Diplodocus hallorum
This dinosaur is a subsspecies of Diplodocus.Seismosaurus (Diplodocus) is the second longest dinosaur of all time. It was named Earth Shaking Lizard by David Gilette. It was thought to reached a length of 54 metres but due to wrong placements of the tail vertebrea and the complication of another dinosaur's bone in it's tail it's length was revised to a slightly shorter size.
Size
Now Seismosaurus is said to have an unbeatable length of between 39-52 metres (45 metres is the most widely accepted length) which eclipses all other dinosaurs, even supersaurus in size (does not apply for Amphicoelias). It's weight varies from 50- over 100 tonnes. It is now thought to weigh up to 113 tonnes when fully grown which is slightly heavier than Argentinosaurus. Seismosaurus could reach 16 metres off the ground to the tops of trees making it extremely tall too.
When
Seismosaurus lived in the Late Jurassic 150 mya when dinosaurs reached their greatest size and Seismosaurus and Amphicoelias both prove that fact and both lived around what is today Colorado and New Mexico, USA.
Defences
Seismosaurus would lived in small groups although, even theropods like Saurophaganax, which could reach up to 12.8 metres would been able to bring one down even in a pack. Seismosaurus also had an immense whip like tail to defend itself although it's size alone would povide that.
Diet and Feeding
Seismosaurus used it's incredibly long neck both reach for leaves no the tops of trrees and also to crop low growing plants. It would have probably had a varied diet eating ferns, horsetails, moss, leaves and cronifers which were plentiful at the time.