skull is template for all skulls in the genus. The horizontal apatosaur holding of the neck would then look a little out of place because we are so used to seeing brachiosaur skulls vertical and apatosaur skulls horizontal to the ground. Since I have not seen all the remains, and not all of the species had full skulls including M. constructus (which had no skull), M. hochuanensis, M. sinocanadorum (partial skulls), M. jingyanensis (majority of skull), and M. anyunensis (no mention of skulls), though, I cannot just toss about theories today on what all of the skulls looked like. Paul has illustrated the M. hochuanensis as having a skull which is intermediate between the skulls of apatosaur-like Mamenchisaurs like shown by Hallett and Martin and the brachiosaur-like skull of M. youngi.
One last thing to notice before I leave it alone for the rest of the day is that the skull of M. youngi clearly was magnificent regardless of whether it is the actual template or the intermediate skull of M. hochuanensis is the template for the genus. Just look at those robust teeth! This thing probably could have eaten mammals and lizards with ease to be honest. I am sure that they probably ate mainly tough plants by stripping the leaves or ferns, but those teeth could be a weapon or an omnivore's teeth to a degree for sure. They wouldn't be as good as a theropod's teeth, but they're pretty fantastic.