You can determine a lot about the physiology of an animal from the fossil remains including what they ate and how they moved. Most soft tissue is a guess since that does not get preserved with the bone. They do a good job of explaining the reasoning behind their scale model in videos throughout the exhibit.
They have videos explaining how they found the fossils and how they recreated it. Also they have a replica of it and an employee standing there to answer questions. From my understanding, based on the size of the spine they deduced the size if the snake
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