Hi @seismosaurus! So far, the longest dinosaur that ever lived is, as I am sure that you may have already known, Seismosaurus. Of course, this is subject to much debate and controversy among scientists. The Amphicoelias was supposedly another large dinosaur that was discovered in 1877. However, only a partial skeleton was ever found, and the pieces that were discovered were in such poor shape, that the existence of Amphicoelias is still questionable.

Seismosaurus belongs to the family diplodocidae. Diplodocus was once thought to be the longest dinosaur ever. However, scientists later discovered much longer relatives that also roamed the earth during the Jurassic period. Many diplodicae were discovered during the late 1800s and early 1800s in many different parts of the world. Technically speaking, Diplodocus is still the longest dinosaur that scientists have yet to locate a complete skeleton for. It is believed that most diplodocidae, including Seusmosaurus, lived at the end of the Jurassic period.

Seismosaurus was a sauropod, which means that it ate plants and walked on four legs. As you can imagine, it also had a very long neck. It is estimated that Seismosaurus could reach between 150 and 197 feet in length and weighed between 80 and 100 tons.

Based on my searches, the longest dinosaur that ever lived is the Amphicoelias.



The ten longest known dinosaurs, based on published length estimates.
Amphicoelias: 40-60 m (131–198 ft)
Supersaurus: 33 to 34 m (108 to 112 ft)
Futalognkosaurus: 32–34 m (105–112 ft)
Sauroposeidon: 28–34 m (92–112 ft)
Diplodocus: 25.6–33.5 m (84–110 ft)
Argentinosaurus: 30 m (98 ft)
Puertasaurus: (comparable to Argentinosaurus)
Hudiesaurus: 30 m (98 ft)
Turiasaurus: >30 m (98 ft)
Barosaurus: 26 m (85 ft)

It's difficult to answer this question, because only estimated lengths are available. That said, there is a great list at Wikipedia.

From;
http://en.wikipedia.org/wiki/Dinosaur_size#Longest_dinosaurs

Longest dinosaurs

See also Longest sauropods The ten longest known dinosaurs, based on published length estimates.

1. Amphicoelias: 40-?60 m (131–198 ft)1
2. Supersaurus: 33 to 34 m (108 to 112 ft)9
3. Futalognkosaurus: 32–34 m (105–112 ft)4
4. Sauroposeidon: 28–34 m (92–112 ft)167
5. Diplodocus: 25.6–33.5 m (84–110 ft)13
6. Argentinosaurus: 30 m (98 ft)1
7. Puertasaurus: (comparable to Argentinosaurus)5
8. Hudiesaurus: 30 m (98 ft)10
9. Turiasaurus: >30 m (98 ft)8
10. Barosaurus: 26 m (85 ft)11

http://upload.wikimedia.org/wikipedia/commons/thumb/f/f4/Amphicoelias_altus_scale.png/800px-Amphicoelias_altus_scale.png