Hi! Would you be interested in hearing about a new giant mamenchisaurid?
I remember we had a small exchange about M. sinocanadorum on one of my deviations so I thought you may be interested in this.
The diameter of this animal's femur is exactly twice that of Mamenchisaurus youngi (15 cm according to the supplementary material of Benson 2014), and assuming relative geometric similarity it would be 8 times more massive.
(Greg Paul's estimation for M. youngi was 7 tonnes, but his model is somewhat shrinkwrapped and a healthy model is probably closer to 8).
So based on the healthy model at least, this animal would be around 64 tonnes.
Sizes like this are also supported by the length of the hindlimb - while it is incomplete it is 3 meters as preserved, and would be even larger complete.
On his website, Greg Paul estimates a Mamenchisaurus hochuanensis with a 135 cm femur to weigh 14 tonnes, and given that femur length, his skeletal yields a hindlimb length of 2 meters (link). Scaling that up to the incomplete hindlimb length alone yields over 47 tonnes, and would be even larger using the full hindlimb to scale (which would be 358 cm based on Mamenchisaurus youngi given a 112.5 cm femur and 66.5 cm tibia for M. youngi as well as the greater diameter). Given that hindlimb length, it would be about 80.3 tonnes scaling from M. hochuanensis.
As for what it is, it could be a new species, but given its size and where it was discovered I wouldn't be surprised if it turns out to be a new specimen of Mamenchisaurus sinocanadorum.
Either way, I'd say there's a good chance that this hindlimb belongs to a specimen that's of comparable size to the giant Tokyo mount or at least approaching the scale of it.