You'd have to do a volumetric study to get a realistic weight estimate for the seismosaur's neck but, visually, you can line up three to five 10,000 lb elephants alongside the guy's neck and figure 40K lbs and you're probably ballpark.
Considering that elephant are a bit thicker and bigger that the neck of of a
Seismosaurus, I'd say that your estimate is far, far, far higher than the actual weight.
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Idiot