LATE JURASSIC MORRISON FORMATION
Diplodocus.
updated 060412
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Diplodocus gets its name from the middle tail bones which had fore and aft bony skids that protected the blood vessels if its tail dragged on the ground.
The tail, supported by huge tendons, was actually held horizontally, well above the ground. A find of skeleton and skin impressions indicated that it (and possibly other sauropods also) had a row of spines down its back.