We all know about the
Blue Whale, which is known to be the biggest creature that ever lived on
Earth, but paleontologists discoveries raised up some doubts about this claim.
While we can't be 100% sure that there was any ever lived creature bigger than the blue whale, some clues are worth a look at. Let's see what were
these creature, along with a few impressive other, smaller than the blue whale, but interesting nonetheless.
1- Leedsichthys problematicus
Leedsichthys was a giant pachycormid (an extinct group of Mesozoic bony fish) that lived in the oceans of the Middle Jurassic period and those
remaining fossils have been found in the Callovian of England, northern Germany, the Oxfordian of Chile, and the Kimmeridgian of France.
Size estimation: 30 to 53 feet (9 to 16 meters)
Although the remains of over seventy individuals have been found, most of them are partial and fragmentary, which has made it difficult to
estimate the fish's length. Arthur Smith Woodward, who described the specimen in 1889, estimated it to be 30 feet (around 9 metres) long, by comparing
the tail of Leedsichthys with another pachycormid, Hypsocormus.
In 1986, Martill compared the bones of Leedsichthys to a pachycormid that he had recently discovered, but the unusual proportions of that specimen
gave a wide range of possible sizes.
More recent estimates, from documentation of historical finds and the excavation of the most complete specimen ever from the Star Pit near Whittlesey,
Peterborough, support Smith Woodward's figures of between 30 and 33 feet (9 and 10 meters).
Recent work on growth ring structures within the remains of Leedsichthys have also indicated that it would have taken 21–25 years to reach these
lengths, and isolated elements from other specimens indicate that a maximum size of just over 16 metres (53 feet) is not unreasonable.

Source
2- Livyatan melvillei
Livyatan melvillei is an extinct species of physeteroid whale, which lived during the Miocene epoch, approximately 12-13 million years ago and those
remaining fossils were discovered in the sediments of Pisco formation at Cerro Colorado, 35 kilometres (22 mi) south-southwest of Ica, Peru. The
remains include a partially preserved skull with teeth and mandible.
Size estimation: 44 to 57 feet (13.5m to 17.5m)
Livyatan melvillei had a body length about the same as a modern adult male sperm whale. The skull of Livyatan melvillei is 3 metres (10 ft) long.
Unlike the modern sperm whale, Physeter macrocephalus, L. melvillei had functional teeth in both its jaws.
The jaws of L. melvillei were robust and its temporal fossa was also considerably larger than in the modern-age sperm whale. L. melvillei is one of
the largest raptorial predators yet known, with whale experts using the phrase "the biggest tetrapod bite ever found" to explain their find. The teeth
of L. melvillei are up to 36 centimetres (1.18 ft) long and are claimed to be the largest of any animal yet known. Larger 'teeth' (tusks) are known,
such as walrus and elephant tusks, but these are not used directly in eating.
Source
Photos source
3- Amphicoelias fragillimus
Amphicoelias is a genus of herbivorous sauropod dinosaur that includes what may be the largest dinosaur ever discovered, A. fragillimus.
Size estimation: 130 to 200 feet (40 to 60 meters)
The third named Amphicoelias species, A. fragillimus, is known only from a single, incomplete 1.5 m tall neural arch (the part of a vertebra with
spines and processes), either last or second to last in the series of back vertebrae, D (dorsal) 10 or D9, that would have measured 2.7 m (8.8 ft)
long in life. In addition to this single vertebra, Cope's field notes contain an entry for an "immense distal end of femur” located only a few tens
of meters away from the giant vertebra, and it is likely that this undescribed leg bone belonged to the same individual animal as the neural spine.
The vertebra was in poor condition, but astonishingly large, measuring 1.5 metres (4.9 ft) up to 2.7 metres (8.9 ft) in height
Source
Photo source
As a comparison scale for all these creature, I took the longest dinosaurs charts from Wikimedia and added to it two of the three creatures above plus
the actual Blue Whale:
edit on 9-7-2012 by elevenaugust because: (no reason given)